From Nick Greene,
Your Guide to Space / Astronomy.
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There are different definitions of Astronomy. Some define it as a branch of science dedicated to the study of the motions and natures of celestial bodies, like planets, stars, and galaxies. Others say it is the study of objects and matter outside the earth's atmosphere and of their physical and chemical properties. Still others would say it is the study of everything, because it's a study of the universe and everything is part of the universe.

All three definitions are accurate, but however you define it, there are many fields within Astronomy. Cosmologists study the Universe as a whole, including its beginnings. Astrometrists measure great distances. Planetologists study planets within our own Solar System as well as those orbiting distant stars. Radio Astronomers use radio-telescopes to study the Universe. There are also Mathematical Astronomers who use numbers, calculations and statistics to explain the universe.

Astronomy is not a "stand-alone" science. It combines areas from a number of other fields, including mathematics, chemistry, geology, biology and physics. In fact, physics is such an integral part of the field that many astronomers are also known as Astrophysicists.

The study of our universe is not new, astronomy is the oldest science. People have been looking up, trying to explain the universe for as long as there have been people. Some of the earliest Astronomers were priests and holy men, studying the movement of celestial bodies to determine celebrations and planting cycles.

The ancient Greeks were the first to start developing astronomy theories about the design of the Universe.

There have been many astronomers throughout history who have played important roles in the science. Many have later been proved wrong as our knowledge and technology improved, but if not for the earlier work, later scientists would have had nothing on which to base their research.

This is only a partial list:

• Ptolemy
• Copernicus
• Tycho Brahe
• Galileo Galilei
• Johannes Kepler
• Isaac Newton
• Edmond Halley
• William Herschel
• Albert Einstein
• Willem de Sitter
• Georges-Henri Lemaitre
• Edwin P. Hubble
• Thomas Gold
• Carl Sagan
• Stephen Hawkings

For more in-depth information about astronomy, check out our Astronomy 101, free email basic astronomy course. You can earn a certificate.