Ancient cultures have long been interested in the motions of the stars and planets. Many of them constructed calenders of one form or another (such as Stonehenge) to predict seasons. Certainly the heavens held religious significance for many peoples. At the same time, the regularity of the sky patterns and motions were useful for predictive purposes - the dry and rainy seasons. These cultures were concerned with survival issues.
The ancient Greeks moved astronomy from purely predictive uses to something akin to modern science. The Greeks suggested models for motions in the sky, and some of the ideas were testable. Here are few selected highlights from Greek contributions:
1.) Pythagorus
Considered the Earth, sun, and planets to move around a central "hearth".
2.) Aristotle
Offered arguments for Earth Sphericity. One was that Earth's shadow against the moon was always curved, and never stripe as might be expected from a flat Earth casting a shadow when on edge. A stronger argument (I think) was that people at different latitudes saw different constellations.
3.) Aristarchus
Applied Euclid's geometry to Astronomy. Made predictions for the relative sizes and distances of the moon and sun. Claimed that the earth went around the sun. Quite a modern view really. However, this Sun-Centered claim was reject by his contemporaries because of the failure to observe stellar parallax (the apparent shift in a star's position because the Earth is moving in space around the Sun).
This is a valid reason for rejecting that claim. What his contemporaries failed to appreciate was the possibility that stars are so far away, that parallax could not be seen with the human eye (which turns out to be the case!)
4.) Erastothenes
Estimated the circumference on the earth from a shadow experiment.
5.) Hipparchus
Invented the magnitude system. He also measured the precession of the Earth. This precession is like the wobble of a spinning top. The Earth's rotation axis is fixed at 23.5 degree but drifts in its orientation, executing a slow circular path in the sky. The North star of today will not always be the North Star. The period is about 26,000 years.
6.) Ptolemy
Represents the culmination of Greek thought at his time. The motions of Sun, Moon, planets, and stars were understood to be real motions of bodies around the Earth, or a geocentric model. Greeks required that the bodies move along circular paths (because circles are perfect) with uniform motion (constant speed).
The problem is that planets show retrograde motion, meaning that they sometimes "back up" and go in reverse the sky, and then go forward (prograde) again. That is impossible for circular motion at fixed speed. So the Ptolemaic and shifted circles to help account for retrograde motions of planets in the sky. These complicated models were described in a book called "The Almagest".
The ancient Greeks moved astronomy from purely predictive uses to something akin to modern science. The Greeks suggested models for motions in the sky, and some of the ideas were testable. Here are few selected highlights from Greek contributions:
1.) Pythagorus
Considered the Earth, sun, and planets to move around a central "hearth".
2.) Aristotle
Offered arguments for Earth Sphericity. One was that Earth's shadow against the moon was always curved, and never stripe as might be expected from a flat Earth casting a shadow when on edge. A stronger argument (I think) was that people at different latitudes saw different constellations.
3.) Aristarchus
Applied Euclid's geometry to Astronomy. Made predictions for the relative sizes and distances of the moon and sun. Claimed that the earth went around the sun. Quite a modern view really. However, this Sun-Centered claim was reject by his contemporaries because of the failure to observe stellar parallax (the apparent shift in a star's position because the Earth is moving in space around the Sun).
This is a valid reason for rejecting that claim. What his contemporaries failed to appreciate was the possibility that stars are so far away, that parallax could not be seen with the human eye (which turns out to be the case!)
4.) Erastothenes
Estimated the circumference on the earth from a shadow experiment.
5.) Hipparchus
Invented the magnitude system. He also measured the precession of the Earth. This precession is like the wobble of a spinning top. The Earth's rotation axis is fixed at 23.5 degree but drifts in its orientation, executing a slow circular path in the sky. The North star of today will not always be the North Star. The period is about 26,000 years.
6.) Ptolemy
Represents the culmination of Greek thought at his time. The motions of Sun, Moon, planets, and stars were understood to be real motions of bodies around the Earth, or a geocentric model. Greeks required that the bodies move along circular paths (because circles are perfect) with uniform motion (constant speed).
The problem is that planets show retrograde motion, meaning that they sometimes "back up" and go in reverse the sky, and then go forward (prograde) again. That is impossible for circular motion at fixed speed. So the Ptolemaic and shifted circles to help account for retrograde motions of planets in the sky. These complicated models were described in a book called "The Almagest".
