Earth’s second moon?

There is an asteroid in elliptical orbit around the Sun that would seem to mimic Earth’s revolution. Since observation showed it to be following our planet it was dubbed Earth’s second moon.

Formally introduced as 3753 Cruithne, it has been further defined to be an Aten asteroid. (An Aten asteroid belongs to a group of near Earth asteroids, the first of which was discovered in 1976. They are described as having semi-major axes of less than 1 astronomical unit. Asteroids that have an orbit completely with in Earth’s orbit would be an Apoheles. - There are 13 known Apoheles and 815 Aten asteroids.)

It is a minor planet in solar orbit; an orbit that goes from inside the orbit of Mercury and outside the orbit of Mars. It has a horseshoe-like shaped movement around the Earth and completes it within a smooth 770 years.

Cruithne was discovered on October 10, 1986 by Duncan Waldron through a photography plate taken with the UK Schmidt Telescope in Australia. The strange orbit was not discovered for almost 11 more years. 

Although sometimes called Earth’s second moon, it maintains a normal elliptic around the Sun yet with a period of revolution of 364 days - almost equal to that of Earth.BUT for the record, it is not a moon and does not orbit the Earth. It migrates away from Earth and then approaches Earth again from the opposite side alternating about every 380-390 years or so.

Not an Earth shattering existence, but a curious one none the less. It has even made an appearance in Sci Fi, playing a major role in Stephen Baxter’s novel “Manifold: Time”.