Read Frosty Drew Observatory and Science Center's Update on SARS-CoV-2 / Coronavirus Disease 2019 and our Reopening Plan. Updated: August 5, 2020

Earth's Strange Companion

Everyone can tell you the name of Earth's satellite - it's the Moon. However I doubt one person in a million could name Earth's companion - Cruithne [pronounced croo-EEN-ya]. Until 1997 no one knew anything about Cruithne until an astronomer named Paul Wiegart examined the orbital data for an asteroid numbered 3753. He was amazed to discover that Cruithne shares the same orbit as the Earth and the Moon. Cruithne is in a solar orbit rather than a terrestrial orbit so it isn't a satellite. Yet Cruithne is dominated by Earth's gravity placing it a special class of objects called companions.

Cruithne's orbit is stretched out into an elliptical shape tilted steeply to the Earth's plane. From the vantage of the celestial north pole the tilt cancels out the ellipse making Cruithne's orbit appear almost circular and a good fit to the Earth's orbit. No one ever considered the possibility that such an elongated tilted orbit could operate in synchrony with Earth's relatively circular orbit. It didn't seem possible that a stable configuration could emerge.

This diagram supposes that we are observing from the direction of the north celestial pole turning once a year so that to us the Earth - Sun line is fixed. As Cruithne overtakes the Earth, Earth's gravity slingshots Cruithne into a wider slower orbit. Cruithne then begins to lose ground to the Earth. Eventually Cruithne loses a total revolution and Earth begins to overtake Cruithne from behind. Now the gravitational slingshot works the other way causing Cruithne to enter a closer faster orbit. Now Cruithne begins to gain ground on the Earth. From the viewpoint of someone on Earth, every time Cruithne gets close, it reverses direction and pulls away from Earth. The resulting orbit is shaped like a horseshoe or a ring with a section missing.

Cruithne passes the Earth by traveling under our South Pole. At Cruithne's closest approach to Earth, northern hemisphere inhabitants can't see it. Even our friends below the Equator don't get much of a view since Cruithne looks like a Block Island sized lump of coal. Its surface is quite dark. Cruithne never get brighter than the 15th magnitude [4000 times too faint to be seen by an unaided eye]. Even in large telescopes it is only a dim point of light.

Earth is the only planet with a co-orbital companion, but we aren't quite unique in the solar system. Two of Saturn's moons are also co-orbital companions with Janus acting as Earth and Epimetheus as Cruithne.

Cruithne's average orbit is lightly inside of Earth.This means it circles the Sun ever so slightly faster than the Earth. The difference is only about one quarter of a percent leading to a long term cycle of almost 400 years between each closet approach. There is no likelihood that Cruithne will hit the Earth. Its aversion to our planet keeps it at least 40 times as far away as the Moon from us.

The long planetless sky drought is beginning to pass. Venus is now low in our western sky, and we have the three outermost planet in our sky every evening. Unfortunately, none of these planets has spectacular surfaces like Jupiter, Saturn or Mars. However both Uranus and Neptune do have moons which are interesting to see and Venus shows phases which look like the Moon. The center of our galaxy is up these nights and we can show you dozens of wonders in that direction.

Leslie Coleman
Leslie Coleman
Entry Date:
Jul 1, 2000
Published Under:
Leslie Coleman's Columns
Subscribe to Leslie Coleman's Columns RSS Feed