Solar System

            The solar system consists of the sun and all the planets and other bodies that revolve around it. It is shaped like a disk with the sun at its center.

            The largest objects orbiting the sun are the nine planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Some planets have moons – natural bodies that revolve around them. Other bodies orbiting the sun include thousands of tiny objects called asteroids and comets – chunks of ice, rock, and dust. The solar system also contains dust and gases left over from when it was formed.

            All the planets and most asteroids revolve around the sun in nearly circular orbits (paths) that are in nearly the same plane. They all move in counterclockwise orbits, as seen from “above.” Some moons have clockwise motion.

Theories about the solar system

            Five planets – Mercury, Venus, Mars, Jupiter, and Saturn – are regularly visible from Earth and have been known since ancient times. Around 300 B.C. some ancient Greek scientists suggested that the sun was the center of the solar system. Until the mid-1500s, however, most people believed that the Earth was the center of the solar system, and that everything else revolved around it. Then, in 1543 the Polish astronomer Nicolaus Copernicus (1473-1543) figured out that the Earth and the other planets traveled around the sun in circular orbits.

            By 1618 further discoveries by the German astronomer Johannes Kepler (1571-1630) led to what became known as Kepler’s three laws of planetary motion. The English scientist Isaac Newton (1642-1727) used a form of math called calculus to show that gravity, the force that holds objects to the Earth, also holds the planets in their orbits round the sun.

            The German astronomer Johannes Bode (1747-1826) discovered a mathematical relationship in the distances of the planets from the sun. In 1781, when the English astronomer William Herschel (1738-1822) discovered Uranus, it fit into Bode’s pattern, as did many asteroids when they were discovered. Astronomers discovered Neptune in 1846 and Pluto in 1930.

Astronomy

Astronomy is older than recorded history. It was first used to figure out when the seasons would change. Today it is used to calculate the age of the universe.

Astronomy is the study of everything in the universe beyond Earth. It is one of the oldest sciences. For thousands of years people have gazed at the sky to try to learn about the stars, the planets, and everything else in the universe.

The night sky changes as the year passes. Starting about 10,000 years ago, people studied the changing positions of the sun and the stars to decide when to plant and harvest crops. The earliest astronomers lived before recorded history. Our first knowledge of ancient astronomy dates from about 2,500 years ago, when Greek astronomers carefully recorded what they had learned. They believed that the Earth was at the center of the universe and that the stars and other planets circled the Earth.

Beginning of modern astronomy

            The Polish scientist Nicolaus Copernicus (1473-1543) was the first person to suggest correctly that the Earth revolved around the sun, not the other way around. The Italian astronomer Galileo Galilei (1564-1642) was the first to use a telescope to study the sky, and he saw things no one else had seen. Although Galileo believed, like Copernicus, that the planets orbited the sun, he could not explain why.

            The person who answered that question was the English mathematician Isaac Newton (1642-1727). Newton discovered that all objects possess a force called gravity that attracts other objects. A large object such as the sun produces enough gravity to hold the planets in orbit around it. Moons orbit planets for the same reason. Because of Newton’s discoveries scientists came to accept the idea that the Earth and other planets revolve around the sun.