5 Misconceptions about the Moon


The Moon holds a special place in our imaginations — and why not? Along with the Sun and the five classical planets, the Moon is one of the few celestial objects whose apparent motion in the night sky can be seen.

You can even see its disk with the naked eye — and unlike the blinding white-hot disk of the Sun or the barely-discernible-to-human-vision disks of Jupiter or Venus, what a disk is the Moon! Throughout history, we’ve stared at the shadows and highlights of the lunar surface and envisioned everything from a man to a magic rabbit.

Mercury
In case you’re wondering, that’s Mercury, not the Moon.

It’s not surprising, therefore, that we’ve built up some misconceptions about the Moon. Here are the five misconceptions about the Moon that I find to be the most prevalent, most fun, most frustrating, or just the most annoying.

#5. The Moon creates a tidal bulge on both sides of the Earth

Most of us have been told that the Moon causes the ocean tides. Our science textbooks probably even contained an image similar to the following:

Tidal field
Tidal fields. Created by Krishnavedala and uploaded to Wikimedia under the CC-BY-SA 3.0 license.

As the caption suggests, though, those two bulges are the tidal field exerted on the Earth by the Moon. If the Earth were just a planet-sized blob of ocean floating in space, the tides might actually look like this. In reality, ocean tides are a little more complicated.

M2 tidal constituent
M2 tidal constituent. Public domain image created by the US government

With things like continents and inlets (not to mention the solid mass of the Earth) to contend with, it’s much harder to find two distinct tidal bulges in the ocean. This came up in a question on the Physics StackExchange. The questioner noted that tide tables from certain ports only a couple hundred kilometers apart were six hours offset — inexplicable if there are only two tidal bulges. The accepted answer does a good job of explaining the complications.

Reality is always messier than the models. That’s why we have the models.

#4. An American flag flies at the Apollo 11 landing site.

Buzz Aldrin salutes the US flag.

No, this isn’t about the stupid Apollo Moon Landing Hoax conspiracy theory. The Apollo 11 crew definitely planted the US flag on the Moon — but it’s probably not still standing.

NASA has a page detailing the expected current condition of the six flags on the Moon. According to Aldrin, that iconic nylon flag was knocked over by the rocket exhaust during the lunar module’s ascent.

If the flag wasn’t destroyed then, it has undoubtedly been bleached white and disintegrated from 45 years of exposure to UV light and extreme temperatures. Most think that the flags planted by the other Apollo missions fared no better.

Aldrin’s account of the flag being knocked over is backed up by images from the Lunar Reconnaissance Orbiter. The LRO imaged all six Apollo landing sites. At the Apollo 16 and 17 landing sites, the images show shadows suggestive of a flag. There is no evidence of a flag flying at the Apollo 11 landing site.

Ironically, there might now be a bleached white flag flying at the site where Gene Cernan and Harrison Schmitt left the Moon for the last time in 1972.

That’s not irony!

#3. The Moon has only two phases, and only comes out at night.

Most people know that the moon goes from New Moon, through a waxing crescent phase to First Quarter, then through a waxing gibbous phase to the Full Moon. Afterward it goes through a waning gibbous phase to Last Quarter, waning crescent, and back to New Moon. All of this happens in the roughly 29.5 days of one lunar month.

Phases of the Moon. Illustration created by Orion 8 and released under the CC-BY-SA 3.0 license.

But from reading literature or watching television, you might think that the Moon is always either full or a crescent. Werewolf attacks will sometimes occur over the course of several consecutive nights. (Or even more strangely, on holidays, even though Halloween, US Thanksgiving, and Christmas cannot all occur during a full moon.) Romance movies always seem to time their dates to the phase of the Moon as well — when was the last time you saw young lovers in the movies gaze into each others’ eyes beneath the light of a silvery waxing gibbous moon?

Likewise, the Moon orbits the Earth, and its rise does not always (or even usually) coincide with sunrise and sunset. Yet a recent country song by Florida Georgia Line included the lyric about meeting a girlfriend “when the Moon comes up and the Sun goes down.” Presumably the two don’t date during the New Moon.

TV Tropes documents this on the Weird Moon trope page. (Warning: TV Tropes will ruin your life.)

#2. The Moon can be a testbed for a manned Mars mission.

Well… only a little. We needed a big honkin’ rocket to reach the Moon, and we’ll similarly need one to reach Mars. Sure, the rocket that carries humans to Mars needs to be a little bigger and a little honkin’er than the Saturn V, but a big honkin’ rocket that can get us to Mars can definitely get us to the Moon.

Delta-Vs for the inner Solar System.

Trouble is that the Moon and Mars are very different, and I don’t mean New York-style pizza vs Chicago-style pizza.

Mars actually has an atmosphere, and though the surface pressure is only 0.6% (that’s zero-decimal-six percent, or less than one percent, for the numerically challenged) that of Earth, it’s still thick enough to perform aerobraking to enter Martian orbit. That saves us having to carry along all the fuel we need to enter orbit, and when dealing with the rocket equation, every kilogram counts.

The Moon has only a negligible atmosphere: Mars’ atmospheric pressure is about 0.006 times that of Earth, whereas the Moon’s atmospheric pressure is around 0.00000000000001 that of Earth. You can’t use that for aerobraking: you would continue past the Moon having lost virtually none of your original velocity. Or you would end up lithobraking.

Once you’re on the surface, the differences are even more pronounced. One of the key points of the Mars Direct plan was in-situ resource utilization — using the natural resources of Mars to supply the mission, rather than having to carry 100% of the mission’s supplies from Earth. The plan is very specific to Mars — a “Moon Direct” plan would focus on the very different resources found on the lunar surface.

Manned missions to the Moon and Mars would also be very different. Future missions to the Moon might be focused on scientific endeavors such as vast farms of solar panels or radio astronomy on the far side of the Moon. Missions to Mars would likely be focused on long-term colonization, and the search for life.

With so little overlap in missions and in local resources, the Moon makes a difficult testbed for Mars.

I may have stirred up a hornet’s nest, so…

Honorable Mention

Before I cover my pick for #1 misconception, here are a few runners-up.

  • “Shoot for the moon. Even if you miss, you’ll land among stars!” No you won’t. If you’re smart, you chose a free return trajectory, so if you miss, you’ll land upon the Earth again. Though the Earth is in space, so it would be just as correct to say “Plop down on the couch. You’re already among the stars!”
  • The Moon doesn’t exist! I’m pretty sure this started out as a parody of the Apollo Hoax conspiracy theory. Not much to say about this, just read and enjoy a laugh.
  • The Moon’s official name is “Luna”… or is it “Selene”? Sci-fi is quite fond of using Luna, the Latin name for the Moon. Selenology is the term for the study of the Moon. But the actual name of the Moon, as used by the IAU in numerous publications is… (ready for this?) “The Moon.”

Hey, it’s no dumber than our planet being named “Earth”.

#1. We never see the dark side of the Moon because it doesn’t rotate.

Pink Floyd notwithstanding, there is no “dark side of the Moon”. The Moon rotates on its axis once every 27 days or so. Some craters near the pole never see the Sun, but that’s hardly the same as having a dark side.

And yes, the Moon does rotate on its axis, once every 27ish days, as mentioned in the last paragraph. I remember a teacher in grade school claiming that we only see one side of the Moon because it doesn’t rotate. Yes, it does. It has what is known as a synchronous rotation — the time it takes to rotate on its axis is synchronized with the time it takes to orbit the Earth factoring out picky details like synodic vs sidereal days etc. — and that is why we only see one side of the Moon.

“But Billy!” I hear some of you shouting. “What that means is that the Moon doesn’t rotate relative to the Earth!”

First of all, my name isn’t Billy.

Second, rotation is not a relative quantity. Position is relative. Velocity is relative. But rotation, like acceleration, is one of those quantities that can be measured as an absolute, without an outside reference. This is a consequence of Newton’s laws: a rotating reference frame will see the appearance of a centrifugal force. Rotation can even be measured, for example, with a Sagnac interferometer.

Conclusion

So there you have it. Five stubborn, annoying, and/or prevalent misconceptions about Earth’s nearest celestial neighbor. Did I miss something? Overlook your favorite Moon misconception? Just completely get something completely, unequivocally wrong? Let me know!

XKCD 123, “Centrifugal Force”
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