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Students should be able to explore a scientific model to illustrate the origin of the universe.

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Reflection on the developments in my thinking in relation to the specification, NoS and inquiry within it.

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Science pedagogy, teacher attitudes, and student success

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Science pedagogy, teacher attitudes, and student success

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This paper helped me understand the importance of scientific inquiry within the classroom. It is important that teachers represent information and abstract ideas in an understandable way for their students or else science would not be effective.

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Useful Resource- Sun, Earth and Moon model.

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Useful Resource- Sun, Earth and Moon model.

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This resource gave me the idea of building a model of the earth-sun-moon model. Students could visually learn about the relationship between each object by creating a model of it. It also allowed for active learning within the classroom.

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astroEDU is an open-access peer-review platform for educators to discover, review, distribute, improve, and remix the best astronomy education activities around the world.

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I used this article when I was explaining what effect the Earth being titled on its axis may have. Some students may not have been aware of the term 'solstice.'

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Most of us probably never think of ourselves as being passengers on-board a spinning planet that whips around on its axis of rotation once every 24 hours, or that travels 970 million kilometres on its 365-day journey around the sun.

f the earth's axis of rotation was perpendicular to the sun-earth line, we would have no solstices and no strong seasonal effects. However, the 23.5o tilt of the earth's axis gives rise to different orientations of earth relative to the sun during the year, giving us both.

At the summer solstice, the northern hemisphere is tipped most directly towards the sun, leading to more concentrated sunlight and shorter shadows at noon. We observe the sun as reaching its maximum height in the sky and the day of the solstice has the greatest number of daylight hours in the year.

On the same day in the southern hemisphere, which is tipped away from the sun, the sun's rays at noon are less concentrated and shadows are longer - it is their mid-winter. The situation in the two hemispheres is reversed at the winter solstice.

The exact moment of the summer solstice, which occurs between June 20 and June 22, varies from year to year because of the influence of other planets on earth's orbit and the slight wobble of its axis. This year, the summer solstice is on June 21. In Dublin, sunrise is at 04:57 and sunset at 21.57, giving just over 17 hours of daylight.

Be glad we don't live on Venus, where the very small axial tilt of 3o provides no seasonal respite from the 462oC temperature caused by its proximity to the sun and its runaway greenhouse effect.

Professor Lorraine Hanlon, is a professor at the School of Physics, University College Dublin

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Summer solstice the longest day of the year but why?

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Most of us probably never think of ourselves as being passengers on-board a spinning planet that whips around on its axis of rotation once every 24 hours, or that travels 970 million kilometres on its 365-day journey around the sun.

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Useful Resources- Teaching the different type of eclipses.

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Useful Resources- Teaching the different type of eclipses.

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NoS 3, 7
PW 1, 2, 3

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My cooperating teacher allowed me log into her account. This resource was useful as it helped me design PowerPoints. It also contained quizzes which I used to evaluate students learning in the classroom.

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This exciting new resource aims to overcome the challenges presented by the new syllabus.Offers a unique and interactive learning experience for students of Junior Cycle Science.

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This short Youtube video was used to help explain the difference in an asteroid, comet and a meteor.

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Less Than Five - What's the Difference Between Comets, Asteroids, Meteoroids, Meteors & Meteorites?

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This web page helped me gather information and form a better understanding of the different celestial bodies. Some information from this web page was included in my PowerPoints and helped me draw up worksheets.

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The path through the solar system is a rocky road. Asteroids, comets, Kuiper Belt Objects-all kinds of small bodies of rock, metal and ice are in constant motion as they orbit the Sun. But what's the difference between them, anyway? And why do these miniature worlds fascinate space explorers so much? The answer is profound: they may hold the keys to better understanding where we all come from.

1. Asteroids

Asteroids are rocky, airless worlds that orbit our Sun. They are remnants left over from the formation of our solar system, ranging in size from the length of a car to about as wide as a large city. Asteroids are diverse in composition; some are metallic while others are rich in carbon, giving them a coal-black color. They can be "rubble piles," loosely held together by their own gravity, or they can be solid rocks.

Most of the asteroids in our solar system reside in a region called the main asteroid belt. This vast, doughnut-shaped ring between the orbits of Mars and Jupiter contains hundreds of thousands of asteroids, maybe millions. But despite what you see in the movies, there is still a great deal of space between each asteroid. With all due respect to C3PO, the odds of flying through the asteroid belt without colliding with one are actually pretty good.

Other asteroids (and comets) follow different orbits, including some that enter Earth's neighborhood. These are called near-Earth objects, or NEOs. We can actually keep track of the ones we have discovered and predict where they are headed. The Minor Planet Center (MPC) and Jet Propulsion Laboratory's Center for Near Earth Object Studies (CNEOS) do that very thing. Telescopes around the world and in space are used to spot new asteroids and comets, and the MPC and CNEOS, along with International colleagues, calculate where those asteroids and comets are going and determine whether they might pose any impact threat to Earth.

For scientists, asteroids play the role of time capsules from the early solar system, having been preserved in the vacuum of space for billions of years. What's more, the main asteroid belt may have been a source of water-and organic compounds critical to life-for the inner planets like Earth.

2. Comets

Comets also orbit the Sun, but they are more like snowballs than space rocks. Each comet has a center called a nucleus that contains icy chunks of frozen gases, along with bits of rock and dust. When a comet's orbit brings it close to the Sun, the comet heats up and spews dust and gases, forming a giant, glowing ball called a coma around its nucleus, along with two tails - one made of dust and the other of excited gas (ions). Driven by a constant flow of particles from the Sun called the solar wind, the tails point away from the Sun, sometimes stretching for millions of miles.

While there are likely billions of comets in the solar system, the current confirmed number is 3,535. Like asteroids, comets are leftover material from the formation of our solar system around 4.6 billion years ago, and they preserve secrets from the earliest days of the Sun's family. Some of Earth's water and other chemical constituents could have been delivered by comet impacts.

3. Meteoroids

Meteoroids are fragments and debris in space resulting from collisions among asteroids, comets, moons and planets. They are among the smallest "space rocks." However, we can actually see them when they streak through our atmosphere in the form of meteors and meteor showers.

This photograph, taken by an astronaut aboard the International Space Station, provides the unusual perspective of looking down on a meteor as it passes through the atmosphere. The image was taken on Aug. 13, 2011, during the Perseid Meteor Shower that occurs every August. Image credit: NASA

4. Meteors

Meteors are meteoroids that fall through Earth's atmosphere at extremely high speeds. The pressure and heat they generate as they push through the air causes them to glow and create a streak of light in the sky. Most burn up completely before touching the ground. We often refer to them as "shooting stars." Meteors may be made mostly of rock, metal or a combination of the two.

Scientists estimate that about 48.5 tons (44,000 kilograms) of meteoritic material falls on Earth each day.

The constellation Orion is framed by two meteors during the Perseid shower on Aug. 12, 2018 in Cedar Breaks National Monument, Utah. Image credit: NASA/Bill Dunford

5. Meteor Showers

Several meteors per hour can usually be seen on any given night. Sometimes the number increases dramatically-these events are termed meteor showers. They occur when Earth passes through trails of particles left by comets. When the particles enter Earth's atmosphere, they burn up, creating hundreds or even thousands of bright streaks in the sky. We can easily plan when to watch meteor showers because numerous showers happen annually as Earth's orbit takes it through the same patches of comet debris. This year's Orionid meteor shower peaks on Oct. 21.

6. Meteorites

Meteorites are asteroid, comet, moon and planet fragments (meteoroids) that survive the heated journey through Earth's atmosphere all the way to the ground. Most meteorites found on Earth are pebble to fist size, but some are larger than a building.

Early Earth experienced many large meteorite impacts that caused extensive destruction. Well-documented stories of modern meteorite-caused injury or death are rare. In the first known case of an extraterrestrial object to have injured a human being in the U.S., Ann Hodges of Sylacauga, Alabama, was severely bruised by a 8-pound (3.6-kilogram) stony meteorite that crashed through her roof in November 1954.

7. Dwarf Planets

Don't let the name fool you; despite their small size, dwarf planets are worlds that are just as compelling as their larger siblings. Dwarf planets are defined by astronomers as bodies massive enough to be shaped by gravity into a round or nearly round shape, but they don't have enough of their own gravitational muscle to clear their path of other objects as they orbit the Sun. In our solar system, dwarf planets are mostly found in the Kuiper Belt beyond Neptune; Pluto is the best-known example. But the largest object in the asteroid belt is the dwarf planet Ceres. Like Pluto, Ceres shows signs of active geology, including ice volcanoes.

Have exactly one minute? Find out more about dwarf planets: Space Shorts: What Is a Dwarf Planet?

8. Kuiper Belt Objects

The Kuiper Belt is a disc-shaped region beyond Neptune that extends from about 30 to 55 astronomical units -- that is, 30 to 55 times the distance from the Earth to the Sun. There may be hundreds of thousands of icy bodies and a trillion or more comets in this distant region of our solar system.

Besides Pluto, some of the mysterious worlds of the Kuiper Belt include Eris, Sedna, Quaoar, Makemake and Haumea. Like asteroids and comets, Kuiper Belt objects are time capsules, perhaps kept even more pristine in their icy realm.

This chart puts solar system distances in perspective. The scale bar is in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. One AU is the distance from the Sun to the Earth, which is about 93 million miles or 150 million kilometers. Neptune, the most distant planet from the Sun, is about 30 AU. Image credit: NASA/JPL-Caltech

9. Oort Cloud Objects

The Oort Cloud is a group of icy bodies beginning roughly 186 billion miles (300 billion kilometers) away from the Sun. While the planets of our solar system orbit in a flat plane, the Oort Cloud is believed to be a giant spherical shell surrounding the Sun, planets and Kuiper Belt Objects. It is like a big, thick bubble around our solar system. The Oort Cloud's icy bodies can be as large as mountains, and sometimes larger.

This dark, cold expanse is by far the solar system's largest and most distant region. It extends all the way to about 100,000 AU (100,000 times the distance between Earth and the Sun) - a good portion of the way to the next star system. Comets from the Oort Cloud can have orbital periods of thousands or even millions of years. Consider this: At its current speed of about a million miles a day, NASA's Voyager 1 spacecraft won't reach the Oort Cloud for more than 300 years. It will then take about 30,000 years for the spacecraft to traverse the Oort Cloud, and exit our solar system entirely.

10. The Explorers

Fortunately, even though the Oort Cloud is extremely distant, most of the small bodies we've been discussing are more within reach. In fact, NASA and other space agencies have a whole flotilla of robotic spacecraft that are exploring these small worlds up close. Our mechanical emissaries act as our eyes and hands in deep space, searching for whatever clues these time capsules hold.

A partial roster of current or recent missions to small, rocky destinations includes, from NASA:

OSIRIS-REx - Now approaching the asteroid Bennu, where it will retrieve a sample in 2020 and return it to the Earth for close scrutiny.

New Horizons - Set to fly close to MU69 or "Ultima Thule," an object a billion miles past Pluto in the Kuiper Belt on Jan. 1, 2019. When it does, MU69 will become the most distant object humans have ever seen up close.

Psyche - Planned for launch in 2022, the spacecraft will explore a metallic asteroid of the same name, which may be the ejected core of a baby planet that was destroyed long ago.

Lucy - Slated to investigate two separate groups of asteroids, called Trojans, that share the orbit of Jupiter - one group orbits ahead of the planet, while the other orbits behind. Lucy is planned to launch in 2021.

Dawn - Finishing up a successful seven-year mission orbiting planet-like worlds Ceres and Vesta in the asteroid belt.

Plus these missions from other space agencies:

The Japan Aerospace Exploration Agency (JAXA)'s Hayabusa2 - Just landed a series of small probes on the surface of the asteroid Ryugu.

The European Space Agency (ESA)'s Rosetta - Orbited the comet 67P/Churyumov-Gerasimenko and dispatched a lander to its surface.

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10 Things: What's That Space Rock? - Solar System Exploration: NASA Science

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The path through the solar system is a rocky road. Asteroids, comets, Kuiper Belt Objects-all kinds of small bodies of rock, metal and ice are in constant motion as they orbit the Sun. But what's the difference between them? Why do these miniature worlds fascinate space explorers so much?

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First year Earth and Space unit

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This resource helped the students gather information when comparing all the planets with each other.

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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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Useful Resources- Our Solar System and The Planets

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Useful Resources- Our Solar System and The Planets

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This article helped me gather new information and allowed me explore the other planets.

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The Solar System is made up of the Sun and all of the smaller objects that move around it. Apart from the Sun, the largest members of the Solar System are the eight major planets. Nearest the Sun are four fairly small, rocky planets - Mercury, Venus, Earth and Mars.

Beyond Mars is the asteroid belt - a region populated by millions of rocky objects. These are left-overs from the formation of the planets, 4.5 billion years ago.

On the far side of the asteroid belt are the four gas giants - Jupiter, Saturn, Uranus and Neptune. These planets are much bigger than Earth, but very lightweight for their size. They are mostly made of hydrogen and helium.

Until recently, the furthest known planet was an icy world called Pluto. However, Pluto is dwarfed by Earth's Moon and many astronomers think it is too small to be called a true planet.

An object named Eris, which is at least as big as Pluto, was discovered very far from the Sun in 2005. More than 1,000 icy worlds such as Eris have been discovered beyond Pluto in recent years. These are called Kuiper Belt Objects. In 2006, the International Astronomical Union decided that Pluto and Eris must be classed as "dwarf planets".

Even further out are the comets of the Oort Cloud. These are so far away that they are invisible in even the largest telescopes. Every so often one of these comets is disturbed and heads towards the Sun. It then becomes visible in the night sky.

Last modified 07 August 2014

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The Solar System is made up of the Sun and all of the smaller objects that move around it. Apart from the Sun, the largest members of the Solar System are the eight major planets. Nearest the Sun are four fairly small, rocky planets - Mercury, Venus, Earth and Mars.

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First year lesson plan- The Big Bang Theory and Expanding Universe.

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lesson plan 2, first years big bang theory

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This resource helped me gain a better understanding of what the Big Bang Theory is. It also gives a nice explanation to what a 'theory' is.

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For centuries, humans have gazed at the stars and wondered how the universe developed into what it is today. It's been the subject of religious, philosophical, and scientific discussion and debate. People who have tried to uncover the mysteries of the universe's development include such famous scientists as Albert Einstein, Edwin Hubble and Stephen Hawking. One of the most famous and widely accepted models for the universe's development is the big bang theory.

Although the big bang theory is famous, it's also widely misunderstood. A common misperception about the theory is that it describes the origin of the universe. That's not quite right. The big bang is an attempt to explain how the universe developed from a very tiny, dense state into what it is today. It doesn't attempt to explain what initiated the creation of the universe, or what came before the big bang or even what lies outside the universe.

Another misconception is that the big bang was a kind of explosion. That's not accurate either. The big bang describes the expansion of the universe. While some versions of the theory refer to an incredibly rapid expansion (possibly faster than the speed of light), it's still not an explosion in the classic sense.

Summing up the big bang theory is a challenge. It involves concepts that contradict the way we perceive the world. The earliest stages of the big bang focus on a moment in which all the separate forces of the universe were part of a unified force. The laws of science begin to break down the further back you look. Eventually, you can't make any scientific theories about what is happening, because science itself doesn't apply.

So what's the big bang theory in a nutshell?

The Big Bang Theory FAQ

Who discovered the big bang theory?

How did the big bang happen from nothing?

What is the big bang theory in simple terms?

What does the big bang theory tell us?

How did the big bang start?

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For centuries, humans have gazed at the stars and wondered how the universe developed into what it is today. It's been the subject of religious, philosophical, and scientific discussion and debate. People who have tried to uncover the mysteries of the universe's development include such famous scientists as Albert Einstein, Edwin Hubble and Stephen Hawking.

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This site may earn affiliate commissions from the links on this page. Terms of use.

There was no doubt Pluto was a planet back when Clyde Tombaugh spotted it in 1930, but scientists rethought that after learning more about the solar system. In 2006, a new definition of what makes a plane from the International Astronomical Union (IAU) saw Pluto demoted to dwarf planet status. That decision has never set right with some members of the public and scientists. A new study from University of Central Florida planetary scientist Philip Metzger makes the case that Pluto should regain its planet status.

In decades past, Pluto seemed like a slightly odd planet, but clearly still a planet. However, scientists have since learned about the scale of the Kuiper Belt and even spotted several Kuiper objects that are larger than Pluto. The 2006 IAU definition of a planet included several key requirements. A planet needs to be spherical, which Pluto is. However, it also needs to have cleared its orbit to become the primary gravitational force. That's where Pluto failed the test. It crosses the orbit of Neptune, the gravity of which affects Pluto. There are also multiple large Kuiper belt objects in space near Pluto.

You could call Metzger a skeptic of dwarf planets. He reviewed scientific literature over more than 200 years to determine if the IAU definition makes sense. According to the paper, Metzger found just a single instance of orbit clearing as a planetary descriptor, and that was in 1802. The reasoning used in that paper has also long since been disproven. Metzger claims that the IAU definition applies standards that aren't even used in planetary research. He cites more than 100 instances of researchers using the term "planet" to refer to objects that don't fit the IAU definition in recent publications. He goes so far as to call the IAU definition "sloppy."

This isn't the first time astronomers have attempted to develop a definition for planets. In the 1950s, Gerard Kuiper (of Kuiper belt fame) published a paper that used the way objects were formed to determine whether or not they were planets. Metzger says this reasoning is also insufficient.

Metzger and his co-author instead say that an object should be classified as a planet if it has enough gravity to become spherical. This makes more sense because it's an intrinsic property of the planet rather than something changeable like orbit, according to the paper. This definition would make Pluto a planet once more, but it could also mean objects deeper in the Kuiper Belt and the dwarf planet Ceres in the asteroid belt would also get a promotion.

So, if you want Pluto back, it's bringing friends to the planet party.

Now read: Pluto Might Have Formed as a Giant Comet, Not a Planet, Unexpected, mysterious x-rays found emanating from Pluto, and Pluto's smooth surface is probably a sea of slow-flowing nitrogen ice

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New Research Makes the Case for Pluto as a Planet - ExtremeTech

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This site may earn affiliate commissions from the links on this page. Terms of use. There was no doubt Pluto was a planet back when Clyde Tombaugh spotted it in 1930, but scientists rethought that after learning more about the solar system.

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After the sun, the most noticeable celestial object is the moon. Its motions, phases, and occasional eclipses add delightful variety to our sky. It's also the closest astronomical object by far.

The Moon's Motions

The moon's motion through our sky is similar to that of the sun, but different:

Like the sun, the moon rises in the east and sets in the west (with some exceptions for observers in earth's extreme northern and southern regions).

Like the sun, the moon doesn't move across our sky quite as fast as the stars do.

However, whereas the sun creeps eastward only one degree per day with respect to the stars, the moon moves eastward, with respect to the stars, by about 13 degrees per day (see illustration below). This means that if you pay attention, you can easily notice the moon's motion, with respect to the stars, during a single night.

As a consequence, the moon moves eastward with respect to the sun by about 12 degrees per day (13 minus 1). This means that the moon rises later and later each successive day, sometimes rising in the morning, sometimes in the afternoon, and sometimes at night. Similarly, the moon sets later each day, and can set at any time of the day or night, depending on where it is with respect to the sun.

At a rate of 12 degrees per day, the moon completes a full circle, with respect to the sun, about once every 30 days (actually 29.5). This time period was originally called one month (guess where the word comes from), although our modern calendar is not directly tied to the moon and defines different months to have different lengths.

The moon is always seen near the ecliptic, but its motion carries it about five degrees to either side during the course of each month.

This simulated multiple-exposure image shows the positions of the sun and moon with respect to the stars over a nine-day period. While the sun moves eastward (from right to left) only one degree per day, the moon moves eastward by 13 degrees per day. The yellow line is the ecliptic, from which the moon never strays by more than about five degrees. (The sizes of both the sun and moon are exaggerated for emphasis.)

Question: Suppose, for the purpose of this question, that a month is exactly 30 days. How many minutes later, on each successive day, would you expect the moon to rise, on average? Remember that after a full month (30 days), the moon should rise at the same time as originally (neglecting seasonal variations).

The Moon's Phases

Besides moving around the sky, the moon is famous for its phases: its variety of apparent shapes, from crescent to half to gibbous to full. There is also a "new" phase, when we don't see the moon at all for a few days. What causes these striking changes in the moon's appearance?

Here are some big clues:

First, the moon goes through its complete cycle of phases exactly once each (lunar) month, that is, in the same time period as its motion with respect to the sun.

Second, the moon's phase is always new when it is near the sun in our sky, andfull when it is opposite the sun, with intermediate phases at intermediate angles (between the moon and sun).

Third, when the moon is in its crescent phase and the sky is sufficiently dark, you can sometimes make out the rest of the moon's dimly lit disk. This tells us that the moon isn't really changing shape; part of it is merely much brighter than the rest.

Finally, at any phase between new and full, you can check that the illuminated side of the moon is always the side that's facing toward the sun.

The simple explanation of all these observations is that the moon shines by reflected sunlight. It's spherical in shape, with half of the sphere illuminated by the sun at any given time. How much of this half we see, however, depends on our viewing direction. When the moon and sun are near each other in our sky, we're looking at the moon's dark side so we don't see it at all ("new moon"). When the moon is opposite the sun in our sky, we're looking at the moon's illuminated side so we see a full moon. At other angles we see part, but not all, of the moon's illuminated side. The illustration below shows how you can simulate the moon's phases using a ball and a strong directional light source to simulate the sun.

Simulation of moon phases using a styrofoam ball illuminated by a strong, directional light source. The four photos were taken with different angles between the "moon" and the light source.

The moon's new and crescent phases occur when the sun is more or less behind the moon, from our perspective. Just looking at these phases, therefore, tells us immediately that the moon must be closer to us than the sun. There's even a way to estimate how much closer it is, as I'll explain below.

Question: Assuming you live at a mid-northern latitude, where would you look in the sky to see a crescent moon at sunset?

Question: At what time of day or night does the moon rise when it's full?

Question: Many scenic photographs of the moon are faked, by combining two separate photos. Think of some ways you can tell when the moon has been artificially added to a photo.

Solar Eclipses

By an amazing coincidence, the moon's apparent size in our sky is almost exactly the same as the sun's: about half a degree. This means that it's possible, as the new moon passes by the sun in its monthly trip around the ecliptic, for the moon's disc to completely cover the sun. When it does, our sky becomes dark for a few minutes and we can see the thin, hot gases surrounding the sun, called the solar corona. This dramatic event is called a total solar eclipse.

Left: Total solar eclipse, showing the solar corona and several red prominences around the edge ( Luc Viatour). Center: Partial solar eclipse ( Michael Mortensen). Right: Annular solar eclipse, when the moon is too far away to completely cover the sun ( Sancho Panza).

A solar eclipse occurs when the moon passes between the sun and the earth. Only a small part of the earth, at most, is covered by the moon's shadow. (Diagram not to scale.)

Remember, though, that the moon can wander as much as five degrees to either side of the ecliptic. In most months, therefore, the new moon misses the sun (from our perspective) by a few degrees, to one side or the other, as it passes. But the moon's path crosses the ecliptic roughly twice each month, and a solar eclipse occurs when the sun happens to be there during the crossing. There's a good chance of this about once every six months. Even then, however, perfect alignment requires that you be somewhere along a narrow path across earth's surface, usually less than 300 km wide. And even then, the moon's disc isn't always quite big enough to cover the sun, because both of their apparent sizes vary as their distances from earth vary slightly. When the moon is too far and/or the sun is too close, yet the alignment is good, we call the event an annular eclipse. Otherwise, if the moon covers only part of the sun, we call it a partial solar eclipse.

Whether the eclipse is total (or annular) or not, a partial solar eclipse is usually visible over a wide area about twice each year. I've seen several partial solar eclipses over the years, and if you're old enough, you probably have as well. I've never been in the right place to see a total solar eclipse, but I'm looking forward to the one that will come through much of the U.S. on August 21, 2017.

Lunar Eclipses

An equally interesting, if less dramatic, event occurs when the sun and moon are exactly opposite each other in our sky. This is when the moon is normally full, and very bright. But if the alignment is precise enough, the earth will block the sun's light from reaching the moon. This event is called a lunar eclipse. As with solar eclipses, there's a good chance for a lunar eclipse to occur about once every six months. Some are only partial, with the earth's shadow never covering more than part of the moon, because the alignment is imperfect.

A lunar eclipse occurs when the moon and sun are on opposite sides of the earth. Often the entire moon is covered by earth's shadow. (Diagram not to scale.)

A sequence of three photos of the total lunar eclipse of August 16-17, 1989, taken over a time period of about half an hour. The exposure time was shortest for the bright partial phase (left), and longest for the dark red total phase (right). The middle photo shows how the eclipsed portion of the moon is much darker than the small sliver that's receiving direct sunlight.

Your chances of having seen a total lunar eclipse are pretty good, because they're visible from everywhere on the night side of the earth, and the totality can last over an hour. Clouds can get in the way, of course, and you sometimes need to be willing to get up in the middle of the night. The next lunar eclipse as of this writing will be a total one on December 10, 2011; best viewed from the Asia-Pacific region, it will also be visible in the morning, as the moon is setting, from western North America. For the next two years the lunar eclipses will be only partial, with the next total lunar eclipse not occurring until April 15, 2014.

Although you might expect the moon to become totally dark during a lunar eclipse, in fact a little sunlight still gets to it-by bending around the earth, through earth's atmosphere. The same thing happens to us every evening after the sun sets, when we still see light in the western sky. And just as with sunsets, it's the red component of the sunlight that best penetrates earth's atmosphere and reaches the eclipsed moon.

The Moon's Size

The earth's shadow, at the moon's location, is nearly three times as wide as the moon itself.

Besides just being a cool sight to watch, a lunar eclipse gives us a way to determine the moon's true size.

As shown at right, the curvature of earth's shadow on the partially eclipsed moon tells us the size of earth's shadow, in comparison to the moon. Specifically, the diameter of earth's shadow is a little less than three times the diameter of the moon. In other words, the moon's diameter is a little more than one third that of the shadow.

If earth's shadow were the same size as the earth itself, we could immediately conclude that the moon's diameter is a little over a third that of the earth. But it's not that simple, because earth's shadow tapers off, getting narrower as you get farther and farther away (as shown in the lunar eclipse diagram above). To take this tapering into account, you have to do a rather intricate calculation using several principles of geometry and algebra. The bottom line is that the earth's shadow, at the moon's location, is about 3/4 as wide as the earth itself. Therefore, since the moon is a little over 1/3 as wide as the shadow, it must be a little over 1/4 as wide as the earth. In miles, the earth's diameter is about 8000 so the moon's diameter is about 2000.

This ingenious method of determining the moon's size was devised by the Greek astronomer Aristarchus of Samos, in the third century B.C. Although the method is absolutely sound, Aristarchus used rather inaccurate measurements so his numerical answer for the moon's size wasn't especially good. But later Greek astronomers repeated the calculation, using better measurements.

Even without doing any calculations, however, you can just look at a partial lunar eclipse and see that the moon is probably just a few times smaller than the earth. Of course, you can also see that earth's shadow is curved. Aristotle, the great Greek philosopher, used this observation to argue that the earth must be a sphere.

The Distance to the Moon

The closer an object is, the greater its angular size.

Knowing the moon's true diameter (1/4 that of the earth), we can use its measured angular diameter (half a degree) to determine its distance. Qualitatively, the idea is this: For a given true size, a larger angular diameter would mean that the moon must be closer; whereas a smaller angular diameter would mean that the moon must be farther away. Because we know the moon's angular diameter, we can calculate just how far it is.

The calculation takes a few steps, and requires some effort and practice to fully understand. But similar calculations come up so often in astronomy that it's a good investment to start practicing now.

Big circle diagram for calculating the moon's distance. (The half-degree angle has been greatly exaggerated.)

A useful trick for visualizing the calculation is to sketch a big, imaginary circle, centered on you (the observer), passing through the moon. Then label the angle that represents the moon's measured angular width (half a degree in reality, but it's fine to exaggerate it in the picture as I have). Also label the moon's known diameter, 1/4 (in units of earth's diameter).

The calculation proceeds in three steps:

1. Ask yourself how many hypothetical moons it would take, placed side by side, to reach all the way around the circle. Since a full circle is 360° but each moon takes up only 1/2°, the answer is 360 divided by 1/2, or 720 hypothetical moons.

2. Now calculate the circumference of the circle. Since each moon has a width of 1/4 e.d., and it takes 720 of them to go all the way around, the answer is 720 times 1/4 e.d., or 180 earth diameters. (You could use units of miles instead of earth diameters if you prefer, but then the numbers get a little more cumbersome: 720 × 2000 = 1440000.)

3. The distance to the moon is the radius of the circle. But for any circle, the circumference is 2π times the radius, where π is a number that's equal to a little more than 3. If you don't need an especially accurate answer, it's fine to pretend that π equals 3 and just say that the circumference is six times the radius. Then the radius (or distance) is simply the circumference divided by 6, or in this case, 180 e.d. divided by 6, or 30 earth diameters.

I like to call these kinds of calculations big circle problems. The general idea is that if you know the angular size and the true size of an object, you can calculate its distance by drawing a big circle and going through these three steps. A slight variation, which comes up just as often, is when you already know the distance and the angle, and want to calculate the true size. Please make sure you understand each step of the calculation, and practice until you can put them together and do a whole big circle problem on your own.

Question: Imagine that you live on an alien world with a moon that appears only half as wide as ours: 1/4 degree. You also know that the true diameter of this moon is 500 miles. Sketch a big circle, centered on you and passing through the moon, and label it with the angle and true diameter. How many hypothetical moons would fit around the full circle?

Next step: What is the circumference of the circle, in miles?

Final step: Using the approximation π = 3, what is your estimate of the distance to this moon, in miles?

The next two questions are harder, so you may want to skip them the first time your read this. Be sure to come back and work them, for practice, when you have time!

Question: Bruno the Bully is chasing you across the playground. Being the nerdy type, you decide to stop running away and figure out just how far from you he is (so you can then calculate how long you have to live). You know that Bruno is 6 feet tall, and, holding up your outstretched fist, you estimate his angular height to be five degrees. Using the approximation π = 3, what is your estimate of Bruno's distance in feet?

Question: According to your map, the mountain on the horizon is 30 kilometers away. Holding out your fist, you estimate its angular height to be three degrees. How tall is the mountain, in kilometers? (Again, please use the approximation π = 3.)

Coming back to the moon, our calculation gave a distance of 30 earth diameters (or 60 earth radii, or about a quarter million miles). Here is a picture of the earth and moon, to scale:

Scale drawing of the earth and the moon, showing that the moon is 1/4 the earth's diameter, and 30 earth diameters away.

Comprehending this enormous distance is a challenge even today, when automobile odometer mileages often exceed a quarter million. But imagine how astonishing this distance must have been to the ancient Greeks, who traveled only by the power of muscle and wind. Equally astonishing is that the Greeks could determine this distance without the benefit of space travel or even a telescope. As we'll see over and over again, the human intellect can reach out to measure things that are far beyond our physical reach.

The Distance to the Sun

Besides calculating the moon's distance, Aristarchus also devised an equally ingenious method to estimate the distance to the sun.

When the moon is half full, the sun must be somewhere directly to its side. By measuring the observed angle between the moon and the sun, you can tell that the sun is many times farther away than the moon.

Wait until the moon is exactly half full, then look up at it. If the illuminated half of the moon's sphere is on your right, this means that the sunlight must be coming directly from the right, at a 90-degree angle to your line of sight. So on the diagram, you know that the sun must lie somewhere along the dashed orange line.

To determine where along the dashed orange line, all you have to do is measure the angle between the moon and the sun, from your perspective. Point to the moon with one hand and the sun with the other, and estimate the angle between your arms. Then, on the diagram, measure out the same angle and extend a line from you toward the sun. Draw the sun where that line intersects the dashed orange line.

If you actually try this procedure, though, you'll find that the angle is indistinguishable from 90°. It has to be slightly less, of course, but you can't tell how much less because it's impossible to tell when the moon is exactly half full. But you can see, at least, that since the angle is close to 90°, the sun must be many times farther away than the moon.

Aristarchus wrote that the angle between the half-moon and the sun was 87°, and from this, estimated that the sun is about 19 times as far away as the moon. What he probably should have said is that the sun is at least 19 times as far away. And since the sun and moon have about the same angular size, this means that the sun's true diameter is at least 19 times that of the moon. Thus, Aristarchus was able to prove that the sun must be much bigger than the earth-a result that must have been hard for anyone to believe at the time.

We now know that the sun is almost 400 times as far away as the moon-an even more astonishing result. Because this distance is so large, nobody was able to measure it until after the invention of the telescope, 1900 years after the time of Aristarchus. But even without a telescope, you can use the method of Aristarchus to see that the sun must be many times farther away than the moon. Next time you see a half-full moon, look at where the sun is and then try to visualize them in three dimensions!

Question: Suppose you see a half-full moon in the southern sky, about half-way between the horizon and zenith. As you look toward it, the illuminated half is on the right. What time of day or night is it?

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After the sun, the most noticeable celestial object is the moon. Its motions, phases, and occasional eclipses add delightful variety to our sky. It's also the closest astronomical object by far.

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This article was used as a debating resource in one of my lessons. Students debated against each other on whether or not Pluto should be reclassified as a planet.

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Should Pluto be reclassified a planet again? UCF scientist Philip Metzger says yes based on his research. Credit: NASA

The reason Pluto lost its planet status is not valid, according to new research from the University of Central Florida in Orlando.

In 2006, the International Astronomical Union, a global group of astronomy experts, established a definition of a planet that required it to "clear" its orbit, or in other words, be the largest gravitational force in its orbit.

Since Neptune's gravity influences its neighboring planet Pluto, and Pluto shares its orbit with frozen gases and objects in the Kuiper belt, that meant Pluto was out of planet status.However, in a new study published online Wednesday in the journal Icarus, UCF planetary scientist Philip Metzger, who is with the university's Florida Space Institute, reported that this standard for classifying planets is not supported in the research literature.

Metzger, who is lead author on the study, reviewed scientific literature from the past 200 years and found only one publication-from 1802-that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.

He said moons such as Saturn's Titan and Jupiter's Europa have been routinely called planets by planetary scientists since the time of Galileo.

"The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research," Metzger said. "And it would leave out the second-most complex, interesting planet in our solar system.""We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it's functionally useful," he said."It's a sloppy definition," Metzger said of the IAU's definition. "They didn't say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit."

The planetary scientist said that the literature review showed that the real division between planets and other celestial bodies, such as asteroids, occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed.

UCF Scientists Philip Metzger is a co-author on the paper questioning the logic behind Pluto's classification. Other authors include Mark Sykes, of the Planetary Science Institute; Alan Stern, of the Southwest Research Institute; and Runyon of Johns Hopkins University Applied Physics Laboratory. Credit: University of Central Florida

However, even this reason is no longer considered a factor that determines if a celestial body is a planet, Metzger said.

Study co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, said the IAU's definition was erroneous since the literature review showed that clearing orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets.

"We showed that this is a false historical claim," Runyon said. "It is therefore fallacious to apply the same reasoning to Pluto," he said.Metzger said that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet's orbit."Dynamics are not constant, they are constantly changing," Metzger said. "So, they are not the fundamental description of a body, they are just the occupation of a body at a current era."

Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.

"And that's not just an arbitrary definition, Metzger said. "It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body."

Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he said.

"It's more dynamic and alive than Mars," Metzger said. "The only planet that has more complex geology is the Earth."

Provided by: University of Central Florida

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New research suggest Pluto should be reclassified as a planet

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The reason Pluto lost its planet status is not valid, according to new research from the University of Central Florida in Orlando. In 2006, the International Astronomical Union, a global group of astronomy experts, established a definition of a planet that required it to "clear" its orbit, or in other words, be the largest gravitational force in its orbit.