Discover the cosmos! Each day we feature a different image or photograph of our fascinating universe, along with a brief explanation written by a professional astronomer.
September 14, 1995
The Far Side
Credit: The Soviet Lunar Program
Explanation: This historic picture was humanity's first glimpse of the far side of the Moon. It was taken by the Soviet spacecraft Luna 3 in October of 1959. Luna 3 followed closely on the heels of another Soviet probe, Luna 2, which had become the first spacecraft to impact the Moon on September 13th of that same year.
Why does the Moon have a far side? Gravitational tidal forces within the Earth-Moon system have synchronized the Moon's period of rotation around its axis with its orbital period at about 28 days. So, as the Moon moves around its orbit its rotation exactly compensates, keeping the same face toward the Earth.
Tomorrow's picture: Space Station Mir Over Earth