Astronomy is the study of the universe and the objects in it. Astronomers observe the sky with telescopes that gather not only visible light but also invisible forms of energy, such as radio waves. They investigate nearby bodies, such as the sun, planets, and comets, as well as distant galaxies and other faraway objects. They also study the structure of space and the past and future of the universe.

Astronomers seek answers to such questions as: How did the universe begin? What processes release energy deep inside stars? How does one star “steal” matter from another? How do storms as big as Earth arise on Jupiter and last for hundreds of years?

To answer such questions, astronomers must study several subjects besides astronomy. Almost all astronomers are also astrophysicists because the use of physics is essential to most branches of astronomy. For example, some parts of cosmology, the study of the structure of the universe, require an understanding of the physics of elementary particles, such as the bits of matter called quarks that make up protons and neutrons. Astronomers use chemistry to analyze the dusty, gaseous matter between the stars. Specialists in the structure of planets use geology.

Astronomy is an ancient science. Like today’s researchers, ancient scholars based their ideas of the universe on what they observed and measured and on their understanding of why objects move as they do. However, the ancients developed some incorrect ideas about the relationships between Earth and the objects they saw in the heavens. One reason for their errors was that they did not understand the laws of motion. For example, they did not know that a force–which we know as gravitation–controls the movements of the planets. Another reason was that their measurements did not reveal the movements of the planets in sufficient detail.

The ancients noted that the positions of the sun, moon, and planets change from night to night. We know that these movements are a result of the revolution of the moon about Earth and the revolution of Earth and the other planets about the sun. The ancients, however, concluded that the sun, moon, and planets orbit a motionless Earth. In many places, religious teachings supported this conclusion until the 1600′s.

Although ancient people misinterpreted much of what they saw in the heavens, they put their knowledge of astronomy to practical use. Farmers in Egypt planted their crops each year when certain stars first became visible before dawn. Many civilizations used the stars as navigational aids. For example, the Polynesians used the positions of the stars to guide them as they sailed from island to island over thousands of miles or kilometers of the Pacific Ocean.