Sunday, February 27, 2011

How many moons does Earth have?

     I was watching Qi clips on youtube on the BBC channel and as usual I was laughing and learning at the same time, and in the midst of it all a question came up that was so easy I was confused by it. How many moons does Earth have? Someone answered one, as expected, but to my surprise was told he was wrong! Stephen Fry explained that Earth has a second moon called Cruithne that is 3 miles wide and takes 700 years to orbit the earth. I got suspicious and did a little research of my own.

     Earth has 1 moon and 2 co-orbital satellites. As you're probably aware, Earth has 1 moon, but did you know there are 2 additional asteroids locked into co-orbital orbits with Earth? They are called 3753 Cruithne and 2002 AA29.

     3753 Cruithne is 5 km across, and sometimes called Earth's second moon although it clearly orbits the sun more then the Earth. It doesn't actually orbit the Earth, but has a synchronized orbit. It has an orbit that makes it look like it's following the Earth in orbit, but it's actually following the it's own, distinct path around the Sun. It looks to me that it takes 385 years to complete it's orbit and not the said 700. Not only is it not a moon it's not Earth's. Steven Fry lied to me!

     If anything 2002 AA29 is earth's second moon, although it is to small to be a moon. It is only 60 meters across, and makes a horseshoe orbit around the Earth that brings it close to the planet every 95 years. In about 600 years, it will apear to circle Earth in a quasi-satellite orbit. It is constantly chasing us and might hit again! That's right I said again. Some time around 4.5 billion years ago it helped create the moon.

I also came across this in my search: Impact Risk
I couldn't make much sense of it but it looks like we might get hit by an asteroid. 


  1. Fascinating stuff. You have to love QI for the laughing and learning alright.

  2. very interensting. thanks for sharing

  3. Could you imagine if we had an additional smaller moon in orbit also? That would be so awesome! I would never leave my lawn!

  4. Very interesting. I didn't actually know about this specific asteroid. Cool.

    Oh, and don't be too worked up about being hit by one. It actually happens pretty commonly, however they end up breaking and burning up in atmo. and what does hit the ground doesn't do much damage.

  5. Wooo those animations are cool. interesting post.

  6. very interesting, what a shame, those asteroids sure have some ugly names.