Ancient Greek Astronomy

The Greeks knew the following (among other things):

1. the earth is round,

2. the stars are very far away compared to the planets,

3. the causes of eclipses, and

4. the causes of the phases of the moon.

They could do many rather sophisticated astronomical operations, such as:

1. Measure the radius of the earth by observing the apparent increase in elevation of the sun at noon when traveling from north to south. (The ``elevation'' or ``altitude'' of an object in the sky is the angle between the direction towards the object, and the direction to the place on the horizon directly below the object.)

2. Measure the size of, and distance to the moon by timing the passage of the moon through the earth's shadow during lunar eclipses.

3. Measure the ratio (diameter of/distance to) for both the sun and the moon, by observing the apparent angular size of each.

4. Predict the positions and motions of the planets over many years to a precision of better than 1^o .

5. Catalog all the visible stars according to brightness (Hipparchus' catalog).

Probably the most famous and complex Greek contribution was Ptolemy's geocentric solar system which served for almost 2000 years as the best way to predict the planetary motions. Ptolemy's theory grew out of observation of the motion of the sun. This theory had the sun moving on an epicyclic path in order to explain its (slightly) variable daily procession along the ecliptic. Ptolemy had the planets moving on complex epicyclic paths which were combinations of two circular motions for each planet.