Link #161: Does The Moon Have a Sister?


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Link #161: Does The Moon Have a Sister?

Artist's conception of Earth with two moons.
Image credit: Grebenkov/CC BY-SA 3.0

In our last post, we spoke about how in Fiji’s history, there was a tradition which forced a brother to strangle his sister to death because she had lost her husband. Speaking of sisters, it was claimed recently that our moon had a sister too.

In 1999, our moon was discovered to have a sister called Cruithne. However shortly after giving the Moon a sister, astronomers took her away. To find out why read on!

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What Is The Myth Of The Moon Having A Sister?

Horseshoe orbit of Cruithne from the perspective of Earth
Image credit: Jecowa/GFDL.

The myth of the moon having a sister revolves around an object in space called 3753 Cruithne. 3753 Cruithne was discovered before the last century ended in 1999. 3753 Cruithne is actually an asteroid that is about five kilometres wide. When it was discovered, astronomers believed that 3753 Cruithne orbited around the Earth in a path shaped like a horseshoe.

According to estimates of the time, 3753 Cruithne would take approximately 770 years to complete its orbit around Earth. As a result of these misconceptions, astronomers dubbed Cruithne as moon’s sister. However, they were mistaken about the nature of 3753 Cruithne.

Why Does Cruithne Not Qualify as Moon’s Sister?

Orbits of Cruithne and Earth
Image credit: Jecowa/GFDL.

3753 Cruithne isn’t moon’s sister. Moon is a satellite of Earth which means that it orbits around our planet. So, for 3753 Cruithne to be moon’s sister, it needs to orbit around Earth too. That, however, isn’t the case.

Astronomers were mistaken earlier when they claimed that 3753 Cruithne revolved around the Earth in a horseshoe shaped orbit. The truth is that the 3753 Cruithne actually revolves around the sun. Why, then, did the astronomers think that 3753 Cruithne revolved around the Earth?

The reason is that 3753 Cruithne’s orbit around the sun is very similar to Earth’s. The similarity exists with regard to the time 3753 Cruithne takes to complete one orbit around the sun. Both Earth and 3753 Cruithne take about the same time to revolve around the sun but their orbits don’t have the same shape.

This situation means that 3753 Cruithne is actually a quasi–satellite of Earth’s unlike a full-fledged satellite like the moon. In fact, the name of this quasi satellite also reveals that it isn’t a moon because only asteroids have numerical elements to their name.

Could The Moon Have Other Siblings?

Since 3753 Cruithne isn’t moon’s sister and Earth’s second satellite, the big question is if there are some heavenly bodies that qualify. In simple words, on some occasions, Earth does acquire temporary satellites before sending them on their way.

These temporary moons are called mini-moons because they only revolve around the Earth for a short period of time. Bodies in space exercise their gravitational power to capture smaller bodies. Earth acquires mini-moons by plucking them out of their normal paths in space.

However, these mini-moons almost always have weird paths because other objects in space also affect their movements. These include the Sun, other planets, and even our own moon!

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