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How Many Moons Does Earth Have? One? Two? A Brief Explanation

Updated on April 15, 2014

In a Hurry?

Scroll down to Conclusion / Summary at the bottom.

Earth History

Let's start at the beginning. It is the beginning after all.

Way back when, the Earth was just a young-ling, a protoplanet1, an embryo found within protoplanetary discs2.

Protoplanetary discs' consist of dense gas that surround newly made stars, that formed from planetesimals3.

Planetesimals are made up of cosmic dust that collect until becoming approximately one kilometer when they attract other planetesimals that then collide.

It impacted with another protoplanet the size of Mars called Theia4.

Earth
Earth | Source
The Moon - Surface
The Moon - Surface | Source

Moon History

That impact caused debris, which would end up becoming The Moon, the main one we all know and are indifferent towards.

It is believed the debris collated and formed smaller bodies, some caught up in the orbit of Earth and some in that of The Sun.

Two of these, bigger sized bodies collided. This would explain the differences between the two sides of the moon. One side is mainly flat, the other is hilly. So the impact flattened the near side I hear you say. If you didn't say that, please go back and read it out loud. Nope.

As the two were sharing the same orbit and travelling at around the same speed when they collided - the smaller moon just coated the big one with a solid crust creating the mountainous areas.

So that's how two became one.

But How Did One Become Two, and Then One Again?

Cruithne was discovered way back in 1986. Surprising right, even more surprising is that it was considered a second moon until 1997 and yet is still pretty much unheard of. It was considered a moon for so long because it appeared to orbit the earth, but it actually orbits the Sun and is affected by the Earths gravitational pull.

It was only once it's true orbital path was discovered (in 1997) that it was downgraded.

But that's not it for Cruithne. It's believed it will spend about the next 5,000 years in it's current orbit, but then could possibly move into a true orbit with Earth for around 3,000 years - thus finally getting that moon promotion it's been pushing for.

So that's how one became two and then one again.

But How Did One Become Two Again Again?

Now to be honest, the other moon/s we have nowadays are hardly that, they're only one or two metres in diameter, but they are natural satellites - so they orbit us just like our main moon.

Discovered in September 2006, RH120 orbited us until June 2007, it stayed for ten months and made three revolutions which is typical of these bodies and we should have at least one doing so at any given time.

The importance though, isn't necessarily in whether it's a moon or not, it's that we could possibly capture this body and bring it back to earth to study.

It probably won't be too long until the super rich will have toilets made out of moon rock, just because they can.

A toilet. My toilet. Not a moon toilet. An ordinary toilet.
A toilet. My toilet. Not a moon toilet. An ordinary toilet.

Which Is Your Favourite Moon?

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Conclusion / Summery

Bullet Points:

  • The Moon was formed from the debris of Earth's collision with another protoplanet called Theia.
  • The Moon also collided with a smaller body creating the mountain ranges.
  • Cruithne was discovered in 1986 and considered Earth's second moon until 1997.
  • In 1997 it was discovered that Cruithne only appeared to orbit Earth, but actually orbits The Sun.
  • However Cruithne is expected to truly orbit Earth in ~5,000 years, for about 3,000 years.
  • Discovered in 2006, Natural satellite RH120 did orbit Earth alongside The Moon for ten months making three rotations.
  • There is at least one small natural satellite orbiting Earth at all times. These measure approximately one or two metres in diameter.


So there you have it. Who knew the idea of Earth having one moon was so debatable. Well, you do now.

Go forth and spread the knowledge. For me, for you, for Earth and The Moon.

BBC - QI: A bit more about Cruithne [Strong Language]

© 2014 Jamie Marsh

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