What is an astronomical unit? why is this unit used to measure distances in the solar system?

One AU is roughly the mean distance from the earth to the moon, more specifically its approximately 93 million miles(150million km). AU is used for measuring the distance of planets and other objects in our solar system. it is used because it provides a convinet way to express and relate distances and objects.

The soviet union beat the us to the first successful launch, with the october 4, 1957, orbiting of sputnik 1, and later beat the us to have the first human in earth orbit, yuri gagarin, on april 12, 1961. the "race" peaked with the july 20, 1969, us landing of the first humans on the moon with apollo 11