Astronomy is the study of planets and their satellites (moons), comets and meteors, star systems, galaxies, and clusters of galaxies.

The curiosity of ancient people about day and night and the Sun, moon, and stars led eventually to observations that heavenly bodies appear to move in a regular pattern. Ancient people needed to figure out the proper times for planting and harvesting crops and for religious celebrations (they didn’t have a calendar at that point), so by keeping track of the movement of the stars and planets, they could keep track of time.

Ancient astronomers noticed the bright Sun, which divided daytime from nighttime, rose every morning from one direction, the east, moved steadily across the sky during the day, and set in the opposite direction, the west. At night, more than 1,000 stars that are visible to the naked eye followed a similar path. The stars appeared to rotate in groups around a point in the sky. The ancient people called these groups of stars constellations.

Useful calendars were developed by several ancient civilizations including the Egyptians, the Maya, and the Chinese, but the Babylonians accomplished even greater things. The Babylonian civilization thrived from the 18^th to the 6^th century BC in what is now Iraq. Babylonian astronomers studied the motions of the Sun and moon. They made a calendar where the new moon became the beginning of each month. The Babylonians could predict the time of the new moon and the day on which the new month would begin.

Aristarchus of Samos, a Greek, believed that motions in the sky could be explained by the Earth turning around its axis once every 24 hours and, along with other planets, revolving around the Sun. Other Greek philosophers who believed that the Sun, moon, planets, and stars revolved instead around the Earth rejected his explanation, and they believed the incorrect view for over 1,500 years.

The history of astronomy took a dramatic turn in the 16^th century as a result of the Polish astronomer, Nicolaus Copernicus. He showed that the motion of the planets can be explained by assuming a central position for the Sun rather than for the Earth (he believed Aristarchus’ theory!).

Little attention was paid to Copernicus’ theory until Italian astronomer Galileo discovered evidence to support it. Galileo saw his chance to test the Copernican theory of a moving Earth when the telescope was invented. In 1609, Galileo made a small telescope, turned it towards the sky, and discovered the phases of Venus, indicating that this planet revolved around the Sun. He also discovered four moons revolving around Jupiter and the rings of Saturn.

Johannes Kepler formed the laws of planetary motion, stating that the planets revolve around the Sun in elliptical orbits, not circular orbits. Sir Isaac Newton explained Kepler’s laws of motion, arguing that an attractive force exists between the Sun and each of the planets—GRAVITY!

Improved telescopes allowed the scanning of planetary surfaces and the discovery of millions of stars, galaxies, and galaxy clusters.

With the use of telescopes, many new members of the solar system were discovered, including Uranus in 1781, Neptune in 1846, and Pluto in 1930. The number of known natural satellites (moons) continues to increase as unmanned probes fly by the outer planets. A spacecraft discovered rings around Jupiter and new moons around Jupiter, Saturn, and Uranus.

Astronomers have discovered 28 stars with planets orbiting them (extrasolar planets) and 10 more stars surrounded by disks of dust that may form planets. Before these discoveries, most astronomers believed that the chance of finding planets around stars like our Sun was very slim. Many astronomers now think that solar systems are relatively common.