Tonight’s sky holds a half-moon that looks somewhat like the picture on the right. Can you tell me, just by the picture, is the moon getting bigger or getting smaller?
Of course the moon isn’t really changing size. We use the shorthand “bigger” and “smaller” to refer to how much of the moon we see. Or more properly, how much of the moon is illuminated by the sun. Or even more properly, how much of the moon’s surface facing the Earth is illuminated by the sun.
You can see why we often just say it’s getting “bigger” or “smaller!” Or we can use the terms waxing (getting bigger) and waning (getting smaller).
So can you tell, from the picture, is the moon tonight waxing or waning?
Would it help if I added that it was 9 PM, and the moon is in the western part of the sky? How about if I noted that it appeared in the sky as night fell and would set (in the west, of course) at about midnight?
That last hint—what time it sets—is a direct consequence of it being a half-moon and is only true at that point. The first two hints are generally true throughout the current phase, and all three are due to which phase it’s in. (In the opposing phase, the moon rises later at night and fades from the sky as day comes. And the half-moon in that phase rises around midnight.)
The answer is that the moon is waxing. It’s halfway between being a new moon (which is mostly invisible) and a full moon. When it’s halfway through the other half of the cycle—waning from full to new—the illuminated part is on the left.
[I apologize to my Australian and other southern readers. The directions are from the point of view of the northern hemisphere, since most readers are from the USA or Europe. Just know you are not forgotten. I have long been a fan of McArthur’s Universal Corrective Map of the World!]
If you cared about moon phases, you could just memorize two facts: right side means waxing, left side means waning. Or if (like me) you suck at rote facts, it’s possible to figure it out by understanding the dynamics of the Sun–Earth–Moon system. And there’s an odd mnemonic I use to keep it straight which I’ll share later. (Or you can just go to a page, like this or this.)
This sideband is about the dynamics of the Sun–Earth–Moon system. It’s also about the fact that today is the autumnal equinox. The actual equinox moment passed much earlier today (sometime this morning for those of us in the USA), but today is the halfway point between the solstices. Today is one of two days during the year when the day and the night are both (roughly) 12 hours long.
I wrote recently that I mourn summer solstice (the longest day), because it means the days are getting shorter. And I celebrate the winter solstice (along with much of the world), because it means the days are getting longer.
As with the summer solstice, today is also a time to mourn. After today, for the next half-year, the days are shorter and the nights are longer. It won’t be until the spring equinox, the vernal equinox, that the days win once again.
There is one cool thing about the equinox, however. They are the two days during the year when the sun rises dead east and sets dead west. That makes it handy for aligning roads and buildings. (One can use the North Star, but then one must work at night!) If you saw the HBO miniseries, of Ken Follett‘s Pillars of the Earth, you saw Ian McShane using the sun to align the baseline for his cathedral.
The equinoxes are the only times the day/night terminator bisects both poles. That aligns the terminator with the lines of longitude. And that means that, on the terminator (dawn or dusk), the lines of latitude (east and west) line up with the sun.
In another recent post, I wrote that I had something planned for Sideband #42. If you click on the image above, you’ll get a chart I’ve made for you that breaks down the phases of the moon. (You might want to open it in a new window so you have it handy as I explain what the chart shows.)
Basically the chart displays a single cycle of the moon from new, waxing to full and then waning back to new (where the cycle repeats). The full cycle takes roughly 28 days (it’s sidereal period is 27.3 days, it’s synodic period is 29.5 days). The chart has four vertical sections, each representing one-quarter of the cycle.
The moon’s appearance is shown for each phase across the chart. Each image of the moon is labeled (in yellow) below. If you are unfamiliar with the terms gibbous and crescent, they refer to whether more than half or less than half of the moon is illuminated. A gibbous moon is a large moon, a crescent moon is a small one.
Below the yellow labels are some black arrows with orange labels. These show two aspects of the moon’s cycle. The upper one shows the gibbous and crescent phases. The lower one shows the waxing and waning phases.
Above the images of the moon phases, just below the list of days, is a row of diagrams that show the moon’s position relative to the Earth. This is from the point of view of hovering high above the North Pole. The black and white icon representing the moon shows which half is illuminated by the sun. (The sun’s light comes from the top of the chart. A slice of the sun is shown to indicate this.)
The red arrow on the moon icons serve two purposes. The direction of the arrow shows which way the moon is orbiting (counter-clockwise as seen from above the North Pole). The red line divides the moon’s near face (which we see from Earth) from its far face (which we can’t see from Earth). If you can imagine yourself standing on Earth and looking at the moon, the red line shows you the same information as the moon phase images below.
The far face is sometimes incorrectly called the “dark side” of the moon, but as the diagram shows, the far side gets just as much sunlight as the near side.
Finally, at the bottom, two sine waves show similar information to the big black arrows, but in a more precise sense and with different labels. The red line shows the moon’s position in terms of being inside or outside the Earth’s orbit. The cyan line shows the moon’s position in terms of being behind or ahead of the Earth. The inside/outside cycle directly corresponds to the crescent/gibbous cycle, and the behind/ahead cycle directly corresponds to the waxing/waning cycle.
I also created an animated diagram that shows similar information. If you click the image at the top left, you’ll get the animated version.
I mentioned a mnemonic I use to quickly identify which is which, because dry facts, left-right stuff, don’t stick well with me. I remember it like this: The moon chases the sun… and loses. I try to explain it below, but the explanation comes out hideously complicated even though it’s so simple in my head. But for what it’s worth…
Imagine you’re in a building in the center of a round race track. You’re looking out a window, so you can see only a section of the track. Two cars are racing: the Sun car and the Moon car. And, unlike every NASCAR race I’ve ever seen, the cars are going around the track clockwise! Each time you see them pass your window, they zip past going from left to right.
The Moon car is just a bit slower than the Sun car. If you can imagine them pass your window neck-and-neck on lap one, on the next lap the Moon car will be behind the Sun car. On the lap after than it will be further behind. Each lap it will be further behind. Eventually it will be on the opposite side of the track, out of sight behind you.
As the laps continue, the Moon car will be almost an entire lap behind, which means that it will appear to be in front of the Sun car as they pass your window. In one of the laps after that, they will again be neck-and-neck, but the Moon car is actually a full lap behind now. As the race continues, this cycle repeats.
That is exactly what’s happening with the sun and moon, if you imagine that the piece of track you can see out your window is the southern sky. As the sun passes your window (at noon), during a new moon, the moon is in the same part of the southern sky as the sun. (You can’t see it, of course, because it’s noon.)
Each day the moon will be “behind” the sun (to the left) until it’s a quarter “lap” behind. Then it’s a half-moon (waxing). Roughly seven days later it’s a half-lap behind, making it a full moon. It’s on the opposite side of the sky from the sun now. Another seven days, and it’s now a waning half-moon three-quarters behind the sun.
Okay, that was horribly complicated. Maybe someday I’ll make a diagram. In the meantime, as a reward for reading, click the long, skinny image for a large one that shows the distance of the Earth and moon relative to their image sizes.