Does the Moon Have Its Own Moon?

Updated on November 9, 2017
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Leonard Kelley holds a bachelor's in physics with a minor in mathematics. He loves the academic world and strives to constantly explore it.



William Henry Pickering was one of the first to consider if the moon could have an object orbiting it. In 1887, he wondered if the moon could have captured an asteroid or meteoroid as it had approached Earth. He knew the likelihood such an event was low but also the chances of spotting one from Earth, for it would be challenging because a full moon would make conditions too bright to see a small object, but a new moon would also be a problem because the moonlet could be behind the moon. Clearly, a middle ground was needed, and the US Army decided to have Clyde Tombaugh hunt for it (Baum 106).

Famous for his discovery of dwarf planet Pluto, Clyde utilized Pickering’s work in his hunt. Using Heinrich d’Arrest’s calculations for the max distance a Martian moon could be from Mars (70 arcminutes, as seen from Earth), Pickering calculated the max distance a moonlet could be from the moon, as seen from the Earth, to be 9 degrees and 47 arcminutes, or a total distance of 59,543.73 kilometers (107).

But what about the size? He decided to use some estimation techniques to decide on a reasonable expectation. Using a -25.5-magnitude value of the sun (which is 600,000 times the brightness of the full moon) gives a max magnitude of -11.1 (whose modern value is actually a little brighter, at -12.7). If the moonlet had a diameter of 209 meters, it would reflect 1/275,000,000 the light of the full moon, based off the earlier distance calculations (108).

Now, the question of when the best time to see the moonlet was attacked. As mentioned before, the full moon and new moon are out as options but if the moon was 1/3 full, then the moonlet could be seen at 12th magnitude as it crossed the terminator of the moon’s surface in shadow. The best controlled scenario for this would be an eclipse, for you get the bonus of the moonlet potentially entering and exiting the Earth’s shadow as well. The only condition this doesn’t take into account is if the moonlet is tidally locked on the other side of the moon, for then we would never see it as it orbited the moon at the same rate as the moon spun around us (109).



But getting the proper exposure to record any moonlets would prove to be tricky but if you sync up your camera to move with the moon then the moonlet would appear to be a streak near the moon. And you want to look around 3 degrees to either side of the moon, for anything greater would just appear as a point of light during the exposure. With the techniques in mind, Pickering give sis a go on January 29, 1888 during a lunar eclipse. Using a Bache telescope with Voight lens 20 centimeters in diameter and focal length of 115 centimeters, Pickering was foiled by cloudy skies and unable to gather any reliable data. That is because some plates seemed to show a mystery object that wasn’t in the right area to be a moonlet and also seemed to jump around the sky. Others looked at the plates and decided they were not reliable (110-114).

Jump ahead to the March 10 and September 3, 1895 eclipses. Barnard decided not to track the moon with a mechanism but instead by hand, for it seemed to make his plates less blurry. Though March 10 was a hazy night, September 3 was a clear night and 6 good plates were taken. None showed any satellite to the moon (115).

Pickering even tried as late as 1903 to hunt for a 5th magnitude object, assuming it was about 320 kilometers above the surface of the moon. Despite gathering many photographic plates, the results were a negative. He was forced to conclude that if the moon has a moon, it is likely smaller than 3 meters across its longest dimension (Cheung).

Case Revisited

In 1983, Stanley Keith Duncan pondered the moon’s moon scenario once again and thought about the initial conditions surrounding the moon. It is possible that 3.8 to 4.2 billion years ago, up to 3 little moonlets were orbiting the moon but once they hit the Roche limit, gravitational forces pulled them apart and the pieces of them impacted the moon and formed the maria that we currently see. Most feel that these impact features are a result of comets or asteroids, but that implies a random distribution which Duncan claims is not the case. Instead, we see clusters around the equator. Another piece of evidence is the small magnetic field of the moon. Apollo rocks hint at a prior magnetic field which was twice that of Earth’s but the moon doesn’t have a dynamo effect like we do because of its size. Duncan instead points to impactors not only bringing radioactive materials to strengthen the magnetic field but also change the axis of the fields in rocks near the impactors, which again the Apollo rocks demonstrate. It could also signify the moon’s axis changing because of a large enough impactor by say another moon (Baum 104-5).

Works Cited

Baum, Richard. The Haunted Observatory. Prometheus Books, New York: 2007. Print. 104-15.

Cheung. “The Earth’s Second Moon, 1846-Present.” University of California, Feb. 5, 1998. Web 31 Jan. 2017.

© 2017 Leonard Kelley


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