Eratosthenes

276–195 B.C.

This Greek astronomer was the first to measure the size of Earth accurately. He determined that the earth's polar diameter was about 7,850 miles. (In fact, the distance is actually 7,900.)

Claudius Ptolemaeus (Ptolemy)

A.D. 120–189

The ancient Greek astronomer Ptolemy theorized that Earth was the center of the universe, and the Sun, Moon, planets, and stars revolved around it. Called the Ptolemaic System, Ptolemy's vision of the universe was accepted for the next 1,500 years.

Nicolaus Copernicus

1473–1543

The Polish astronomer Copernicus was the first to theorize that Earth orbits the Sun. This meant the Sun and not Earth was the center of our universe—a controversial idea that was at first strongly denounced. This sun-centered theory is called the heliocentric theory (helios means Sun in Greek).

Johannes Kepler

1571–1630

The German astronomer Kepler discovered that the orbits of the planets are elliptical (oval) rather than round. Before Kepler's discovery, astronomers thought the orbits of all the planets were perfect circles.

Galileo Galilei

1564–1642

Galileo is considered the first astronomer to use the telescope. He discovered four of Jupiter's satellites and the craters on Earth's moon. The telescope also allowed Galileo to verify Copernicus's theory that the planets circle the Sun (the heliocentric theory), and not Earth (the Ptolemaic theory). The idea that the Sun was at the center of our universe was so revolutionary that in 1616 Galileo was condemned by the Inquisition and forbidden to discuss Copernicus's theories.

Sir Isaac Newton

1643–1727

This British astronomer is best remembered for discovering the principle of gravity. Legend has it that, seeing an apple fall from a tree, Newton concluded that a force, gravity, pulled it to the ground. He then used his theory of gravity to explain how the moon is held in its orbit around Earth.

Newton's theory of universal gravitation states that every particle of matter (anything that takes up space) attracts every other particle of matter. The force becomes weaker as the distance between particles is increased.

Edmond Halley

1656–1742

This British astronomer was the first to calculate the orbit of a comet. The comet was later named after him. Halley's comet passes close enough to Earth to be seen about every 76 years. It was last seen in 1986 and should return in 2061.

Sir William Herschel and Sir John Herschel

1738–1822 and 1792–1871

These English astronomers, father and son, are the first to have conducted a thorough study of stars and nebulae. William Herschel identified more than 800 double stars (pairs of stars in orbit around a common center of gravity) and about 2,500 nebulae. Only about 100 nebulae had been discovered before him. His son, John Herschel, moved to South Africa for a number of years in order to catalog the stars in the Southern Hemisphere, which were not visible in England. He also discovered many new double stars and nebulae.

Edwin Hubble

1889–1953

The American astronomer Edwin Hubble classified the different types of galaxies in the universe and developed the theory that the universe is expanding, which is called Hubble's Law. The Hubble space telescope, which was carried aboard the space shuttle in 1990, is named in his honor.

Hubble Space Telescope