Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing "Hoax" - Philip Plait (2002)
Part II. From the Earth to the Moon
Chapter 6. Phase the Nation: The Moon's Changing Face
never know whether to be surprised at the fact that, of the all the topics touched by bad astronomy, the Moon has the longest tally.
I'm surprised because the Moon is probably the most obvious of all astronomical objects. Some might argue the Sun is, but you can never really look right at the Sun. It's always in the corner of your eye but never fully in it.
The Moon is a different story. When the night is dark, and even the crickets have gone to sleep, the full Moon shines down in blazing contrast to the black sky. Even as the thinnest of crescents the Moon commands attention, hanging low in the west after sunset. Whether high in the sky or low near the horizon, it dominates the night.
So it surprises me that there is so much misunderstanding about the Moon. I would think that since it's such a common sight, it would be the best understood.
But perhaps that's naive. After all, the more we know about something, the more room there is to misunderstand it. So it is with the Moon.
Why does the Moon look bigger near the horizon than when it's overhead? Why does it have phases? How does it cause tides? How can it be up during the day? Why does it show only one face to the Earth? Which part is the dark side?
These topics all have some pretty hefty bad astronomy associated with them, and I promise we'll get to all of them. But first things first. The most obvious aspect of the Moon is that it changes. Even the least attentive of sky watchers will notice that sometimes the Moon is a thin crescent and sometimes it's a big fat white disk hanging in the sky. In between those times, it can be half full or partially full. Sometimes it's gone altogether! These shape changes are called the phases of the Moon. What causes them?
A lot of people think it's due to the shadow of the Earth falling on the Moon. The Moon is a big sphere, so when it's almost all the way in the Earth's shadow, the thought goes, the Moon is a crescent. When it's fully out of the shadow, it's full.
That's a clever idea, but incorrect. The Sun is the major source of light in the solar system. That means the Earth's shadow always points away from the Sun. That, in turn, means the Moon can only be in the Earth's shadow when it's on the opposite side of the sky from the Sun. But the Moon can't always be in Earth's shadow, especially when it's near the Sun in the sky. We also know that when the Moon gets directly between the Earth and the Sun we get a total solar eclipse. That's a pretty rare event, yet the Moon's phase changes every night. Clearly, the Earth-shadow theory cannot be correct, and something else must be going on.
So what do we know about the Moon? Well, it's a big ball, and it orbits the Earth once a month. Actually, the word "month" is derived from the same root as the word "Moon." The phases change as the Moon goes around us, which is a clear indication that they must have something to do with the orbit. In science, it's usually best to take stock with what you see before trying to figure out why you're seeing it. So, let's take a look at the phases and start at the start.
New Moon marks the beginning of the lunar cycle of phases, which is why it's called new. When the Moon is new, it's completely dark. This happens when it's near the Sun in the sky. Since the Sun is so bright and the Moon is dark, the new Moon can be very difficult to see. The Islamic month, for example, begins at the time the very earliest new Moon can be spotted, and so the fol lowers of Islam keep very careful records and have keen-eyed observers ready to see it as early as possible.
First quarter is when the Moon is half lit, confusingly enough. It's called first quarter because the Moon is lit like this when it's one-quarter of the way around the Earth from the Sun, roughly one week after new Moon. For people in the northern hemisphere of the Earth, this means the right-hand side of the Moon-the side facing the Sun-is lit and the left-hand side is dark. For people in the southern hemisphere the reverse is true, since, to the view of people in the north, people in the south are upside-down.
A week later, the Moon is full. The whole disk is evenly illuminated. When the Moon is full it's opposite the Sun in the sky, and it rises when the Sun sets.
A week after that, the Moon is at third quarter. Just like when it's at first quarter, the Moon is half lit, and, also like first quarter, the half facing the Sun is lit. This time, though, it's the other half that's lit. From the northern hemisphere, the left half is lit and the right half is dark. Reverse that if you're south of the equator.
Finally, a week later, the Moon is new again, and the cycle repeats. There are also names for the phases of the Moon when it's between these four major ones. As more of the visible part of the Moon becomes lit, we say it is waxing. When the Moon is between new and first quarter, it's still crescent shaped but it's getting fatter, approaching half full. We say the Moon is now a waxing crescent. After it's half full and approaching full, it's in the gibbous phase, or, more accurately, waxing gibbous. After it's full, it starts getting smaller. This is called waning. The Moon is waning gibbous from full to third quarter, and a waning crescent from third quarter to new.
So now we have names for all those shapes. The question remains, why does the Moon go through phases? Now that we've looked at them, we're closer to figuring that out. However, there's one more thing I want you to do. Go get a ping-pong ball or a baseball. Don't have one? That's okay, you can use your imagination.
Imagine that you are holding a white styrofoam ball. This is our model of the Moon. You will be the Earth and, for this demonstration, a lamp across the room will be the Sun. Before we start the demo, let's think about this for a second: when you hold up the ball, half of it will be lit by the lamp and half will be in shadow. That seems obvious, but it's crucial to understanding phases. No matter how you hold the ball, half will always be lit, and half dark. Got it? Okay, let's set the Moon in motion.
Let's start at new Moon. When it's new, the Moon is between the Sun and Earth. Imagine holding the Moon up so that it lines up with the Sun. From your point of view, the Sun is glowing brightly, but the Moon itself is dark. That's because the side of the Moon being lit by the Sun is facing away from the Earth. From the Earth, we only see the side that is not lit by the Sun, so it's dark.
Now move the Moon one-quarter of an orbit around from the Sun. The Sun is off to the right, and so the right-hand side of the Moon is lit. The left-hand side is dark. Remember, half the Moon is always lit by the Sun, but when it's in this part of the orbit, we only see half of that half. We see one quarter lit up.
Now turn so that the Moon is opposite the Sun. With your back to the Sun, you see the entire half of the Moon facing you lit up, and it's full. (Incidentally, that's why photographers like to take portrait shots with the Sun over their shoulder: that way, your face is fully illuminated by the Sun and there are no shadows on it. Of course, you have to squint because the Sun's in your eyes, but that's a sacrifice you make for a good shot.)
Finally, turn so that the Moon is three quarters of the way around in its orbit. The Sun is now off to the left, and the left-hand side of the Moon is lit. Again, of course, really half the Moon is lit, but you see only half of that half. This time, since the Sun is to the left, you see the left half lit up. The right side is in shadow, and it's dark.
That's what causes the phases. It's not the Earth's shadow at all. The Moon has phases because it's a ball, with one half lit by the Sun. Over a month, its position relative to the Sun changes, showing us different parts of it being lit up.
Once you understand this, an interesting side effect can also be seen. For example, at new Moon, the Moon always appears near the Sun in the sky. That means it rises at sunrise and sets at sunset. When the Moon is full, it's opposite the Sun in the sky. It rises at sunset and sets at sunrise. The Moon is like a giant clock in the sky. If the full Moon is high in the sky, it must be near midnight (halfway between sunset and sunrise). If it's getting low in the west, sunrise cannot be far behind.
You can get even fancier with the quarter-Moon phases. The first-quarter Moon is one-quarter of the way around from the Sun, and is high in the sky at sunset (90 degrees away from the Sun). So it rises at noon, and sets at midnight. It's another common misconception that the Moon is only up in the sky at night. When it's at first quarter it can be seen easily in the afternoon sky; the thirdquarter Moon can also be seen in the sky after sunrise, since it sets at noon.
Another obvious feature of the Moon is that its brightness changes with phase. This seems pretty obvious; after all, there is more of it lit up when it's full than when it's half full. You might think that it is twice as bright then.
The Moon's phases are an effect of geometry, and not due to the Earth's shadow on the Moon. In this diagram, the Sun is off to the right. The position of the Moon is shown in the inner circle, while the phase seen by someone on the Earth is shown in the outer circle. The Moon is new when it is closest to the Sun in the sky, and full when it's farthest from the Sun. The other phases happen as the Moon orbits the Earth.
That turns out not to be the case. Like everything else in astronomy, there's more to this story. Careful measurements of the Moon's brightness show that it can be up to ten times brighter when it's full compared to its first quarter.
There are two reasons for this. One is that when the Moon is full, the Sun is shining straight down on it from our viewpoint. When the Sun is directly overhead here on Earth, there are no shadows, and when it's low in the sky shadows are long. The same is true for the Moon. There are no shadows on the surface when the Moon is full. When it's at first quarter there are lots of shadows, which darken the surface, making the Moon look less bright overall. When the Moon is full, those shadows aren't there, and so it has more than twice as much lit surface from our view than when it's at first quarter.
The other reason has to do with the Moon's surface. Meteorite impacts, ultraviolet radiation from the Sun, and the violent temperature changes from day to night on the Moon have eroded the top centimeter or so of the lunar surface. The resulting powder is extremely fine, like well-ground flour. This powder has a peculiar property: it tends to reflect light directly back to the source. Most objects scatter light every which way, but this weird soil on the Moon focuses much of the light back toward the source. This effect is called back-scatter.
When the Moon is half full, the Sun is off to the side as seen by us. That means the lunar soil tends to reflect that light back toward the Sun, away from us. When the Moon is full, the Sun is directly behind us. Sunlight that hits the Moon gets reflected, preferentially back to the Sun, but we are in that same direction. It's as if the Moon is focusing light in our direction. This effect, together with the lack of shadows, makes the full Moon much brighter than you might expect.
Even the new Moon can be brighter than you expect. Normally, the new Moon is dark and difficult to spot. But sometimes, just after sunset, you can see the crescent Moon low in the sky. If you look carefully, sometimes you can see what looks like the outline of the rest of the Moon, even though it's dark.
Your eyes aren't playing tricks on you. This effect is called earthshine. From the Moon, the Earth goes through phases, too. They are opposite the Moon's phases, so when the Moon is full as seen from the Earth, the Earth would be new as seen from the Moon, and so on. The Earth is physically bigger than the Moon, and it also reflects light more efficiently. The full Earth as seen from the Moon would look many times brighter to you than the full Moon does on the Earth.
This brightly lit Earth illuminates the new Moon pretty well, faintly lighting what would normally be the dark part of the Moon's surface. If you look through a telescope or a pair of binoculars, there's even enough light to spot craters on the surface. The effect is even more amplified if the lit side of the Earth is covered by clouds, making the Earth an even better reflector of sunlight.
Earthshine is a pretty name for this, but there's an even more poetic one: it's called "The old Moon in the new Moon's arms."
The phases of the Moon are both more complicated and more subtle than you might have thought. If you had any misconceptions about them before reading this section, let's hope it was just a phase.