What are planets? The planet Venus from space Before there were planets For several hundred million years after the Big Bang, there were no planets, only stars. Planets couldn't form until the [...]
Main sequence stars: A starry sky with the Milky Way galaxy (thanks to Bernd Nies) What is a main sequence star? Nine out of ten stars that you see in the sky [...]
Earth from space: How old is the Earth? When did the Earth form? Like Mercury and Venus and the other planets that go around the Sun, the Earth probably formed a little more than four and a [...]
Kids playing tetherball using centrifugal force Who first explained centrifugal force? When something is going straight, it always keeps going straight unless something else stops it or turns it. (The [...]
This is a sphere. A sphere is a three-dimensional circle. Or you could say that a sphere is the set of all the points that are at the same distance from the center [...]
