The ancient Babylonians were pioneers of astronomy, and the Greeks came into contact with Babylonian astronomical data which they used extensively. Their first contribution was the construction of geometric models to represent the motion of the moon, sun and planets. The Greeks believed that stars were part of an outer sphere that rotated once every twenty-four hours, with the sun, moon and planets moving in fixed orbit round the earth. In contradiction to the ardent belief at that time that the earth was the centre of the universe, Pythagoreans developed a model which assigned the earth an orbit around a central fire, though they did not recognise the sun to be this centre.

Eudoxus (ca. 355 BCE)

A Greek mathematician of Plato’s time, Eudoxus used observation to approximate the periods of orbit of the planets and the angles they make with one another. He used this to come to the conclusion that the universe was a series of concentric hollow spheres, with the outermost sphere containing the stars and the centre being the earth. This is probably the first ever attempt at mathematically defining the circular orbits of the planets. Hence, Eudoxus can be considered the father of scientific astronomy. 

Aristarchus (3^rd century BCE) 

Aristarchus gave geometric form to the computation of the distance and size of the moon and the sun, without, however, providing an actual solution for these values. He provided methods for ascertaining the ratio between the distances of the moon and the sun from the earth. His method for calculating these values was indisputable; given the right observational apparatus, he would be able to provide accurate values for these ratios.

More importantly, according to Archimedes and Plutarch, he hypothesised that the sun was the centre of the solar system, and the earth rotated along with the other planets. However, none of his works on this subject seem to have survived. 

Hipparchus (ca. 160 BCE) 

Hipparchus created the first star catalogue, with the position of about 850 stars, along with their relative brightness. During a total eclipse of the sun, he used the parallax method to find the distance and diameter of the moon. By comparing the fraction of the sun that was not covered by the moon at different places he was able to compute the arc subtended by the sun on the earth. Approximating the length of this arc to the diameter of the sun brought Hipparchus to his result.

As with most other civilisations of the time, the Greeks followed a lunar calendar, with twelve months each varying between twenty-nine and thirty days in length. As a result, the year lasted only 354 days, which the Greeks knew was short of a solar year because of the relation of the months with the seasons. They solved this problem by adding an extra month every two years, which made the year average out to about 369 days, reasonably close to the actual length of 365.25 days. This method, known as intercalation, was used by most other civilisations as well. 

Greek writers were aware of the  discrepancy of around four days between the lunar and the solar months which still remained. They resolved this by adding more months at greater intervals of time to provide a better average year length. For example, Meton of Athens (432 BCE) invented a system in which the appropriate number of months was included every nineteen years.

Much of the ancient world used various kinds of  clocks to keep time. The common type of clock was the sundial. A needle sticking upright on a dial made a shadow that went around the dial as the sun crossed the sky. The position of the shadow indicated how much of the day had passed. The Greeks also used water clocks. The common feature of these clocks was the measurement of time using the constant flow of water from one vessel to another. The collective name given to these clocks is the clepsydra.

Ctesibius of Alexandria (ca. 270 BCE) made significant modifications to the clepsydra which caused it to have an even flow regardless of the height of water in the vessel. He also built a large water clock, the flow of which was used to run different gadgets like bells and singing birds, making it the world’s first cuckoo clock.¹

1. Peter James and Nick Thorpe, “Ancient Inventions”, (Ballantine Books, 1994), p. 125.