Everyone has seen the Moon, Earth’s only natural satellite, in the sky going through its phases as it revolves around our planet, but there are also other classes of objects in space that have orbits similar to Earth. Some of these objects revolve around the Sun, but pass very close to the Earth on regular intervals due to an orbital resonance. The result, when viewed from Earth is an oddly-shaped orbit centered on a point near Earth. One of these objects is the asteroid 3753 Cruithne, one of five known quasi-satellites of Earth.
Cruithne is a small asteroid, roughly 5km in diameter, it is never bright in the sky and it requires a 12-inch telescope to view it. During close approaches, Earth’s gravity alters its orbit slightly so Cruithne spends a good deal of time either catching up to the Earth or falling behind it until the next close pass alters its orbit in the exact opposite way. The pattern is stable for hundreds of millions of years.