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
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.
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.
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 third quarter 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.
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
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.
7.
The Gravity of the Situation: The Moon and the Tides
“There is a tide in the affairs of men…”
If I had a nickel for every time I am asked about tides… I’d have a
There are a lot of misconceptions about tides. Anyone who has spent a day at the beach knows about tides; the difference between high and low tide can be substantial. But the details of tides can be a bit weird. For example, there are roughly two high tides and two low tides a day. I get questions about this all the time. Most people have heard that the Moon’s gravity causes tides, so why are there two high tides each day? Shouldn’t there be only one high tide, when the Moon is overhead, with a low tide on the opposite side of the Earth?
When I wrote a web page about tides, and again while researching them for this chapter, I couldn’t find a single source that made any sense. Different web pages and different books all had different explanations. Some made sense for a while, then said something clearly wrong. Others started off wrong and got worse from there. Most are close, showing that the explanation relies on several different factors. What’s worse: I wrote a draft of
