Ptolemy comes up with a complete model of the motion of the heavens based on orbits and sub-orbits called epicycles, which explains why planets sometimes make strange loops.

Nicolaus Copernicus shows that the earth is not the center of the universe and is not even fixed but is rotating very fast about its own axis.

Galileo Galilei uses his telescope to show that the Milky Way is composed of distant stars and that Venus has phases just like the Moon--proving Copernicus's theory.

William Herschel discovers Uranus, the first new planet to be discovered since ancient times.

John Couch Adams analyzes pertubations of Uranus's orbit and predicts the position of an undiscovered planet within two desgrees. His calculations are repeated by Urbain Leverrier a couple years later.

Johann Galle discovers Neptune using the predictions of Leverrier and Couch Adams.

Grove Karl Gilbert shows that the Moon's large craters are different than those of Earth's volcanic craters and are made by meteor impacts.

Clyde Tombaugh discovers Pluto by comparing two photographs made by a telescope donated to the Lowell Observatory.

Russian Luna II probe is the first successful space mission to another planet, circling the Moon and revealing its dark side for the first time.

The Mariner II probe flies by Venus and shows that the surface of the planet is 700^oF - 750^oF hotter than expected.

Carl Sagan realizes that the high temperatures on the surface of Venus are due to a greenhouse effect, created by the fact that its atmosphere is 97% carbon dioxide (CO₂).

Apollo missions bring back rocks from the Moon, revealing that the Moon's rocks were formed at the same time as the Earth's.

The Pioneer 10 probe becomes the first space probe to encounter Jupiter and reveals a lot of detail about its magnetosphere and radiation belts.

The Pioneer 11 space probe provides the first close-up images of Saturn and a lot of information about its moons and rings.

Voyager I goes past Saturn's moon Titan and reveals that although it is very cold, it has an atmosphere like that of the early Earth. It also discovers 8 new moons of Saturn.

Voyager II flies past Uranus and shows the planet is tilted on its side. It also discovers 10 new moons of Uranus.

Voyager II becomes the first space craft to encounter Neptune and also observes geysers spewing nitrogen gas(N₂) on the surface of Neptune's moon Triton.

The Hubble Space Telescope is launched and once its optics are fixed it provides views of the universe with incredible clarity.

The American Clementine mission provides the most spectacular high-resolution pictures of the Moon's surface ever seen, and provides enough pictures to create a detailed 3D map of the entire surface.

The Mars Pathfinder Sojouner lands on Mars and sends back some really cool live(!) pictures.