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Location: Salisbury, England
Stonehenge is the most famous prehistoric astronomical observatory

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Decoding the ancient astronomy of Stonehenge

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Place of birth: Nicaea, Turkey
Hipparchus was one of the first people to correctly theorize that the Earth orbited around the Sun.

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Hipparchus, also spelled Hipparchos, (born, Nicaea, Bithynia [now Iznik, Turkey]-died after 127 bc, Rhodes?), Greek astronomer and mathematician who made fundamental contributions to the advancement of astronomy as a mathematical science and to the foundations of trigonometry. Although he is commonly ranked among the greatest scientists of antiquity, very little is known about his life, and only one of his many writings is still in existence. Knowledge of the rest of his work relies on second-hand reports, especially in the great astronomical compendium the Almagest, written by Ptolemy in the 2nd century ad.

Lover of truth

As a young man in Bithynia, Hipparchus compiled records of local weather patterns throughout the year. Such weather calendars ( parapēgmata), which synchronized the onset of winds, rains, and storms with the astronomical seasons and the risings and settings of the constellations, were produced by many Greek astronomers from at least as early as the 4th century bc.

Most of Hipparchus's adult life, however, seems to have been spent carrying out a program of astronomical observation and research on the island of Rhodes. Ptolemy cites more than 20 observations made there by Hipparchus on specific dates from 147 to 127, as well as three earlier observations from 162 to 158 that may be attributed to him. These must have been only a tiny fraction of Hipparchus's recorded observations. In fact, his astronomical writings were numerous enough that he published an annotated list of them.

Hipparchus also wrote critical commentaries on some of his predecessors and contemporaries. In Tōn Aratou kai Eudoxou Phainomenōn exēgēseōs biblia tria ("Commentary on the Phaenomena of Aratus and Eudoxus"), his only surviving book, he ruthlessly exposed errors in Phaenomena, a popular poem written by Aratus and based on a now-lost treatise of Eudoxus of Cnidus that named and described the constellations. Apparently his commentary Against the Geography of Eratosthenes was similarly unforgiving of loose and inconsistent reasoning. Ptolemy characterized him as a "lover of truth" ( philalēthēs)-a trait that was more amiably manifested in Hipparchus's readiness to revise his own beliefs in the light of new evidence. He communicated with observers at Alexandria in Egypt, who provided him with some times of equinoxes, and probably also with astronomers at Babylon.

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Solar and lunar theory

Hipparchus's most important astronomical work concerned the orbits of the Sun and Moon, a determination of their sizes and distances from the Earth, and the study of eclipses. Like most of his predecessors- Aristarchus of Samos was an exception-Hipparchus assumed a spherical, stationary Earth at the centre of the universe (the geocentric cosmology). From this perspective, the Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn (all of the solar system bodies visible to the naked eye), as well as the stars (whose realm was known as the celestial sphere), revolved around the Earth each day.

Every year the Sun traces out a circular path in a west-to-east direction relative to the stars (this is in addition to the apparent daily east-to-west rotation of the celestial sphere around the Earth). Hipparchus had good reasons for believing that the Sun's path, known as the ecliptic, is a great circle, i.e., that the plane of the ecliptic passes through the Earth's centre. The two points at which the ecliptic and the equatorial plane intersect, known as the vernal and autumnal equinoxes, and the two points of the ecliptic farthest north and south from the equatorial plane, known as the summer and winter solstices, divide the ecliptic into four equal parts. However, the Sun's passage through each section of the ecliptic, or season, is not symmetrical. Hipparchus attempted to explain how the Sun could travel with uniform speed along a regular circular path and yet produce seasons of unequal length.

Hipparchus knew of two possible explanations for the Sun's apparent motion, the eccenter and the epicyclic models ( see Ptolemaic system). These models, which assumed that the apparent irregular motion was produced by compounding two or more uniform circular motions, were probably familiar to Greek astronomers well before Hipparchus. His contribution was to discover a method of using the observed dates of two equinoxes and a solstice to calculate the size and direction of the displacement of the Sun's orbit. With Hipparchus's mathematical model one could calculate not only the Sun's orbital location on any date, but also its position as seen from the Earth. The history of celestial mechanics until Johannes Kepler (1571-1630) was mostly an elaboration of Hipparchus's model.

Hipparchus also tried to measure as precisely as possible the length of the tropical year -the period for the Sun to complete one passage through the ecliptic. He made observations of consecutive equinoxes and solstices, but the results were inconclusive: he could not distinguish between possible observational errors and variations in the tropical year. However, by comparing his own observations of solstices with observations made in the 5th and 3rd centuries bc, Hipparchus succeeded in obtaining an estimate of the tropical year that was only 6 minutes too long.

He was then in a position to calculate equinox and solstice dates for any year. Applying this information to recorded observations from about 150 years before his time, Hipparchus made the unexpected discovery that certain stars near the ecliptic had moved about 2° relative to the equinoxes. He contemplated various explanations-for example, that these stars were actually very slowly moving planets-before he settled on the essentially correct theory that all the stars made a gradual eastward revolution relative to the equinoxes. Since Nicolaus Copernicus (1473-1543) established his heliocentric model of the universe, the stars have provided a fixed frame of reference, relative to which the plane of the equator slowly shifts-a phenomenon referred to as the precession of the equinoxes. ( See animation.)

Hipparchus also analyzed the more complicated motion of the Moon in order to construct a theory of eclipses. In addition to varying in apparent speed, the Moon diverges north and south of the ecliptic, and the periodicities of these phenomena are different. Hipparchus adopted values for the Moon's periodicities that were known to contemporary Babylonian astronomers, and he confirmed their accuracy by comparing recorded observations of lunar eclipses separated by intervals of several centuries. It remained, however, for Ptolemy (ad 127-145) to finish fashioning a fully predictive lunar model.

In On Sizes and Distances (now lost), Hipparchus reportedly measured the Moon's orbit in relation to the size of the Earth. He had two methods of doing this. One method used an observation of a solar eclipse that had been total near the Hellespont (now called the Dardanelles) but only partial at Alexandria. Hipparchus assumed that the difference could be attributed entirely to the Moon's observable parallax against the stars, which amounts to supposing that the Sun, like the stars, is indefinitely far away. (Parallax is the apparent displacement of an object when viewed from different vantage points). Hipparchus thus calculated that the mean distance of the Moon from the Earth is 77 times the Earth's radius. In the second method he hypothesized that the distance from the centre of the Earth to the Sun is 490 times the Earth's radius-perhaps chosen because that is the shortest distance consistent with a parallax that is too small for detection by the unaided eye. Using the visually identical sizes of the solar and lunar discs, and observations of the Earth's shadow during lunar eclipses, Hipparchus found a relationship between the lunar and solar distances that enabled him to calculate that the Moon's mean distance from the Earth is approximately 63 times the Earth's radius. (The true value is about 60 times.)

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Hipparchus | Biography, Discoveries, Accomplishments, & Facts

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Hipparchus, Greek astronomer and mathematician who made fundamental contributions to the advancement of astronomy as a mathematical science and to the foundations of trigonometry. Although he is commonly ranked among the greatest scientists of antiquity, very little is known about his life.

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Place of birth: Alexandria, Egypt
Founder of Ptolemaic model of  the solar system
Characteristics:
-Earth at center, sun, moon, planets, and stars travel along a circular path
-Epicycles explained the backwards motion of some of the planets
-Consisted of 80 circular paths to explain the motion of all of the known celestial bodies

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Place of birth: Torun, Poland
Founder of the Heliocentric model of the solar system
Characteristics:
-Sun is centered in the middle of the solar system. Planets and moons orbit around the sun
-Earth spins on its axis
-Moons orbit respective planets
-Retrograde motion occurs when Earth “laps” an inferior planet in the sky
-Circular orbits in the sky

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Place of birth: Knutstorp Manor, Sweden
-Most of his observations came before the telescope
He used his own instruments
-Brahe kept detailed records of stars, planets, and

other noteworthy astronomical events
-Brahe’s observations accurately identified positions of

celestial bodies which helped in the creation of the

laws of planetary motion

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Invention and Improvement of the Telescope 

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Invention and Improvement of the Telescope 

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-Telescopes are like giant light collectors
-There are two types of telescopes, reflecting and refracting.
Refracting telescopes:
Refracting uses lenses to bed light from a celestial object to an observer's eye
Reflecting Telescopes: Reflecting uses mirrors to reflect the light from a celestial object to an

observers eye

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Place of birth: Pisa, Italy
-Utilized brand new technology for his work the telescope
-Built his own telescope
Discoveries:
-geography of the moon
-sunspots on the sun
-Jupiter’s moons
-Transit of Venus

He published his work

and came out publicly in support of the heliocentric model of the

solar system, which he was imprisoned for.

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Place of birth: Weil der Stadt, Germany
- Used Brahe’s records to try to find a description of planetary motion
- Created three laws of planetary motion based on Brahe’s planetary data
1st law:
The orbital paths of the planets are elliptical (not circular), with the Sun at one focus
2nd Law:
An imaginary line connecting the Sun to any planet sweeps out equal area in equal amounts

of time
3rd law: The square of a planet's orbital period is proportional to the cube of it’s semimajor axis

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Place of birth: Woolsthorpe Manor House, United Kingdom
-Discovered gravity and the law of universal gravitation
-Acceleration due to gravity g=9.81 m/s^2
Law of inertia:

an object in motion will remain in motion unless acted on by an outside force
Newton's Law of Universal Gravitation:
Universal gravitational

Every particle attracts every

other particle in the universe

with a force directly

proportional to the product

of their masses and inversely

proportional to the distance

between their centers

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Place of birth: Ulm, Germany
-Developed the theories of special and general relativity
Special relativity:
Theory relating space and time. The observance of space and time are relative to each

individual observer.
General Relativity:
Theory of gravitation that explains how objects with mass warp spacetime. Massive

objects bend space and slow down time.

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Place of birth: Oxford, United Kingdom
Predicted the existence of Black Holes and noted that they emit Hawking radiation

Hawking Radiation:

Electromagnetic radiation

emitted by a black hole, due to quantum effects, causing it to lose matter. If a black hole loses

more matter than it gains, it could shrink and eventually vanish.