Astronomy EssentialsSpace

The moon: 5 myths about our natural satellite

Animation showing moon crossing full Earth with moon far side facing camera.
View larger. | Here’s a unique view of the moon as it moved in front of the sunlit side of Earth in 2015. The Deep Space Climate Observatory (DSCOVR) satellite captured it from about a million miles (1.6 million kilometers) out. This image shows what many call the “dark side” of the moon. But it’s not dark at all. In this image, it’s fully illuminated. Read about the myth that the moon has a dark side, and about more moon myths, below. Image via NASA/ NOAA.

We have full moons, blue moons, Harvest Moons, supermoons and any number of culturally relevant references to the moon. Maybe it’s time to unearth a few moon myths and misconceptions. Have you believed any of these myths?

Myth 1: The moon has a permanent dark side

Most grammar school students know that the moon presents only one face or side to the Earth. This is (roughly) true and gives rise to the idea that there is a permanent dark side of the moon, a thought immortalized in Pink Floyd’s music and elsewhere.

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But the side of the moon that is perpetually turned away from Earth is not darker than the side we see. It is fully illuminated by the sun just as often (lunar daytime), and is in shade just as often (lunar night), as is the familiar Man in the Moon side we see.

The Earth-facing side of the moon gives rise to another misconception that many people share, namely that we see only 50% of the moon from Earth. In fact, only about 41% of the moon’s far side (a much more accurate and preferable term than dark side) is invisible to earthly observers. A diligent observer on Earth can, over time, observe about 59% of the moon’s surface. This is because a phenomenon called libration causes the moon’s viewing angle, relative to Earth, to change slightly over its orbit. Basically, this causes our view of the the moon to shift slightly up-and-down and from side-to-side.

Lunar libration is when we can see a bit further over one limb (edge) or the other. The moon occasionally exposes slightly more of its surface on the eastern or western extreme (depending on the location in the orbit). That’s why, as viewed from Earth, about 59% of the moon’s surface is exposed over the course of the moon’s (roughly) monthly orbit around the Earth.

On black background, beam of light hitting a prism and coming out the other side as a spectrum of colors.
Pink Floyd’s Dark Side of the Moon album cover from 1973. Image via Wikimedia Commons (fair use).

Myth 2: The moon is perfectly round

To the eye, the moon appears round. So it’s natural to assume that it is actually spherical in shape – with every point on its surface equidistant from its center – like a big ball. Not so. The shape of the moon is that of an oblate spheroid, meaning it has the shape of a ball that is slightly flattened.

Look at a photo of Jupiter and you will see a good example of this. The moon exhibits very slight oblateness, but more important is the fact that the “side” of the moon that faces Earth is a bit larger than the side turned away from us. This makes it slightly similar to the shape of a typical bird egg that is larger on one “end” than on other. You might think of it as “gumdrop” shaped. So the moon is not exactly spherical. The deviation is small but real.

Full moon with clear view of dark splotches (maria) and craters.
Near side of the moon as seen through a telescope. The moon looks round, but it isn’t. In a dark sky, the moon looks bright white, but this image captures its true asphalt-gray color. Notice that the moon’s near side has dark “maria” or “seas,” while the moon’s far side lacks these features. Image via Wikimedia Commons (CC BY-SA 3.0).

Myth 3: The moon is bright white

Anyone who has seen a full moon high in a clear sky late at night has a right to believe this. Comparatively speaking, however, the moon is neither particularly bright nor actually white. It appears very bright relative to the dark sky, and ordinarily looks white to the eye. Remember the old-style incandescent light bulbs? Now imagine a 100-watt light bulb located about 150 feet (46 meters) away and shining in an otherwise completely dark night. That is approximately how bright the full moon is. Really.

And the color? Well, as with brightness, color is a subjective thing. The moon emits no light of its own, but rather shines by reflecting sunlight. Sunlight is composed of all colors, but peaks in the yellow-green range of the spectrum. The sun looks white when high in the sky, as does the moon, because of the way our eye-brain connection mixes all the colors together. The moon’s color varies somewhat according to its phase and position in the sky, although this color variation generally is too subtle for human eyes. However, the moon is actually gray rather than pure white, on average much like the well-worn asphalt on most streets.

A half-lit moon on a black sky. There are many small craters and big dark areas on the lit right side.
View at EarthSky Community Photos. | Lorraine Boyd in Glen Falls, New York, captured November’s 1st quarter moon on November 20, 2023. Lorraine wrote: “There’s just something about seeing the moon in the 1st quarter phase that puts a smile on my face.” Thank you, Lorraine! Us, too.

Myth 4: There is no gravity on the moon

But of course the moon does have gravity. The idea that the moon has no gravity is, frankly, so ludicrous that I would not even mention it were it not so prevalent. When shown an image of one of the Apollo astronauts jumping high or seemingly floating across the lunar surface, some of my college students will reply that it is because there is no gravity on the moon. In reality, the force of gravity on the moon is only about 1/6 what it is on Earth, but it is still there.

I think that this moon myth, widespread though it may be, is simply a misunderstanding of what the word gravity means in physics. Every physical body, whether it be the sun, Earth, the moon, a human body or a subatomic particle – everything that has substance – has a gravitational pull.

While the practicality of measuring the weight (the pull of gravity) on tiny objects, such as a grain of sand, can be debated, the force exists and can be calculated. Even photons of light and other forms of energy exhibit gravity. Gravity holds galaxy clusters, galaxies, stars, planets and moons together and/or in orbit about each other. If every physical thing did not exhibit gravity, the universe as we know it could not exist.

Myth 5: The moon raises significant tides in people

There is no question that the moon, or rather its gravity, is the major cause of ocean tides on Earth. The sun’s gravity raises tides, too, by the way, but its effect is smaller. Some folks use the indisputable fact of the moon’s effect on the tides to argue that the moon raises tides in the human body. However, to believe that ocean tides and human tides both are caused by the moon betrays a major misunderstanding about how gravity works to produce ocean tides.

In short, gravity depends on two things: mass and distance. Tides occur only when the two objects involved (say, Earth and the moon) are both of astronomical size (far larger than a human!), and also close (astronomically) in distance. The moon is roughly 30 Earth diameters away from our planet, and roughly 1/80th of the Earth’s mass. Given that, the moon helps raise tides, which on average, are a couple of meters (a few feet) high in the fluid oceans.

If tidal effects were even measurable in the human body, which they aren’t, they would be on the order of a ten-millionth of a meter, or about one-thousandth the thickness of a piece of paper. Those are still tides, you say? Perhaps. But they are far, far smaller tides than are raised within your body when a truck passes you on the highway … or even when another person walks past you on the street.

So while the moon’s gravity can power the tides on Earth, its effect on a human body is utterly inconsequential.

By the way, we often hear people say that nurses in hospitals report an increase in birth rate at times of the month when the moon is full. But studies don’t bear out this correlation. There’s a concise summary of moon/ birth rate studies at Wikipedia. Be sure to click into the references to see that they were published in bona fide science journals, such as the New England Journal of Medicine, American Journal of Obstetrics and Gynecology, and so on.

Of course, the moon does influence us in some ways

The moon can certainly pull on our heartstrings when we see a beautiful crescent bathed in earthshine, or an orange-hued lunar eclipse. But it may have some physical effects on us as well.

Women’s menstrual cycles appear to correlate with the cycle of the moon’s monthly orbit around Earth. If it is a true correlation, and not a coincidence, it’s not yet fully explained.

Another recent study showed that people sleep less leading up to a full moon. Maybe you’ve noticed yourself tossing and turning as the moon nears full phase? Read more: People sleep less before a full moon.

Man standing on a car with hands out to huge glowing full moon.
View at EarthSky Community Photos. | Stojan Stojanovski in Bitola, Macedonia, captured this image on August 30, 2023. Stojan wrote: “My friend shows you the super Blue Moon tonight on the top of the mountain.” Thank you, Stojan!

Bottom line: There are plenty of myths that surround the moon. Here are five of the most popular myths. How many of these myths have you been led to believe?

March 31, 2024
Astronomy Essentials

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