The average distance from the Earth to the moon, 238,855 miles, is over 110 times greater than the moon's diameter of 2,159.2 miles. The moon completes its orbit around the Earth in 27 Earth days.
Although observed since ancient times, the first scientific calculations about the moon were made by Galileo Galilei in 1610, the same year that the astronomer discovered the first moons orbiting another planet. In 1959, the first lunar probes, Luna 1 and Luna 2, landed on the surface of the moon. The moon, which controls the Earth's tides, has orbited the Earth for an estimated 4.5 billion years.