Astronomy Notes

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jan 07, 2012

Little-known facts about the Moon

Moon illuminated by reflected Earth light
Moon illuminated by reflected Earth light

1. There is a dark side to the Moon.
It's a common misconception that the “dark side” of the Moon (that is, the side facing away from the Earth) is so named because it's not visible from the Earth—in other words, just a metaphor. Of course, we all know the far side of the Moon isn't always dark. But there's a very good reason why we call it the dark side.

About 59% of the Moon's surface is visible from the Earth at one time or another. This means that 41% of the Moon's surface is never in complete darkness, because for that part of the Moon the Earth is always in the sky, every minute of the day and night. During a new Moon, when the near side is facing away from the Sun, it still receives a lot of reflected light from the Earth. This can be seen clearly in the photo at right. This photo was taken near dusk. The dark blue areas are being illuminated entirely by light from the Earth. The only time the near side gets really dark is during a lunar eclipse. Even then, there's enough light from the Earth to make it visible from Earth through a telescope.

On the other hand, the far side of the Moon can get really dark, some of the time. No light from the Earth ever reaches it. During a full Moon, when it's facing away from the Sun, the far side of the Moon is very dark indeed. At full moon, no light, heat, or radio waves from the Sun (or the Earth) can reach it. So it really does make sense to call it the “dark side of the Moon.”

2. Is there an earthrise on the Moon?

Even though one side of the Moon always faces us, the moon's rotation is not perfect. About 59% of the Moon can be seen from Earth at one time or another. The two slivers (4½% on each side) are the only portions that ever see Earthrises and Earthsets. In these regions, the Earth barely rises above the horizon. The remaining regions either see the Earth fixed in the sky, or not at all.

Since the Earth's rotation is slowing down, eventually the Earth will be tidally locked with the Moon. This means that some parts of the Earth will see the Moon continuously, while other parts will never see it. All the astronomers will, of course, move to the part that never sees the Moon (assuming the Earth hasn't been burned up by the sun by this time). The combined weight of their equipment will upset the Earth's balance and cause the Earth to tumble aimlessly into space ... just kidding.

3. How much pressure does sunlight exert on the Moon?

There's tons of light on the Moon—literally. I'm talking about radiation pressure. All light exerts a physical pressure on whatever it illuminates. On a reflecting surface, sunlight exerts 9.15 micronewtons of force per square meter. If the light is absorbed, the force is half that, or 4.575 µN/m2. Multiplied by the total area facing the sun, and treating the moon as a flat, light-absorbing disk, this means the total radiation pressure is (very roughly) 43,175,000 newtons, or 4880 tons of pressure (measured as Earth tons). Since the Moon weighs 8.082×1019 tons, this is only 0.06 quadrillionths of (6e−17) of the Moon's mass, not enough to have any noticeable effect.

4. How long is the day on the Moon?

A day on the Moon, the time between one sunrise and the next, is 29.53 Earth days, on average (it varies a bit). This is the same as the cycle of phases of the Moon, which is known as a synodic month.

A “month” on the Moon, for that portion of the Moon that experiences Earthrises and Earthsets, is also 29.53 Earth days. A sidereal month, the time to reach the same position relative to the stars (known officially as the International Celestial Reference Frame), is 27.3217 days. Therefore, the Moon rotates around the Earth 12.36 times in one solar year and 13.37 times in one sidereal year. The speed of the Earth around the Sun is about 30 times the speed of the Moon around the Earth.

As for a year: as Bill Clinton would say, it depends on what “year” is. If by “year” you mean the time to make a complete orbit around whatever it's circling around, it would be 29.53 days. But that's misleading, because physically there's no fundamental difference between the Moon circling the Earth and vice versa. Both circle each other (technically, they circle around their center of mass). A “year” on the Moon, that is, the time it takes to circle the Sun, is about the same as one Earth year.

5. How bright is it on the Moon when the Earth is full?

The albedo of the Moon is 0.12. In other words, the Moon reflects 12% of the light that hits it. This means the Moon as viewed from the Earth is about as bright as an asphalt road during the daytime. This does not mean, however, that the Moon is 12% as bright as the Sun.

The albedo of the Earth is 0.367. So not only is the Earth bigger in the sky, it reflects over three times as much light per unit area. On the Moon, the Earth appears 3.67 times bigger in width, or 13.46 times bigger in area, than the Moon appears on Earth. Therefore, the Earth seen from the Moon is 13.46×0.367/0.12 = 41.2 times brighter than the Moon seen from the Earth.

How does this compare to the sun? The Sun has a visual (apparent) magnitude of −26.74. Earth's visual magnitude, as seen from the Sun, is −3.86 [1,4]. The Moon's visual magnitude from the Sun is +0.21. The Moon's apparent magnitude on the Earth is −12.74.

We can use the relationship that says one magnitude is a 2.512× increase in brightness. This is a logarithmic scale, so we use the formula Magnitude = −2.5 log10(f1/f2). From this formula, we can calculate that the Sun is 398,359 times brighter than the Moon as viewed from the Earth, but only 9,673 times brighter than the Earth when viewed from the Moon.

6. What kind of atmosphere is on the Moon?

Because it reflects less light, and absorbs three times as much light per unit area as the Earth, the Moon is (on average) 16°C warmer than the Earth. This surprises many people, because we all know the Moon can get very cold indeed when it gets dark. Part of the reason is its thin atmosphere. The Moon's atmosphere is only 2×10−12 torr. Since 1 torr = 1.316×10−3 Earth atmospheres, the Moon's atmosphere is 380 trillion times thinner than Earth's.

According to NASA [2], the Moon's atmosphere consists of the following gases:

Gas Percent of total
Helium 25%
Neon-20 25%
Hydrogen (H2) 22%
Argon-40 19%
Neon-22 3.2%
Argon-36 1.2%
Methane 0.6%
Ammonia 0.6%
Carbon dioxide 0.6%

The percentage of carbon dioxide in the atmosphere is actually 17 times higher on the Moon than the percentage on the Earth. The total mass of the Lunar atmosphere is only 25000 kilograms, or 27.5 tons. This means there are 1,011 pounds of CO2 in the lunar atmosphere, compared with 6,952,000,000,000,000 pounds in Earth's atmosphere. But the Moon is rich in helium-3, a light isotope of helium, which is becoming valuable on Earth.

A human adult at rest breathes 7 to 8 liters per minute. This equates to 9 to 10 grams of air. Therefore, in 5 years a person inhales and exhales the same amount of gas (in terms of weight) as the entire atmosphere of the Moon. Put another way, the Earth's 7 billion people would need 271,428 years to breathe all the air on the Earth at least once. If they could breathe on the Moon, they would breathe up its entire atmosphere in 0.0225 seconds.

7. Is there water on the Moon?

Yes, but the quantity is very uncertain. There is spectroscopic evidence for some hydroxyl radicals (·OH). Radar evidence from NASA probes also suggests there might be water. Water could only exist as ice, underground or in permanent shadows, because the low atmospheric pressure would cause it to evaporate, and the strong ultraviolet radiation would split H2O into ·H and ·OH radicals.

8. Is the Moon upside-down in Australia?

No, it is the Australians who are upside-down. The Moon is pointing the correct way.

[4] Milone and Wilson, Solar System Astrophysics, vol. 1.