Why did ancient astronomers observe the sky?

Beginning around 600 BC, ancient philosophers and scientists developed a number of important astronomical ideas by simply observing the sky, since they didn't have any modern tools or instruments to tell the time, location of a place or the nature of the Universe.

The ancient Greeks were able to figure out the shape and size of the Earth by calculating the height of the Sun or the stars. About 230 BC, Eratosthenes estimated the size of the Earth by using the Sun as a reference point. The sundial was another simple tool that depended on the Sun to let people know what time it was.

Sailors at sea used the stars above to guide them. In fact, Many early tribes of Polynesia and New Zealand have sailed great distances using the stars for navigation. The sun's position in the sky also allowed sailors to determine the latitude at which they were sailing. Another instrument called the 'qiblah' used the sun to find the direction of Mecca, so that Muslims could face in that direction for their daily prayers. So, you see ancient astronomers had many good reasons, to watch the sky, the Sun, stars and the Moon, day and night.