Astronomy has been around since the beginning of civilization. Ancient civilizations stared and the night sky over and over and pondered it. They noticed regularities in the moon, night and day, and other heavenly bodies. These civilizations had problems defining time and direction. They needed an accurate way to know when to plant their crops and perform their religious ceremonies.

The regularities of the moon, sun, and the planets provided a solution. The sun always rose in the east, moved across the sky during the day, and set in the west. As the sun set a plethora of stars moved across the sky in exactly the same manner. People living in the North Temperate Zone noticed that night and day were not of equal length. On long days the sun rose north of east and rose high in the sky. On short days, the sun rose south of east, and didn’t rise quite so high. Observation of the stars in the early morning sky before sunrise and in the evening sky after sunset showed that the sun changes its position among the background of stars. The Egyptians may have been the first people to calculate that it takes 365 days for the sun to return the same spot among the stars.

The sky also holds the moon and five other planets. They move in a narrow path across the sky named the Zodiac. It takes approximately 29.5 days for the moon to overtake the sun. This is known as the synodic month. The sun and the moon always move in the same direction across the zodiac, east to west. The five planets: Mercury, Venus, Mars, Jupiter and Saturn, also move in a generally eastward direction, however, during certain times of the synodic period, they move westward, or retrograde. People of ancient times sought a connection between celestial events, especially planetary movements and their fortune. This belief led to the formation of astrology. While today your horoscope may be little more than a few random sentences, the mathematics required for prediction of celestial and planetary events actually furthered the science of astronomy.

Constellation maps were created by many ancient people, including Egyptians, Mayans, and Chinese. However, the Babylonians accomplished greater achievements. Babylonian custom was to designate the first day of each month the day after the new moon, when the crescent first appears. First done by observation, later they wanted to calculate this event in advance. Around 400 B.C, after Babylonia became a part of Persia, astronomers observed that the speed of the moon and sun around the zodiac do not have constant speed. The moon accelerated for half of each revolution, to a maximum, then decelerated to the former minimum. They attempted to represent this cycle by giving the moon a fixed speed for the first half, and a different fixed speed for second half. Later, the moon’s speed was represented as a factor that increases linearly to the maximum and decreases to the minimum. With these calculations, astronomers could predict the time of the new moon and day the new month would begin. Since they new that, they could also know the position of the moon and sun on a daily basis.

Using a similar process, stargazers also calculated the positions of the planets, both eastward and retrograde. Archeologists have unearthed cuneiform tablets showing the calculations. Some tablets discovered were determines to be from the cities of Babylon and Uruk (Erech) on the Euphrates River. On them was the name of Naburiannu or Kidinnu, possibly astronomers that invented the system for calculation.

The Greeks made some of the greatest contributions to astronomy. Several Greek works mention celestial objects. Homer’s The Odyssey refers to the constellations of the Great Bear, Orion, and the Pleiades and tells how to they can be used for navigation. Farming was helped out by the poem Works and Days by Hesiod, which described which constellations to look for in the morning sky at different seasons to alert the farmer when it was the proper time for plowing, sowing, and harvesting.

They were the first to provide theories for the orbits and observation of the planets and sun.

It’s legend that Thales accurately predicted a total solar eclipse on May 28, 585 B.C., but that might be mostly apocryphal. Around 450 B.C. Greece dove face first into planetary observation. Philolaus, around 5th century, B.C, a follower of Pythagoras, believed that the sun, earth, moon and planets revolved around a central fire hidden from view by an interposed counterearth. The revolution of the earth around this fire every 24 hours accounted for the daily motion of the sun and stars. Eudoxus of Cnidus proposed a theory marked by a huge sphere containing all the stars on its inner surface in a daily rotation about the earth. To account for solar, planetary, and lunar movements, he proposed smaller interconnected transparent spheres that revolved in different ways.

The most radical theorists was a man by the name Aristarchus of Samos. He actually thought that the observations could be explained by the earth rotated along its axis in 24-hour intervals, and along with the other planets, orbit around the centralized sun. He must have been crazy! Not surprisingly, most Greek philosophers rejected this hypothesis. They regarded the earth as a huge, heavy motionless body around which the light and bodies revolve. This theory is known as the geocentric system. It remained unchanged for nearly 2000 years!

In the 2nd century AD, Greece decided to combine all of their theories. Two people, Ptolemy and Hipparchus determined the positions of over 1000 stars. They postulated a series of circles surrounding the earth to explain the eastward motions of the sun, moon, and planets. To explain the retrograde motion of the planets, they also created a second circle, called the epicycle, the center of which rested on the first circle. By properly choosing the diameters and the speeds of the circular motions associated with each object, it matched the observations. This system was explained in his great work the Almagest.