In 1997 an asteorid was found that shares the same orbit around the Sun as us! 3753 Cruithne was suddenly announced as our second moon - but it is in no real way a moon of the Earth.
A real moon (or satellite) travels around (or orbits) a planet - a bit like this diagram shows
What do you think makes something a moon? Probably a big round rocky thing that goes around a planet? The Moon we see in the sky is held in orbit by the Earth's gravity and travels around us once each month - absolutely a true satellite. Now let's look at a typical orbit of Cruithne, as shown here...
This diagram is a view from above the solar system. The Sun is the dot in the centre, the pinkish circle is the orbit of Mercury, the green is Venus, the blue is us, the Earth and red shows the orbit of Mars. And there is the yellow orbit of asteroid Cruithne - it's a massive track that goes out as far as Mars and in as far as Mercury! However, the single most important thing to notice is that Cruithne's orbit is NOT around the Earth. Another fact is that if you watched over time this yellow line would wander to the right and to the left along the orbit of the Earth once every 385 years! At times the orbit brings Cruithne underneath the south pole of the Earth (40 times further away than the Moon), and at other times it can be on the other side of the Sun. It's all terribly complicated and my calculator blew up last time I tried to work out where Cruithne actually was! Needless to say you cannot call Cruithne second a moon of the Earth just because it shares the same orbit as the Earth. If it went around us then it's a different story. So what can we call this strange world? Technically and correctly Cruithne is (sorry about the tech-speak) a Co-orbital Near-Earth Asteroid. But if we call it a Co-Orbital Near and Crossing Earth Asteroid, then it becomes a CONCA (pronounced Con-ker), and if that gets picked up by big sceintist people then I've just invented a new world-famous space mnemonic! And by the way there are now two more asteroids which travel in the same way as Cruithne, called excitingly 1998 UP1 and 2000 PH5 - that brings the total to 3 CONCAs so far. That, as they say, is that.
Images and information have been adapted from Paul Wiegert's excellent article Near-Earth asteroid 3753 Cruithne --Earth's curious companion--. Paul is from Queen's University Astronomy Research Group at the Dept of Physics, Queen's University, Kingston, Ontario K7L 3N6 CANADA
A Final Note: Some strange people who think they know better (and should be avoided) use the Sun as their viewpoint to show that the Moon doesn't actually orbit the Earth. If anyone says such silliness to you, then tell them from the viewpoint of the centre of the Galaxy, the Earth doesn't orbit the Sun either! And from the viewpoint of the Andromeda Galaxy, the Sun doesn't travel around the Galaxy! etc. etc. Honestly! |
