The Earth’s Second Moon – Cruithne

I never knew that the Earth had another moon apart from The Moon. But what is this new moon that has decided to steal our already (beautiful) moon of its honour?

If you believed till now that the Earth’s only moon was the Moon, tough luck, you are wrong! Yes, when I found out about Cruithne, I had NO idea that Earth had a second moon. Now is the time when I shall delve further into this mystical moon that hardly any members of the planet have heard about.

Cruithne or 3753 Cruithne is also given other (quite scientific) names like 1983 UH and 1986 TO. Discovered on the 10th of October 1986, Duncan Waldron in Australia, was the man responsible to locate this quasi satellite. A quasi satellite can be very simply described as an object in space that follows the orbit of its planet and is related to this planet when it comes to gravity and the other complexities but is somehow very eccentric in terms of orbiting. Then why is it that after it’s been discovered, we still can’t see it? It wasn’t till 1997 that it was discovered that Cruithne had an orbit way different than that of the Earth. This highly eccentric orbit is better described as a horse shoe kind of orbit.

Here are a few vital stats about Cruithne:

  • Diameter: 5 km or 3 miles
  • Closest approach to the Earth: 30 times the distance that is between the Earth and the Moon
  • Mass: 130,000,000,000,000kg
  • Years to complete orbiting around the Earth: 770 years
  • Time taken to revolve around sun: About 364 days
  • Temperature: 275 Kelvin (About 2 degree Celsius)

So is there any worry of Cruithne and Earth clashing? Looks like not in a long (and I mean LONG) time because the orbit of Cruithne dictates that it follows the Earth in a synchronised way even though scientists claim that Cruithne’s orbit isn’t stable.  

What is this about the eccentric orbit? Well, to the sun, Cruithne is an elliptic orbit (which is similar to that of the Earth) which means that it feels like it follows the Earth. But to the Earth, Cruithne looks like it follows a kidney bean/horse shoe kind of orbit because the orbital speed and distance of Cruithne to the Sun is a lot more different than that of the Earths. Because the Earth revolves around the Sun in 365 days and Cruithne revolves in 364, it means that in the long run, it is going to be the Earth that ends up following Cruithne because Cruithne is quicker than the Earth when it comes to revolutions. The process is then reversed and it will feel like Cruithne is catching up with the Earth and so on and so forth. Eventually (with a bunch of complex science descriptions that I didn’t understand) because of the speed and time difference, the horse shoe orbit and the Earth will pull away from each other. This happens because there are gravitational energy exchanges that occur every single time Cruithne and Earth come very close to each other.

Well, enough of the boring sciency bits, now with the more interesting parts. When can you next see Cruithne? Cruithne’s last series of close interactions with the Earth happened in 1902 which isn’t really that long back. The next time there will be a set of close meetings will be in the July of 2292 where the Earth greets Cruithne at a (very tiny) 12.5 million km. Cruithne again meets Earth in what seems like every 380 to 390 years. Shame we can never see it, eh?

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18 Responses to “The Earth’s Second Moon – Cruithne”
  1. Littlekid137 Says...

    On August 20, 2009 at 11:14 am

    Interesting. But according to your picture it does not go completly around the Earth. So is it still considered a satellite or moon? I understand that it is held near Earth by gravity but it does not seem like a moon. Still a very good researched paper and very interesting to read, Nice job!

  2. Gourab Modak Says...

    On August 20, 2009 at 11:15 am

    Nice article. I never knew about Cruithne.

  3. Christine Ramsay Says...

    On August 20, 2009 at 1:43 pm

    An interesting piece. I certainly hadn’t heard of another moon orbiting Earth.


  4. Ramalingam Says...

    On August 20, 2009 at 9:26 pm

    I have no idea about Cruithne.Quite informative article.Thank you

  5. chitragopi Says...

    On August 20, 2009 at 9:40 pm

    Rare information. I am sure many of us do not know of this second moon. Thank u for sharing.

  6. tingh Says...

    On August 20, 2009 at 9:48 pm

    Wow never knew that before. Great article. But one question, according the moving diagram, it really isn’t ORBITING Earth, at least it doesn’t look like it to me, cuz the orbit it out side the earth isn’t it?

  7. Leon9972 Says...

    On August 20, 2009 at 11:01 pm

    There is no gravity in space.

  8. Aldrin A Wilding West Says...

    On August 21, 2009 at 2:17 am

    A well written and well presented piece Atikin. I enjoyed this.

    Re the comments above. No, Cruithne doesn’t orbit the Earth in the normal sense that we think of an orbit, but its orbit orbits the earth and so factually Cruithne does orbit the earth too.

    No matter. As someone with a somewhat obscure allegiance* to our better known moon, Luna, I will be watching Cruithne closely to see she doesn’t try and steal the limelight !

    *see this link for explanation –

    Buzz :)

  9. ducroisjosef Says...

    On August 21, 2009 at 3:18 am

    That was great, thank you.

  10. Lucas Dié Says...

    On August 21, 2009 at 4:21 am

    Fascinating piece of information.

    It all depends on the definitions, I suppose, when a moon is a moon and when space debris :)

  11. Lee Says...

    On August 21, 2009 at 5:54 am

    Very nice information. I never know that Earth has a second moon.

    Share this great post at: to gain more views and readers. :)

  12. Nikita K Says...

    On August 21, 2009 at 7:19 am

    HEY: This is from the writer. Just to clear stuff up.

    The moon is a natural sattelite and Cruithne is a quasi-satellite so Cruithne is a second moon too. If you think Cruithne isn’t a moon then it’s up to you but contradict Stephen Fry because its was out of his mouth that I heard that Cruithne is a ‘moon’ so I decided to research it.

    Also, the diagram there is a little hard to understand but actually the horse shoe yellow orbit moves around the blue orbit which is the Earths. If you look carefully at the blue orbit, it is wobbling slightly signifying that the yellow orbit moves around it.

    Hope I’ve clarified stuff up.

  13. Lostash Says...

    On August 22, 2009 at 5:15 pm

    I’m going to investigate this further…..most interesting! I have a feeling this may be a stray asteroid of some sort though?

  14. Lostash Says...

    On August 22, 2009 at 5:18 pm

  15. Adam Henry Sears Says...

    On August 22, 2009 at 5:24 pm

    This is great work, Atikin. I never knew that. Now I do. Thanks for sharing and have a good day.

  16. Jason Kuisel Says...

    On May 18, 2010 at 8:42 am

    After reading this article, the term ‘Moon” didn’t sit very well with me, in an article on wiki it is defined as follows, which seems like a much better fit. Cruithne is an asteroid in orbit around the Sun in 1:1 orbital resonance with that of the Earth. It is a periodic inclusion planetoid orbiting the Sun in an apparent horseshoe orbit.[2] It has been called “Earth’s second moon”, although it is only a quasi-satellite.[2]

  17. Peter Marshall Says...

    On December 28, 2010 at 1:50 am

    So there is no gravity in space, Leon9972?

    Then I wonder what it is that keeps the Earth in orbit around the Sun, and the Moon in orbit around the Earth (not to mention all of those artificial satellites launched by humans)?

  18. mansi Says...

    On August 2, 2011 at 8:23 am

    it is wonderful thing. i am so excited to see cruithne(second moon).

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