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Aristotle was concerned about the physical nature of the system; without unrollers, the outer motions would be transferred to the inner planets. The Eudoxan system had several critical flaws. One was its inability to predict motions exactly. Callippus' work may have been an attempt to correct this flaw. A related problem is the inability of his models to explain why planets appear to change speed. A third flaw is its inability to explain changes in the brightness of planets as seen from Earth.

Because the spheres are concentric, planets will always remain at the same distance from Earth. This problem was pointed out in Antiquity by Autolycus of Pitane c. Apollonius of Perga c. The deferent is a circle carrying the planet around the Earth.

In a deferent and epicycle model, the deferent carries a small circle, the epicycle , which carries the planet. The deferent-and-epicycle model can mimic the eccentric model, as shown by Apollonius' theorem. It can also explain retrogradation , which happens when planets appear to reverse their motion through the zodiac for a short time. Modern historians of astronomy have determined that Eudoxus' models could only have approximated retrogradation crudely for some planets, and not at all for others. In the 2nd century BC, Hipparchus , aware of the extraordinary accuracy with which Babylonian astronomers could predict the planets' motions, insisted that Greek astronomers achieve similar levels of accuracy.

Somehow he had access to Babylonian observations or predictions, and used them to create better geometrical models. For the Sun, he used a simple eccentric model, based on observations of the equinoxes , which explained both changes in the speed of the Sun and differences in the lengths of the seasons.

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For the Moon, he used a deferent and epicycle model. He could not create accurate models for the remaining planets, and criticized other Greek astronomers for creating inaccurate models. Hipparchus also compiled a star catalogue. According to Pliny the Elder , he observed a nova new star. So that later generations could tell whether other stars came to be, perished, moved, or changed in brightness, he recorded the position and brightness of the stars. Ptolemy mentioned the catalogue in connection with Hipparchus' discovery of precession.

Precession of the equinoxes is a slow motion of the place of the equinoxes through the zodiac, caused by the shifting of the Earth's axis. Hipparchus thought it was caused by the motion of the sphere of fixed stars.

Greek Astronomy

In the 3rd century BC, Aristarchus of Samos proposed an alternate cosmology arrangement of the universe: His astronomical ideas were not well-received, however, and only a few brief references to them are preserved. We know the name of one follower of Aristarchus: Aristarchus also wrote a book On the Sizes and Distances of the Sun and Moon , which is his only work to have survived. In this work, he calculated the sizes of the Sun and Moon, as well as their distances from the Earth in Earth radii.

Shortly afterwards, Eratosthenes calculated the size of the Earth, providing a value for the Earth radii which could be plugged into Aristarchus' calculations. Hipparchus wrote another book On the Sizes and Distances of the Sun and Moon , which has not survived. Both Aristarchus and Hipparchus drastically underestimated the distance of the Sun from the Earth. Hipparchus is considered to have been among the most important Greek astronomers, because he introduced the concept of exact prediction into astronomy. He was also the last innovative astronomer before Claudius Ptolemy , a mathematician who worked at Alexandria in Roman Egypt in the 2nd century.

Ptolemy's works on astronomy and astrology include the Almagest , the Planetary Hypotheses , and the Tetrabiblos , as well as the Handy Tables , the Canobic Inscription , and other minor works. The Almagest is one of the most influential books in the history of Western astronomy. In this book, Ptolemy explained how to predict the behavior of the planets, as Hipparchus could not, with the introduction of a new mathematical tool, the equant. The Almagest gave a comprehensive treatment of astronomy, incorporating theorems, models, and observations from many previous mathematicians.

This fact may explain its survival, in contrast to more specialized works that were neglected and lost. Ptolemy placed the planets in the order that would remain standard until it was displaced by the heliocentric system and the Tychonic system:. The extent of Ptolemy's reliance on the work of other mathematicians, in particular his use of Hipparchus' star catalogue, has been debated since the 19th century.

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A controversial claim was made by Robert R. Newton in the s. Newton's theories have not been adopted by most historians of astronomy. Claudius Ptolemy of Alexandria performed a deep examination of the shape and motion of the earth and celestial bodies. He worked at the museum, or instructional center, school and library of manuscripts in Alexandria.

Ptolemy is responsible for a lot of concepts, but one of his most famous works summarizing these concepts is the Almagest, a series of 13 books where he presented his astronomical theories. Ptolemy discussed the idea of epicycles and center of the world. The epicycle center moves at a constant rate in a counter clockwise direction.

Once other celestial bodies, such as the planets, were introduced into this system, it became more complex. The models for Jupiter, Saturn, and Mars included the center of the circle, the equant point, the epicycle, and an observer from earth to give perspective. The discovery of this model was that the center of the Mercury and Venus epicycles must always be colinear with the sun. This assures of bounded elongation. Bowler, , 48 Bounded elongation is the angular distance of celestial bodies from the center of the universe.

Ptolemy's model of the cosmos and his studies landed him an important place in history in the development of modern-day science.

The cosmos was a concept further developed by Ptolemy that included equant circles, however Copernicus model of the universe was simpler. In the Ptolemaic system, the earth was at the center of the universe with the moon, the sun, and five planets circling it. The earth was at the exact center of the cosmos, most likely because people at the time believed the earth had to be at the center of the universe because of the deductions made by observers in the system.

The sphere carrying the moon is described as the boundary between the corruptible and changing sublunary world and the incorruptible and unchanging heavens above it Bowler, , The heavens were defined as incorruptible and unchanging based on theology and mythology of the past. The Almagest introduced the idea of the sphericity of heavens. The assumption is that the sizes and mutual distances of the stars must appear to vary however one supposes the earth to be positioned, yet no such variation occurred Bowler, , 55 , The aether is the area that describes the universe above the terrestrial sphere.

This component of the atmosphere is unknown and named by philosophers, though many do not know what lies beyond the realm of what has been seen by human beings. The aether is used to affirm the sphericity of the heavens and this is confirmed by the belief that different shapes have an equal boundary and those with more angles are greater, the circle is greater than all other surfaces, and a sphere greater than all other solids.

Therefore, through physical considerations, and heavenly philosophy, there is an assumption that the heavens must be spherical.

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The Almagest also suggested that the earth was spherical because of similar philosophy. The differences in the hours across the globe are proportional to the distances between the spaces at which they are being observed. Therefore, it can be deduced that the earth is spherical because of the evenly curving surface and the differences in time that was constant and proportional.

In laymen terms, the earth must be spherical because they change in time-zones across the world occur in a uniform fashion, as with the rotation of a sphere.

The observation of eclipses further confirmed these findings because everyone on earth could see a lunar eclipse, for example, but it would be at different hours. The Almagest also suggest that the earth is at the center of the universe. The basis on which this is found is in the fact that six zodiac signs can be seen above earth, while at the same time the other signs are not visible Bowler, , The way that we observe the increase and decrease of daylight would be different if the earth was not at the center of the universe.

Though this view later proofed to be invalid, this was a good proponent to the discussion of the design of the universe. Ideas on the universe were later developed and advanced through the works of other philosophers such as Copernicus, who built on ideas through his knowledge of the world and God. The Little Book of Black Holes.

Ancient Greek astronomy

Little Green Apples Publishing. A Discourse Addressed to an Infidel Mathematician. Night Sky With the Naked Eye. Newton's Philosophy of Nature. The Stones and the Stars. Works of Oliver Lodge. The Heavens on Earth. The Copernicus of Antiquity: Aristarchus of Samos Illustrated Edition. How to write a great review. The review must be at least 50 characters long. The title should be at least 4 characters long.

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Greek Astronomy by Sir Thomas L. Heath series Dover Books on Astronomy.

Ancient Greek astronomy - Wikipedia

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