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A novice-level video about tides. A good place to start!

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Use this simulation to observe tidal movement caused by both the moon and the sun

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Tide Simulation - VirtualSC Video Portal

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We've talked a lot about the effect of the Moon's gravitational pull on Earth. But what about Earth's much bigger gravitational influence on the Moon? After all, Earth has 80 times the Moon's mass. Well, just as the Moon's pull slightly distorts Earth's sphere, Earth's gravity slightly deforms the Moon. It's not as dramatic as the ocean tides ― think of it as the difference between trying to squish a balloon filled with water and a balloon filled with sand ― but these tides on the Moon are measurable using lasers, and in some cases their effects are visible. Young cliffs on the Moon, called lobate scarps, form due to the combined forces of the Moon contracting as its hot interior cools and Earth's gravity pulling on the surface. The contraction causes the Moon's crust to buckle, pushed together and upwards to form the cliffs, but scientists examining these cracks have observed that their positions are related to the pull of Earth's gravity.

Writer: Tracy Vogel
Graphic Designer: Vi Nguyen
Science Advisors: Vishnu Viswanathan, Joseph Renaud, NASA's Goddard Space Flight Center

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You walk along a beach, seashells, driftwood and seaweed left by the retreating tides at your feet. Look up at the Moon, and you're seeing the main cause of the surge and retreat of oceans from our shores. As distant as the Moon may seem, its gravitational pull on Earth plays a huge role in [...]

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This video shows people making a true scale model of the solar system in the desert. This helps give a sense of how large the solar system is, and how far away objects truly are.

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To Scale: The Solar System | PBS LearningMedia

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Observe a team as they build an accurate scale model of the solar system on a dry lakebed in Nevada in this video from Wylie Overstreet and Alex Gorosh. Use this resource to visualize the abstract concept of the size and scale of the solar system and to develop and use models.

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VIDEO: An animation of the movements of Earth, Sun, and Moon. NOT to scale.

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VIDEO: a demonstration of why tilt affects seasons

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Please be aware that resources have been published on the website in the form that they were originally supplied. This means that procedures reflect general practice and standards applicable at the time resources were produced and cannot be assumed to be acceptable today. Website users are fully responsible for ensuring that any activity, including practical work, which they carry out is in accordance with current regulations related to health and safety and that an appropriate risk assessment has been carried out.

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The seasons: light intensity and latitude

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This Teaching Astronomy and Space video clip, from the Institute of Physics (IOP), Teachers TV and the Science and Technology Facilities Council (STFC), shows a simple demonstration to explain why regions at the equator are hotter than regions nearer the Earth's north and south poles.

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VIDEO: a demo of moon phases

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Please be aware that resources have been published on the website in the form that they were originally supplied. This means that procedures reflect general practice and standards applicable at the time resources were produced and cannot be assumed to be acceptable today. Website users are fully responsible for ensuring that any activity, including practical work, which they carry out is in accordance with current regulations related to health and safety and that an appropriate risk assessment has been carried out.

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This Teaching Astronomy and Space video, from the Institute of Physics (IOP), Teachers TV and the Science and Technology Facilities Council (STFC), illustrates a simple demonstration of the phases of the Moon. Using a light source and a small ball, the Moon's phases can be seen by a central observer representing the Earth.

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VIDEO: Jaclyn disrupts some common misconceptions about rotation, revolution, and tilt

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Easily create beautiful interactive video lessons for your students you can integrate right into your LMS. Track students' progress with hassle-free analytics as you flip your classroom!

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SIMULATION: An interactive model that shows how the moon, earth, and sun interact.

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Drag Earth or the Moon to change the time.

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VIDEO: Moon's rotation from NASA

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Published: December 2, 2022

Historical Date: November 23, 2020

An enduring myth about the Moon is that it doesn't rotate. While it's true that the Moon keeps the same face to us, this only happens because the Moon rotates at the same rate as its orbital motion, a special case of tidal locking called synchronous rotation. The animation shows both the orbit and the rotation of the Moon. The yellow circle with the arrow and radial line have been added to make the rotation more apparent. The arrow indicates the direction of rotation. The radial line points to the center of the visible disk of the Moon at 0°N 0°E.
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The Moon's Orbit and Rotation - Moon: NASA Science

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Animation of both the orbit and the rotation of the Moon.

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DATA SET: Tide data for identifying patterns

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Tide Predictions - NOAA Tides & Currents

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provides measured tide prediction data in chart and table

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LAB: shows how objects interact in space. Useful for identifying patterns, stability, and change.

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VIDEO: Harvard graduates demonstrate misconceptions about the seasons, and the real reason for the seasons is explained.

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VIDEO: The introduction to the unit project

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A visually powerful series that shows the power centers of the universe: black holes, exploding stars, and active galaxies. Requires Amazon Prime.

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This visually dynamic series follows a trail of energy into the power centers of the universe: black holes, exploding stars, active galaxies. Based on the findings of current science, including state of the art supercomputer simulations, the series dives into the heart of a supermassive black hole, flies down onto toxic alien planets and rides on the roiling surface of stars about to go supernova.

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Watch Secrets Of The Universe now on your favorite device! Enjoy a rich lineup of TV shows and movies included with your Prime membership.

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A NOVA documentary series that gives the fascinating story of every planet in our solar system. Requires Amazon Prime.

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Watch NOVA: The Planets now on your favorite device! Enjoy a rich lineup of TV shows and movies included with your Prime membership.

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Astrophysicists show how black holes might hold the answers to how the universe evolved. Requires Netflix.

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Astrophysicists show how black holes might hold answers to how the universe evolved, leading to life on Earth and, ultimately, the human race.

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To determine whether we’re alone in the universe, astrobiologists look at Mars, Jupiter, and extreme environments on Earth. Requires Netflix.

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If life exists in other worlds, our own planet and its neighbors may hold clues as to how it would arise. Watch trailers & learn more.

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As a doctor, engineer, and NASA astronaut, Mae Jemison has always reached for the stars. In 1992, Jemison became the first African American woman to travel in space. She has also written several books and appeared on many television programs including an episode of Star Trek: The Next Generation. In addition to her many awards, Jemison has been inducted into the National Women's Hall of Fame and the International Space Hall of Fame.

Mae Carol Jemison was born on October 17, 1956 in Decatur, Alabama. The youngest of three children, her mother was an elementary school teacher and her father was a maintenance supervisor. A few years after she was born, Jemison and her family moved to Chicago, Illinois. In addition to her love for dance, Jemison knew that she wanted to study science at a very young age. Jemison grew up watching the Apollo airings on TV, but she was often upset that there were no female astronauts. However, Jemison was inspired by African American actress Nichelle Nichols who played Lieutenant Uhura on the Star Trek television show. Jemison was determined to one day travel in space. In 1973, she graduated from Morgan Park High School when she was 16 years old. Once she graduated, Jemison left Chicago to attend Stanford University in California.

As one of the only African American students in her class, Jemison experienced racial discrimination in school. She later served as president of the Black Student Union and choreographed a performing arts production called Out of the Shadows about the African American experience. Jemison graduated in 1977 with a Bachelor of Science degree in Chemical Engineering and a Bachelor of Arts degree in African and African-American studies. After graduating from Stanford University, Jemison attended Cornell Medical School. While in medical school, she traveled to Cuba to lead a study for the American Medical Student Association. She also worked at a Cambodian refugee camp in Thailand. Jemison graduated from Cornell with a Doctorate in Medicine in 1981. Shortly after her graduation, she became an intern at the Los Angeles County Medical Center, and then went on to practice general medicine. Fluent in Russian, Japanese and Swahili, Jemison joined the Peace Corps in 1983 and served as a medical officer for two years in Africa.

After working with the Peace Corps, Jemison opened a private practice as a doctor. However, once Sally Ride became the first American woman in space in 1983, Jemison decided to apply to the astronaut program at NASA. She applied in 1985, but after the Space Shuttle Challenger exploded in 1986, NASA took a break from accepting new people. However, Jemison applied again in 1987 and was one of the 15 people chosen out of over 2,000 applications. She was selected for NASA Astronaut Group 12, which was the first group chosen after the Challenger explosion. After being selected, Jemison trained with NASA and worked on projects at the Kennedy Space Center in Florida and the Shuttle Avionics Integration Laboratory. She received her first mission on September 28, 1989 when she was selected to join the STS-47 crew as a Mission Specialist. On September 12, 1992 Jemison and six other astronauts went into space on the space shuttle Endeavor. This voyage made Jemison the first African American woman in space. The team made 127 orbits around the Earth and returned to the Kennedy Space Center in Florida on September 20, 1992.

Jemison left NASA in 1993 after serving as an astronaut for six years in total. She started The Jemison Group, a consulting company that encourages science, technology, and social change. She also began teaching environmental studies at Dartmouth College and directed the Jemison Institute for Advancing Technology in Developing Countries. After hearing that she was a fan of the Star Trek television show, actor LeVar Burton asked Jemison to appear in an episode. Jemison agreed and became the first real astronaut to be on an episode of Star Trek: The Next Generation. She played Lieutenant Palmer in the episode, "Second Chances." In 1994, Jemison created an international space camp for students 12-16 years old called The Earth We Share (TEWS). She also created a nonprofit organization called the Dorothy Jemison Foundation for Excellence. Jemison later accepted a position as the Andrew D. White Professor-at-Large at Cornell University in 1999. She went on to write her first book in 2001, Find Where the Wind Goes, which was a children's book about her life.

Currently, Jemison is leading the 100 Year Starship project through the United States Defense Advanced Research Projects Agency (DARPA). This project works to make sure human space travel to another star is possible within the next 100 years. She also serves on the Board of Directors for many organizations including; the Kimberly-Clark Corp., Scholastic, Inc., Valspar Corp., Morehouse College, Texas Medical Center, Texas State Product Development and Small Business Incubator, Greater Houston Partnership Disaster Planning and Recovery Task Force, and the National Institute of Biomedical Imaging and Bioengineering. Jemison is a member of the National Academy of Sciences' Institute of Medicine, and has been inducted into the National Women's Hall of Fame, National Medical Association Hall of Fame and Texas Science Hall of Fame. She has received multiple awards and honorary degrees including the National Organization for Women's Intrepid Award and the Kilby Science Award. She currently lives in Houston, Texas.

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Astronaut Mae Jemison became the first African American woman to travel in space. Discover more at womenshistory.org.

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It was almost like a dispatch from another planet: the invitation to the young astronomer to leave Brooklyn and visit the lakes and gorges of upstate New York.

"A letter shows up in my mailbox from Carl Sagan," said Neil deGrasse Tyson, recalling the 1975 invitation at a recent Library of Congress event celebrating Sagan. "I couldn't believe it. Famous people don't write out of the blue to strangers."

But the invitation was real. In response to his Cornell application, Tyson met the famous professor on a college visit soon after. Sagan offered to let the 17-year-old astronomer camp out at his house if a snowstorm knocked out his bus ride home.

Tyson eventually ended up at Harvard instead of Cornell, but he now hosts Cosmos, the 13-part remake of the original series (airing on the National Geographic Channel and Fox on Sundays at 9 p.m. EDT/8 p.m. CDT) that shot Sagan's celebrity sky high.

The original invitation, the visit, and the connection were typical Carl Sagan.

"He worked very hard for his students, got them jobs, worried about their education, many of them very well placed now," says William Poundstone, author of Carl Sagan: A Life in the Cosmos. "If you talk to the people he inspired, who knew him, they are uniformly effusive."

"Sagan was certainly the most famous U.S. scientist of the 1980s and early 1990s," says science journalism expert Declan Fahy of American University in Washington, D.C. Fahy says, "After Cosmos reached half a billion viewers in 60 nations, his fame reached another level. The book of the series spent more than 70 weeks on the bestseller list."

But who was Carl Sagan? Scientist, celebrity, writer, professor, skeptic, and free-thinker, he was much more than the narrator of a TV series.

"Part of what made him great was the number of things he pursued," says NASA's David Morrison, director of the Carl Sagan Center for the Study of Life in the Universe, at the SETI Institute in Mountain View, California. Morrison marveled both at Sagan's breadth of accomplishments and his lack of self-importance.

"He worked very hard, 18-hour days. He had a tremendous appetite for his work," says Poundstone. "He was made for television, sure, and he looked very relaxed and normal in jeans when other scientists didn't. But there was a lot more to him."

As a scientist, Sagan made a real mark on planetary science in the early 1970s as a young Harvard professor, "at a time when planetary science was a bit of a backwater," Poundstone says.

Sagan first predicted that the greenhouse effect made the atmosphere of Venus hot enough to melt lead, at a time when some scientists still speculated that its clouds might hide oceans, says Morrison.

Sagan also identified dark-shaded regions on Mars as highlands and identified lighter areas as desert plains marked by dust storms. Those storms later bedeviled NASA's Mars Viking landers in the 1970s.

"He was a really great big-picture scientist, great with back-of-the envelope calculations, who could see the fundamental premises of science and observations," says Morrison.

On the two Voyager missions launched in 1977 to explore Jupiter, Saturn, Uranus, and Neptune, Sagan was a member of the imaging science team. "That was really before the crest of his fame," says Morrison, a former student of Sagan's. "He was not a superstar then, just one of us."

Sagan led the team that put together the "golden records" attached to the Voyager mission's two spacecraft. The records (sent along with phonograph needles) included cuts of everything from Bach to "Johnny B. Goode," along with greetings and natural sounds from Earth.

To a large extent, Poundstone says, Sagan benefited from filling a scientific niche, planetary science, that was set to explode with new knowledge as a result of NASA's line of planetary probes exploring the solar system starting in the 1960s.

Reporters gravitated toward Sagan on those missions, Poundstone says. "They knew who could explain things." Sagan ended up as a regular on the Tonight Show (as Tyson now is on Comedy Central's Colbert Report), a guest favorite of Johnny Carson.

Parodied by Carson for his consonant-rolling pronunciation of "bill-ions and bill-ions" in the series, Sagan indeed thought big, even opening a line of Cosmos-themed stores that anticipated the museum-themed stores in malls today.

After the 1980 publication of Cosmos and the premiere of the PBS series, "things changed for Carl. He was getting death threats; he had to travel in limousines and keep a closed schedule," Morrison said. "People don't remember that."

The death threats partly derived from Sagan's work on the U.S. Air Force's Project Blue Book, which investigated UFOs in the 1950s and 1960s. "He started out with an open mind, but came to the conclusion that there wasn't any evidence for aliens visiting Earth," Poundstone says. Sagan was a big proponent of the likelihood of life existing elsewhere in the universe, however.

Sagan also waded into one of the wellsprings of today's global warming debate, as the senior author on a 1983 Science journal study of "nuclear winter." The group's climate model found worldwide subzero temperatures an inevitable consequence of the dust clouds resulting from a nuclear exchange between the superpowers.

He had angered both UFO fans and belligerent critics of the nuclear winter report. Morrison recalls that after Sagan received threats, "they hid his office number at Cornell and he used a back door to get to work." In the era of the "Unabomber" mailing explosives to professors, the threats were taken seriously.

"Science is more than a body of knowledge. It's a way of thinking, a way of skeptically interrogating the universe with a fine understanding of human fallibility," Sagan said in his last, 1996 interview with PBS's Charlie Rose. He died that same year from cancer.

The power of skeptical thinking infused his 1995 book, The Demon-Haunted World: Science as a Candle in the Dark, which sought to explain the scientific method to general readers. It included a "baloney-detection kit" for skeptics.

Sagan wasn't skeptical about everything: He was also a marijuana advocate, appearing as "Mr. X," a successful pot smoker, in a book by Harvard's Lester Grinspoon. And he advocated for medical marijuana in the years before his death.

"Do something meaningful," said Ann Druyan, Sagan's third wife, speaking at the Library of Congress event, where Sagan's papers were donated to its collections. "That was at the heart of his whole life."

Druyan, Sagan's third wife and frequent collaborator, was a co-writer on the original Cosmos and is part of the production team on the new series.

Sagan wrote 20 popular books (dictating, not typing them) and hundreds of scientific studies. Nevertheless, his fame brought him criticism from other scientists, and a snub from the National Academy of Scientists, when he was nominated for membership but not accepted.

"That's just human nature: envy and resentment," Poundstone says.

Today, Cosmos is remembered for popularizing thinking big about space and inspiring young scientists. And the memory of Sagan as a prominent defender of science is perhaps his greatest legacy.

"His influence can be seen now in that almost every scientist with a prominent media profile cites him as an influence," Fahy says. "When scientific organizations want to increase public interest in science, one of their first ideas is something like: 'We need more Carl Sagans.'"

If he were around today, Poundstone says, Sagan would likely be speaking out forcefully for science in his inimitable way, regardless of the slights.

"For at least 100 years we have had scientific celebrities," he says. "But Sagan was the first one with a personality made for the age of television, and that was where he found a home."
Follow Dan Vergano on Twitter.

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It was almost like a dispatch from another planet: the invitation to the young astronomer to leave Brooklyn and visit the lakes and gorges of upstate New York. "A letter shows up in my mailbox from Carl Sagan," said Neil deGrasse Tyson, recalling the 1975 invitation at a recent Library of Congress event celebrating Sagan.

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Johannes Kepler: His Life, His Laws and Times

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NASA.gov brings you the latest images, videos and news from America's space agency. Get the latest updates on NASA missions, watch NASA TV live, and learn about our quest to reveal the unknown and benefit all humankind.

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He made major contributions to the fields of physics, astronomy, cosmology, mathematics and philosophy.

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Table of Contents

1. Galileo's Early Life, Education and Experiments

2. Galileo, Telescopes and the Medici Court

3. Galileo Galilei's Trial

4. What Was Galileo Famous For?&nbsp;

Galileo Galilei (1564-1642) is considered the father of modern science and made major contributions to the fields of physics, astronomy, cosmology, mathematics and philosophy. Galileo invented an improved telescope that let him observe and describe the moons of Jupiter, the rings of Saturn, the phases of Venus, sunspots and the rugged lunar surface. His flair for self-promotion earned him powerful friends among Italy's ruling elite and enemies among the Catholic Church's leaders. Galileo's advocacy of a heliocentric universe brought him before religious authorities in 1616 and again in 1633, when he was forced to recant and placed under house arrest for the rest of his life.

Galileo's Early Life, Education and Experiments

Galileo Galilei was born in Pisa in 1564, the first of six children of Vincenzo Galilei, a musician and scholar. In 1581 he entered the University of Pisa at age 16 to study medicine, but was soon sidetracked by mathematics. He left without finishing his degree. In 1583 he made his first important discovery, describing the rules that govern the motion of pendulums.

Did you know? After being forced during his trial to admit that the Earth was the stationary center of the universe, Galileo allegedly muttered, "Eppur si muove!" ("Yet it moves!" ). The first direct attribution of the quote to Galileo dates to 125 years after the trial, though it appears on a wall behind him in a 1634 Spanish painting commissioned by one of Galileo's friends.

From 1589 to 1610, Galileo was chair of mathematics at the universities of Pisa and then Padua. During those years he performed the experiments with falling bodies that made his most significant contribution to physics.

Galileo had three children with Marina Gamba, whom he never married: Two daughters, Virginia (Later "Sister Maria Celeste") and Livia Galilei, and a son, Vincenzo Gamba. Despite his own later troubles with the Catholic Church, both of Galileo's daughters became nuns in a convent near Florence.

Galileo, Telescopes and the Medici Court

In 1609 Galileo built his first telescope, improving upon a Dutch design. In January of 1610 he discovered four new "stars" orbiting Jupiter-the planet's four largest moons. He quickly published a short treatise outlining his discoveries, "Siderius Nuncius" ("The Starry Messenger"), which also contained observations of the moon's surface and descriptions of a multitude of new stars in the Milky Way. In an attempt to gain favor with the powerful grand duke of Tuscany, Cosimo II de Medici, he suggested Jupiter's moons be called the "Medician Stars."

"The Starry Messenger" made Galileo a celebrity in Italy. Cosimo II appointed him mathematician and philosopher to the Medicis, offering him a platform for proclaiming his theories and ridiculing his opponents.

Galileo's observations contradicted the Aristotelian view of the universe, then widely accepted by both scientists and theologians. The moon's rugged surface went against the idea of heavenly perfection, and the orbits of the Medician stars violated the geocentric notion that the heavens revolved around Earth.

Galileo Galilei's Trial

In 1616 the Catholic Church placed Nicholas Copernicus 's "De Revolutionibus," the first modern scientific argument for a heliocentric (sun-centered) universe, on its index of banned books. Pope Paul V summoned Galileo to Rome and told him he could no longer support Copernicus publicly.

In 1632 Galileo published his "Dialogue Concerning the Two Chief World Systems," which supposedly presented arguments for both sides of the heliocentrism debate. His attempt at balance fooled no one, and it especially didn't help that his advocate for geocentrism was named "Simplicius."

Galileo was summoned before the Roman Inquisition in 1633. At first he denied that he had advocated heliocentrism, but later he said he had only done so unintentionally. Galileo was convicted of "vehement suspicion of heresy" and under threat of torture forced to express sorrow and curse his errors.

Nearly 70 at the time of his trial, Galileo lived his last nine years under comfortable house arrest, writing a summary of his early motion experiments that became his final great scientific work. He died in Arcetri near Florence, Italy on January 8, 1642 at age 77 after suffering from heart palpitations and a fever.

What Was Galileo Famous For?

Galileo's laws of motion, made from his measurements that all bodies accelerate at the same rate regardless of their mass or size, paved the way for the codification of classical mechanics by Isaac Newton. Galileo's heliocentrism (with modifications by Kepler) soon became accepted scientific fact. His inventions, from compasses and balances to improved telescopes and microscopes, revolutionized astronomy and biology. Galilleo discovered craters and mountains on the moon, the phases of Venus, Jupiter's moons and the stars of the Milky Way. His penchant for thoughtful and inventive experimentation pushed the scientific method toward its modern form.

In his conflict with the Church, Galileo was also largely vindicated. Enlightenment thinkers like Voltaire used tales of his trial (often in simplified and exaggerated form) to portray Galileo as a martyr for objectivity. Recent scholarship suggests Galileo's actual trial and punishment were as much a matter of courtly intrigue and philosophical minutiae as of inherent tension between religion and science.

In 1744 Galileo's "Dialogue" was removed from the Church's list of banned books, and in the 20th century Popes Pius XII and John Paul II made official statements of regret for how the Church had treated Galileo.

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Galileo's Early Life, Education and Experiments Galileo, Telescopes and the Medici Court Galileo Galilei's Trial What Was Galileo Famous For? Galileo Galilei (1564-1642) is considered the father of modern science and made major contributions to the fields of physics, astronomy, cosmology, mathematics and philosophy.

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Known by some as the "father of modern astronomy"

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Who Was Copernicus?

Nicolaus Copernicus was born on February 19, 1473 in Torun, a city in north-central Poland on the Vistula River. Copernicus was born into a family of well-to-do merchants, and after his father's death, his uncle-soon to be a bishop-took the boy under his wing. He was given the best education of the day and bred for a career in canon (church) law.

At the University of Krakow (today's Jagiellonian University), he studied liberal arts, including astronomy and astrology, and then, like many Europeans of his social class, was sent to Italy to study medicine and law.

While studying at the University of Bologna, he lived for a time in the home of Domenico Maria de Novara, the principal astronomer at the university. Astronomy and astrology were at the time closely related and equally regarded, and Novara had the responsibility of issuing astrological prognostications for Bologna.

Copernicus sometimes assisted him in his observations, and Novara exposed him to criticisms both of astrology and of aspects of the Ptolemaic system - founded by the ancient mathematician and astronomer Ptolemy - which placed Earth at the center of the universe.

Copernicus later studied at the University of Padua and in 1503 received a doctorate in canon law from the University of Ferrara. He returned to Poland, where he became a church administrator and doctor.

In his free time, he dedicated himself to scholarly pursuits, which sometimes included astronomical work. By 1514, his reputation as a learned mathematician, physician and astronomer was such that he was consulted on matters of currency and coinage, and by church leaders attempting to reform the Julian calendar.

Ptolemaic System

The cosmology of early 16th-century Europe held that Earth sat stationary and motionless at the center of several rotating, concentric spheres that bore the celestial bodies: the sun, the moon, the known planets, and the stars.

From ancient times, philosophers adhered to the belief that the heavens were arranged in circles (which by definition are perfectly round), causing confusion among astronomers who recorded the often eccentric motion of the planets, which sometimes appeared to halt in their orbit of Earth and move retrograde across the sky.

In the second century, Ptolemy sought to resolve this problem by arguing that the sun, planets, and moon move in small circles around much larger circles that revolve around Earth. These small circles he called epicycles, and by incorporating numerous epicycles rotating at varying speeds he made his celestial system correspond with most astronomical observations on record.

The Ptolemaic system remained Europe's accepted cosmology for more than 1,000 years, but by Copernicus' day accumulated astronomical evidence had thrown some of his theories into confusion. Astronomers disagreed on the order of the planets from Earth, and it was this problem that Copernicus addressed at the beginning of the 16th century.

Heliocentric Theory

Sometime between 1508 and 1514, Copernicus wrote a short astronomical treatise commonly called the Commentariolus, or "Little Commentary," which laid the basis for his sun-centered or heliocentric theory, a radical departure from the conventional wisdom of his era. The work was not published in his lifetime.

In the treatise, he correctly postulated the order of the known planets, including Earth, from the sun, and estimated their orbital periods relatively accurately.

For Copernicus, his heliocentric theory was by no means a watershed, for it created as many problems as it solved. For instance, heavy objects were always assumed to fall to the ground because Earth was the center of the universe. Why would they do so in a sun-centered system?

He retained the ancient belief that circles governed the heavens, but his evidence showed that even in a sun-centered universe the planets and stars did not revolve around the sun in perfectly circular orbits.

Because of these problems and others, Copernicus delayed publication of his major astronomical work, De revolutionibus orbium coelestium libri vi, or "On the Revolutions of the Heavenly Spheres," nearly all his life. Completed around 1530, it was not published until 1543 - the year of his death.

What Did Nicolaus Copernicus Discover?

In "On the Revolutions of the Heavenly Spheres," Copernicus' groundbreaking argument that Earth and the planets revolve around the sun led him to make a number of other major astronomical discoveries. While revolving around the sun, Earth, he argued, spins on its axis daily. Earth takes one year to orbit the sun and during this time wobbles gradually on its axis, which accounts for the precession of the equinoxes.

Major flaws in the work include his concept of the sun as the center of the whole universe, not just the solar system, and his failure to grasp the reality of elliptical orbits, which forced him to incorporate numerous epicycles into his system, as did Ptolemy. With no concept of gravity, Earth and the planets still revolved around the sun on giant transparent spheres.

In his dedication to "On the Revolutions of the Heavenly Spheres"-an extremely dense scientific work-Copernicus noted that "mathematics is written for mathematicians." If the work were more accessible, many would have objected to its non-biblical and hence heretical concept of the universe.

For decades, "On the Revolutions of the Heavenly Spheres" remained unknown to all but the most sophisticated astronomers, and most of these men, while admiring some of Copernicus' arguments, rejected his heliocentric basis.

Death and Legacy

Nicolaus Copernicus died on May 24, 1543 in what is now Frombork, Poland. Largely unknown outside of academic circles, he died the year his major work was published, saving him from the outrage of some religious leaders who later condemned his heliocentric view of the universe as heresy.

One of those critics was Martin Luther, the infamous Vatican critic who was one of the founders of the Reformation. Luther stated that "This fool wishes to reverse the entire science of astronomy; but sacred Scripture tells us that Joshua commanded the Sun to stand still, and not the Earth." The Vatican did eventually ban "On the Revolutions of the Heavenly Spheres" in 1616.

It was not until the early 17th century that Galileo and Johannes Kepler developed and popularized the Copernican theory, which for Galileo resulted in a trial and conviction for heresy. Following Isaac Newton 's work in celestial mechanics in the late 17th century, acceptance of the Copernican theory spread rapidly in non-Catholic countries, and by the late 18th century the Copernican view of the solar system was almost universally accepted.

Centuries after his burial in an unmarked grave beneath the floor of the cathedral in Frombork, Copernicus' remains were finally given a hero's burial in 2010. His body was identified using DNA analysis of the skull, which matched the DNA found in hairs that were tucked in the pages of books that Copernicus owned.

His black granite tombstone is now marked with a heliocentric model of the solar system featuring a golden sun encircled by six of the planets.

Sources

Nicolaus Copernicus. Stanford Encyclopedia of Philosophy, Stanford University.
Nicolaus Copernicus biography: Facts & discoveries. Space.com.
Vatican bans Copernicus' book. Physics Today.
16th-century astronomer Copernicus reburied as hero in Poland. Associated Press.

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Copernicus: Facts, Model & Heliocentric Theory - HISTORY

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Nicolaus Copernicus was a Polish astronomer who developed a heliocentric theory of the solar system, upending the belief that Earth was the center of the universe.

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An animation that shows just how big the universe is. Make sure you "allow" flash on your browser in order to see it!

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Scale of the Universe: Discover the vast ranges of our visible and invisible world.

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Scale of Universe is an interactive experience to inspire people to learn about the vast ranges of the visible and invisible world.

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A novice-level text to get you started.

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10 Weird Things You Probably Didn't Know About the Milky Way

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10 Weird Things You Probably Didn't Know About the Milky Way

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From National Geographic: "Our star-studded home has plenty of its own strange wonders"

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When we look up, every star we see is in the Milky Way, the spiral galaxy we call home. The Milky Way holds every alien planet humans have ever spotted, and the billions more that likely exist in the galaxy.

On a dark night, the dense plane of the Milky Way winds like a ribbon across the sky. On a really dark night, in areas free from light pollution, that ribbon becomes so intensely spangled with stars that it's possible to see the dark, dusty clouds of dust and gas deep in space that blot out their light. Those clouds are so prominent that Australia's Aboriginal people saw them create the shape of an emu.

Our galactic home is one of trillions of galaxies in the universe. Astronomers have been ardently studying them for almost a century, ever since Edwin Hubble discovered that neighboring Andromeda was not just another nearby dusty nebula, but a galaxy in its own right. And yet, humans are still trying to unravel the secrets of our galactic home and how it fits in the tapestry of the universe.

"I would love to see a movie in time of the assembly of the Milky Way," says Jay Lockman of the Green Bank Observatory, who presented new observations about our galaxy this week at the 231st meeting of the American Astronomical Society in Maryland.

Here are some of the fun, weird facts and questions we have about the 13.6-billion-year-old space oddity we inhabit.

The Milky Way Is (Mostly) Flat

Our galaxy is, on average, a hundred thousand light-years across but only a thousand light-years thick. Within this flattened (though somewhat warped) disc, the sun and its planets are embedded in a curving arm of gas and dust, putting the solar system about 26,000 light-years away from the galaxy's turbulent core. A bulge of dust and stars swaddles the galactic center, looking like a dollop of whipped cream plopped on both sides of a pancake.

Earth Is 18 Galactic Years Old

The solar systemis zooming through interstellar space at around 500,000 miles an hour. Even at that rate, it takes about 250 million years to travel once around the Milky Way. The last time our 4.5-billion-year-old planet was in this same spot, continents fit together differently, dinosaurs were just emerging, mammals had yet to evolve, and the most profound mass extinction in the planet's history- an event called the Great Dying -was in progress.

There's a Monster Black Hole in the Galaxy's Middle

Called Sagittarius A*, the supermassive black hole weighs in at more than four million times the mass of the sun. We've never seen this object directly-it's hidden behind thick clouds of dust and gas. But astronomers have been able to follow the orbits of stars and gas clouds near the galactic center, which allowed them to infer the mass of the cosmic heavyweight hiding behind the curtain. It's thought that supermassive black holes are parked in the cores of most galaxies, and some are feeding on nearby matter so greedily they shoot out jets of powerful radiation visible from millions of light-years away.

You can take a spin through the Milky Way's chaotic center courtesy of a new animation released at the AAS meeting.

The Milky Way Won't Live Forever

In about four billion years, the Milky Way will collide with its nearest neighbor, the Andromeda Galaxy. The two spiral galaxies are currently hurtling toward each other at 250,000 miles an hour. When they do smash into one another, it won't be as cataclysmic as you might imagine-Earth will likely survive, and very few stars will actually be destroyed. Instead, the newly formed mega-galaxy will offer a night skyscape with a spectacular blend of stars and streamers unlike anything we see today.

Our Sun Is One Star Among Several Hundred Billion

There are a hundred billion stars in the Milky Way. Or is it 300 billion? Or 400 billion? That's right-we don't actually know how many stars are in our galaxy. Many of them are dim, low-mass stars that are hard to detect over vast cosmic distances, and there are massive clouds obscuring the bulge of stars nearest to Sagittarius A*. Astronomers have estimated the total number of stars based on the Milky Way's mass and brightness, but more precise numbers are still elusive.

We're Surrounded By a Dark Halo

The Milky Way is embedded in a clump of dark matter that is far larger and more massive than the galaxy itself. In the late 1960s, astronomer Vera Rubin inferred the presence of these invisible halos around galaxies when she observed that stars near the edge of Andromeda were whipping around the galaxy's center at speeds that should send them flying off into space. And yet, they weren't, meaning that some sort of cosmic glue held everything together. That glue, we now know, is dark matter.

We Hang Out With Ancient Stars

The Milky Way is also surrounded by more than 150 ancient groups of stars, some of which are among the oldest in the universe. Called globular clusters, these primordial stellar conglomerates live in the Milky Way's halo and orbit the galactic center. Each is crammed with hundreds of thousands of stars. Also hanging around the Milky Way are dozens of satellite galaxies; most of these are tough to see, but the Small and Large Magellanic Clouds glisten each night in the southern sky.

The Galaxy Is an Island in a Stream of Stars

The Milky Way eats galaxies that come too close. Over the years, scientists studying the galaxy's fringe have detected some two dozen faint streamers of stars that are the remnants of galaxies past. These ghostly stellar rivers formed when the Milky Way's more powerful gravity ripped apart smaller galaxies, leaving behind glittering strands of leftovers. At the AAS meeting, the Dark Energy Survey team announced that it had detected 11 more of these streamers, some of which have been given Aboriginal names.

The Galactic Center Is Blowing Hot Air

The Milky Way is blowing massive bubbles of extremely hot gas and energetic particles. Stretching far above and below the galactic plane, these so-called Fermi bubbles balloon straight out of the galaxy's center, fueled by a wind blowing at two million miles an hour. Unknown until 2010, it's not entirely clear why the bubbles exist, but scientists think they could be linked to the frenzy of star death and formation in the region around Sagittarius A*.

Gas Clouds Are Fleeing the Galaxy

Observed recently with the Green Bank Telescope, more than a hundred hydrogen gas clouds are zooming away from the galaxy's core at 738,000 miles an hour. Scientists studying the deserting swarm say the clouds can act as tracers for the powerful processes that produce the giant Fermi bubbles.

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10 Weird Things You (Probably) Didn't Know About the Milky Way

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When we look up, every star we see is in the Milky Way, the spiral galaxy we call home. The Milky Way holds every alien planet humans have ever spotted, and the billions more that likely exist in the galaxy. On a dark night, the dense plane of the Milky Way winds like a ribbon across the sky.

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Our solar system is a part of a larger galaxy. Learn about it here!

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The band of the Milky Way galaxy can be seen at night in areas with dark skies. Here it is seen with several Atacama Large Millimeter/submillimeter Arra (ALMA) antenna. (Credit: ESO/B. Tafreshi

Our Sun (a star) and all the planets around it are part of a galaxy known as the Milky Way Galaxy. A galaxy is a large group of stars, gas, and dust bound together by gravity. They come in a variety of shapes and sizes. The Milky Way is a large barred spiral galaxy. All the stars we see in the night sky are in our own Milky Way Galaxy. Our galaxy is called the Milky Way because it appears as a milky band of light in the sky when you see it in a really dark area.

It is very difficult to count the number of stars in the Milky Way from our position inside the galaxy. Our best estimates tell us that the Milky Way is made up of approximately 100 billion stars. These stars form a large disk whose diameter is about 100,000 light years. Our Solar System is about 25,000 light years away from the center of our galaxy â€" we live in the suburbs of our galaxy. Just as the Earth goes around the Sun, the Sun goes around the center of the Milky Way. It takes 250 million years for our Sun and the solar system to go all the way around the center of the Milky Way.

We can only take pictures of the Milky Way from inside the galaxy, which means we don't have an image of the Milky Way as a whole. Why do we think it is a barred spiral galaxy, then? There are several clues.

The first clue to the shape of the Milky Way comes from the bright band of stars that stretches across the sky (and, as mentioned above, is how the Milky Way got its name). This band of stars can be seen with the naked eye in places with dark night skies. That band comes from seeing the disk of stars that forms the Milky Way from inside the disk, and tells us that our galaxy is basically flat.

Several different telescopes, both on the ground and in space, have taken images of the disk of the Milky Way by taking a series of pictures in different directions – a bit like taking a panoramic picture with your camera or phone. The concentration of stars in a band adds to the evidence that the Milky Way is a spiral galaxy. If we lived in an elliptical galaxy, we would see the stars of our galaxy spread out all around the sky, not in a single band.

An all-sky image shows the flat plane of the Milky Way galaxy. (Credit: E. L. Wright/UCLA, The COBE Project, DIRBE, NASA)

Another clue comes when astronomers map young, bright stars and clouds of ionized hydrogen in the Milky Way's disk. These clouds, called HII regions, are ionized by young, hot stars and are basically free protons and electrons. These are both important marker of spiral arms in other spiral galaxies we see, so mapping them in our own galaxy can give a clue about the spiral nature of the Milky Way. There are bright enough that we can see them through the disk of our galaxy, except where the region at the center of our galaxy gets in the way.

There has been some debate over the years as to whether the Milky Way has two spiral arms or four. The latest data shows that it has four arms, as shown in the artist's illustration below.

Since we can't get outside the Milky Way, we have to rely on markers of spiral arms like young, massive stars and ionized clouds. This artist's conception of the Milky Way's spiral structure is based on the measured distances of young, hot stars (shown in red) and ionized clouds of hydrogen gas (shown in blue). (Credit: Credit: Urquhart JS, et al.; Robert Hurt, the Spitzer Science Center; Robert Benjamin)

Additional clues to the spiral nature of the Milky Way come from a variety of other properties. Astronomers measure the amount of dust in the Milky Way and the dominant colors of the light we see, and they match those we find in other typical spiral galaxies. All of this adds up to give us a picture of the Milky Way, even though we can't get outside to see the whole thing.

There are billions of other galaxies in the Universe. Only three galaxies outside our own Milky Way Galaxy can be seen without a telescope, and appear as fuzzy patches in the sky with the naked eye. The closest galaxies that we can see without a telescope are the Large and Small Magellanic Clouds. These satellite galaxies of the Milky Way can be seen from the southern hemisphere. Even they are about 160,000 light years from us. The Andromeda Galaxy is a larger galaxy that can be seen from the northern hemisphere (with good eyesight and a very dark sky). It is about 2.5 million light years away from us, but its getting closer, and researchers predict that in about 4 billion years it will collide with the Milky Way. , i.e., it takes light 2.5 million years to reach us from one of our "nearby" galaxies. The other galaxies are even further away from us and can only be seen through telescopes.

Our nearest large neighbor galaxy is the Andromeda galaxy. (Credit: Bill Schoening, Vanessa Harvey/REU program/NOAO/AURA/NSF)

Updated: December 2015
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For Educators

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This site is intended for students age 14 and up, and for anyone interested in learning about our universe.

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An infographic trip through the wonders of the solar system

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Don't let the ugly thumbnail deter you. This is an engaging, interactive simulation of the solar system. You can watch planets orbit the sun and "visit" different destinations in the solar system.

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Online 3D simulation of the Solar System and night sky in real time - the Sun, planets, dwarf planets, comets, stars and constellations

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A helpful interactive for PBS

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Drag Earth or the Moon to change the time.

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This interactive is visually pleasing and has a lot of information, but it is a little difficult to navigate. It is worth the trouble, though! Getting a sense of how the website works before you start reading is best.

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An interactive web documentary that allows users to find answers to the main questions of why, how and when explore the Moon in the words of European scientists who are working on lunar science right now.

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A beautiful interactive infographic

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You've now reached the outer edges of our own solar system

It would take you about 23 million years of continuous scrolling on this scale to get to the farthest regions of the observable universe, another 435,000,000,000,000,000,000,000 km
or 46 billion light years away.

We think we'll stop here.

Live long and prosper

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You've now reached the outer edges of our own solar system It would take you about 23 million years of continuous scrolling on this scale to get to the farthest regions of the observable universe, another 435,000,000,000,000,000,000,000 kmor 46 billion light years away. We think we'll stop here.

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Ask a Biologist: Why do we have seasons?

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Ask a Biologist: Why do we have seasons?

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A scientist answers this question!

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Why Do We Have Seasons?

It's spring. The frost of winter has slowly melted under the warming skies and the flowers are in full bloom. But spring doesn't last forever. In a few months, temperatures will soar as summer arrives. Then, the temperature will slowly cool again through autumn before the land chills back down to winter.

Many parts of the world experience seasons as a normal part of life. But why does the Earth have seasons?

A Leaning Earth

Imagine running a pole through the center of the Earth from the most Southern point to the most Northern point. Instead of standing straight up and down, the pole sits at an angle. Every day, the Earth revolves once around this tilted pole, which is also called an axis.

While the Earth rotates around the sun, the tilt of the axis always faces in the same direction. This means that when the Earth is on one side of the sun, the Northern Hemisphere leans toward the sun. During this time, the sun beams more directly on the Northern Hemisphere than on the Southern Hemisphere.

When the Earth is on the other side of the sun, the Southern Hemisphere leans toward the sun and the sun shines more directly on the Southern Hemisphere. Seasons are caused by the amount of direct sunlight that reaches a certain part of the Earth. The amount of direct sunlight changes in most places as our planet revolves around the sun.

Sunny Side Up

Summers can get so hot that you can feel like you're melting. Winters can get so cold that you feel like a human popsicle. Can the tilt of the Earth really be responsible for such extreme changes in the weather? Yes, it can.

First, let's talk about sunlight. Light from the sun is made up of tiny particles called photons. When a stream of photons, or sunlight, travels from the sun to any surface on Earth, it heats up that surface. That's why our skin grows hotter when we stand out under the sun. This is also why the ground heats up when the sun's out.

However, you warm up faster if you stand under the sun during the summer than during the winter. The reason for this has to do with the angle at which sunlight hits the Earth during each season.

Detangle the Angle

Imagine that the sun is a can of spray paint and that the floor is the Earth's surface. If you hold the can directly above the floor and spray down, the drops of paint will land close together. If you put the can of paint on the floor and spray from an angle, the drops of paint will land far apart from each other.

Sunlight that reaches the Earth's surface works in the same way. Think about how your location moves in relation to the sun every day. When the sun is directly overhead, photons land closer together and deliver more heat. This is why it's so easy to warm up during midday. When the sun is on the horizon, photons land far apart. The spread of those photons delivers less heat to your location, which is why it's harder to warm up at sunrise and sunset.

In June, when the Northern Hemisphere is leaning toward the sun, the Northern Hemisphere receives direct streams of sunlight and experiences the hot temperatures of summer. Knowing this, do you think the Southern Hemisphere experiences summer in June or December?

Why do you think that places near the equator are always hot and places near the poles are always cold? The answer is, once again, because of the Earth's tilt. Throughout the year, land on and near the equator always receives a lot of direct sunlight. Meanwhile, land on or by the poles receive sunlight from a low angle. Because the poles never receive direct sunlight, they can never warm up very much even during summers, when the sun never sets. Learn more about the Earth's tilt with The Seasons.

Additional images by Wikimedia Commons. Winter mountains by Dmitry A. Mottl.

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You might think that most places on Earth experience the changing of seasons because of the distance from the Sun. But, surprise, that's not the cause. Come and learn some new information to lean on about the tilt of the Earth.

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VIDEO: National Geographic Solar System 101

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VIDEO: National Geographic Solar System 101

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A brief and clear overview of solar system features

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Explore National Geographic. A world leader in geography, cartography and exploration.

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An online tour through the solar system. Click around and "visit" other planets.

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Our solar system can be divided into three regions: the inner solar system, the outer solar system, and the Kuiper Belt and Oort Cloud. The inner, rocky planets are Mercury, Venus, Earth, and Mars. These worlds also are known as terrestrial planets because they have solid surfaces. Mercury, Earth, a…

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From SciShow

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From NASA

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Short URL to share this page:
https://svs.gsfc.nasa.gov/10929

Mission:
LRO (Lunar Reconnaissance Orbiter)

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Note: While we identify the data sets used in these visualizations, we do not store any further details nor the data sets themselves on our site.

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Goddard TV Tape:
G2012-028 -- Evolution and Tour of the Moon

GCMD keywords can be found on the Internet with the following citation: Olsen, L.M., G. Major, K. Shein, J. Scialdone, S. Ritz, T. Stevens, M. Morahan, A. Aleman, R. Vogel, S. Leicester, H. Weir, M. Meaux, S. Grebas, C.Solomon, M. Holland, T. Northcutt, R. A. Restrepo, R. Bilodeau, 2013. NASA/Global Change Master Directory (GCMD) Earth Science Keywords. Version 8.0.0.0.0

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Although the moon has remained largely unchanged during human history, our understanding of it and how it has evolved over time has evolved dramatically. Thanks to new measurements, we have new and unprecedented views of its surface, along with new insight into how it and other rocky planets in our solar system came to look the way they do.

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Still stumped? Here are 4 key ideas to understanding moon phases.

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Why does the moon seem to change its shape every night? Why can I see the moon in the daytime?
The answer to both questions is the same. It's that the moon is a world in space, just as Earth is. Like Earth, the moon is always half illuminated by the sun; the round globe of the moon has a day side and a night side. And, like the Earth, the moon is always moving through space. From our earthly vantage point, as the moon orbits around Earth, we see varying fractions of its day and night sides. These are the changing phases of the moon. And the moon is in the daytime sky about half the time. It's just that it's sometimes it's so near the sun we don't notice it. How can you understand moon phases? Here are four things to remember:
1. When you see the moon, think of the whereabouts of the sun 2. The moon rises in the east and sets in the west, each and every day 3. The moon takes about a month (one moonth) to orbit the Earth 4. The moon's orbital motion is toward the east
1. When you see the moon, think of the whereabouts of the sun. After all, it's the sun that's illuminating and creating the dayside of the moon.

Moon phases depend on where the moon is with respect to the sun in space. For example, do you see which moon phase is being shown in the first illustration above? The answer is, it's a full moon. The moon, Earth and sun are aligned with Earth in the middle. The moon's fully illuminated half - its dayside - faces Earth's night side. That's always the case on the night of a full moon.

Don't just take our word for it. Go outside. No matter what phase of the moon you see in your sky, think about where the sun is. It'll help you begin to understand why the moon you see is in that particular phase.

2. The moon rises in the east and sets in the west, each and every day. It has to. The rising and setting of all celestial objects is due to Earth's continuous daily spin beneath the sky.

Just know that - when you see a thin crescent moon in the west after sunset - it's not a rising moon. Instead, it's a setting moon.

At the same time, though ...

3. The moon takes about a month (one moonth) to orbit the Earth. Although the moon rises in the east and sets in the west each day (due to Earth's spin), it's also moving on the sky's dome each day due to its own motion in orbit around Earth.

This is a slower, less noticeable motion of the moon. It's a motion in front of the fixed stars. If you just glance at the moon one evening - and see it again a few hours later - you'll notice it has moved westward. That westward motion is caused by Earth's spin.

The moon's own orbital motion can be detected in the course of a single night, too. But you have to watch the moon closely, with respect to stars in its vicinity, over several hours.

The moon's eastward, orbital motion is easiest to notice from one day (or night) to the next. It's as though the moon is moving on the inside of a circle of 360 degrees. The moon's orbit carries it around Earth's sky once a month, because the moon takes about a month to orbit Earth.

So the moon moves - with respect to the fixed stars - by about 12 to 13 degrees each day.

4. The moon's orbital motion is toward the east. Each day, as the moon moves another 12 to 13 degrees toward the east on the sky's dome, Earth has to rotate a little longer to bring you around to where the moon is in space.

Thus the moon rises, on average, about 50 minutes later each day.

The later and later rising times of the moon cause our companion world to appear in a different part of the sky at each nightfall for the two weeks between new and full moon.

Then, in the two weeks after full moon, you'll find the moon rising later and later at night.
We have more details on individual moon phases at the links below. Follow the links to learn more about the various phases of the moon.
... and here are the names of all the full moons.

Bottom line: Why the moon waxes and wanes in phase.
EarthSky lunar calendars show moon phases for all of 2019. They make great gifts. Order now. Going fast!

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Top 4 keys to understanding moon phases | EarthSky.org

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Why does the moon seem to change its shape every night? Why can I see the moon in the daytime? The answer to both questions is the same. It's that the moon is a world in space, just as Earth is.

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From NASA. This is a beautiful site that gives clear information about our moon. This resource is great for background information.

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Overview The brightest and largest object in our night sky, the Moon makes Earth a more livable planet by moderating our home planet’s wobble on its axis, leading to a relatively stable climate. It also causes tides, creating a rhythm that has guided humans for thousands of years. The Moon was likel…

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A clear, detailed interactive that shows how Earth's revolution affects the seasons. Make use of the text features of this site - they are super helpful!

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An oldie but a goody. This interactive demonstrates how Earth's revolution creates seasons.

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Seasons Interactive

Each year on Earth, we observe patterns of changing temperatures, daylight hours, and seasons. In this activity, you will use a computer simulation to model Earth's orbit around the Sun to help understand why we observe these patterns.

Resources

Student Book Pages and Student Sheets

Interactive

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Each year on Earth, we observe patterns of changing temperatures, daylight hours, and seasons. In this activity, you will use a computer simulation to model Earth's orbit around the Sun to help understand why we observe these patterns. Student Book Pages and Student Sheets