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Ancient Astronomers

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Before the invention of the telescope, modern mathematics, modern physics, and modern ways of science, ancient civilizations were still able to discover an amazing amount of information about astronomy. All the planets in the solar system except for Uranus, Neptune, and Pluto were known to ancients. Ancient Egyptians were able to figure out how long a year was based upon the movements of the sun and the flooding of the Nile River. Greeks were able to estimate the diameter of Earth to within 32 km (20 miles) long before Europeans "knew" the Earth was flat. This page contains this and much more information on ancient astronomy.

Purpose for Astronomy

Like today, there were always those who were interested in just knowing about things for the interest in simply knowing. However, that didn't get ancient civilizations very far, so there had to be a reason for people to study astronomy.

The main one was to try to understand what was going on in order to try to predict what would happen. Often times, ancient astronomers were priests, who were kept alive only as long as they correctly predicted events. Astronomical bodies, such as the sun, moon, and planets, were often seen as gods and goddesses. Predicting planetary positions were a way of predicting the will of the gods, and could then foretell events such as whether or not there would be a good harvest.

Eclipses and comets were two of the most important phenomena to ancient people, for they believed that they were bringers of anything from a plague, defeat or victory in war, or the death of a ruler. Changes in the sky, such as a new star, were also seen as very important predictors of events to come, but were much more rare.


The Akkadians are among the first people credited to have kept astronomical records, and the earliest date from around 2500 B.C. They lived in the northern part of what was later called Babylon. The astronomer-priests that lived later in Babylon were able to use the records of the Akkadians to predict some of the motions of the sun, Earth's moon, and stars.


The Egyptians were one of the first to create an accurate calendar. Unlike most cultures', their calendar was based upon the sun and stars rather than on the moon. This was due to the Nile River. With a calendar, they were able to make accurate estimates of when to plant crops, and when the Nile's annual flooding would occur. Since the first calendars, there have always been seven days in a week, to match the quarter cycles of the Moon, and always twelve months in a year, to match the twelve complete cycles of the Moon per year.

Besides the calendar, Egyptians used astronomy to help build some of the most impressive architecture on Earth: The Pyramids. By using knowledge of the sun and constellations, Egyptians were able to align the pyramids and shafts within the pyramids with the cardinal directions, as well as with star groupings that held spiritual significance.Comet Halley


The Chinese have been observing the sky for several millennia, making them the oldest civilization with a continuous astronomical record. Some of the amazing records that the Chinese hold is the first documented solar eclipse - over 4000 years ago in 2136 B.C. The first recording of any planetary grouping was made by the Chinese in 500 B.C. In the 400's B.C. they made the Book of Silk, the earliest known atlas of comets, and it was discovered in a tomb in 1973.


The Polynesian islanders were among the first people to navigate the oceans, and they used the stars to guide them. The sun marked their day, but very strict records of stars were kept in order to guide them during the night; this practice started around the dawn of the Christian era -- over 2000 years ago.

Often, the way they would keep track of stars was by creating mythologies around them, much as the Greeks did. The Pleiades (an open cluster), Jupiter, and Aldebaran (a bright red star), are central to the founding story of the polynesian culture.

By using fixed locations on the horizon, sailors memorized hundreds of stars and were able to determine their longitude and latitude to a high accuracy. By memorizing the positions, brightness, colors, and time of year that stars were visible, sailors were equipped to navigate at any time during the year.


One of the most important - and infamous - contributions of the Greeks was the geocentric model of the universe. The geocentric model means that the Earth, "geo," is at the center, "centric," of the universe, and that all other bodies move around it with the Earth at a fixed location. This idea was based upon the teachings of Aristotle (384-322 B.C.).

This view held so much sway because of many of the philosophies of the ancient Greeks, including Pythagoras and the Pythagoreans. They believed that the circle is the perfect form, and that the simplest model that made sense must be the correct one. Since the heavens were perfect, everything must move upon a circle, and since the simplest model was that the Earth stood still and everything moved around it, then that must also be true. After all, we can't feel the Earth moving, so why should be believe that it does without any extraordinary evidence?

However, evidence against this system was obvious even to the Greeks 2500 years ago. It had to do with the motion of the planets. For periods of time, the planets seem to orbit in an eastward direction across the stars. However, for brief periods of time, they switch and go in a westward direction. This is called retrograde.

The explanation for this now is simple, and is discussed in the Scientific Revolution section. However, back then, philosophers added extra little circles to the orbits of the planets. The planets now orbited on a little circle, whose center orbited on the larger circle that was around the Earth. This circle upon a circle was called an "epicycle," and the larger circle was called a "deferent."

PtolemyThe Greek Aristarchus of Samos (310-230 B.C.) was one of the first to actually present the heliocentric - sun-centered - model of the solar system. However, Aristotle's views were too wide-spread and well-known, and he had too many followers for anyone to listen to an idea that didn't put our egotistical race at the center of the universe.

Around A.D. 140, the Greek astronomer Ptolemy (left) solidified the geocentric model, elaborating and formalizing the view in a manner that closely approximated the movements of the sun and planets. In Ptolemy's model of the universe, Earth was the center sphere, surrounded by eight other spheres, which were, in order, the moon, Mercury, Venus, the sun, Mars, Jupiter, Saturn, and then the "fixed stars."


Every major civilization had their own type of calendar: China, India, and Rome, just to name a few. Most were based upon the cycle of the moon (about every 27.3 days); many days were skipped from our 365-day year. In Ancient Rome, if the calendar was too far off (judging by the seasons), the Emperor at the time would declare that the next day would be "x" days sooner or later than normal (e.g. if today was July 12, tomorrow might be August 2).

Some calendars based upon the rotation of stars. For example, the Ancient Egyptians used the movements of Sirius (AKA The Dog Star) to tell when the floods of the Nile would come.

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