We all know and love the moon. We’re so assured that we only have one that we don’t even give it a specific name. It is the brightest object in the night sky, and amateur astronomers take great delight in mapping its craters and seas. To date, it is the only other heavenly body with human footprints. What you might not know is that the moon is not the Earth’s only natural satellite. As recently as 1997, we discovered that another body, 3753 Cruithne, is what’s called a quasi-orbital satellite of Earth. This simply means that Cruithne doesn’t loop around the Earth in a nice ellipse in the same way as the moon, or indeed the artificial satellites we loft into orbit. Instead, Cruithne scuttles around the inner solar system in what’s called a “horseshoe” orbit.
Cruithne’s Orbit
To help understand why it’s called a horseshoe orbit, let’s imagine we’re looking down at the solar system, rotating at the same rate as the Earth goes round the sun. From our viewpoint, the Earth looks stationary. A body on a simple horseshoe orbit around the Earth moves toward it, then turns round and moves away. Once it’s moved so far away it’s approaching Earth from the other side, it turns around and moves away again. Horseshoe orbits are actually quite common for moons in the solar system. Saturn has a couple of moons in this configuration, for instance.
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