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Moon in motion: how to use a ping-pong perspective to understand lunar phases.

Sometimes my local astronomy club holds a public "star party" when the Moon is in the sky. Sooner or later the matter of lunar phases comes up, and someone in the crowd expresses the idea that the dark part of the Moon must be caused by Earth's shadow. "Nope," a club member will reply. "Earth's shadow only falls on the Moon during a lunar eclipse. The dark part of the lunar disk is simply the night side, which we see more or less of at different times of the month."

And sometimes my club holds a star party when the Moon isn't in the sky. Then someone in the crowd is sure to ask, "Where's the Moon?" I can tell by the puzzled (or even worried) tone of these questions that a lot of people believe the Moon is visible every hour of every night. But few of us really pay much attention to whether the Moon is up or not at the moment. "Actually, if you go out at a random hour on a random night," I respond, "it's more likely that the Moon will not be visible."

Misunderstandings about the Moon's phases and motions are commonplace. The good news, however, is that there are some simple observations you can make yourself to clear up all the confusion.

For starters, go out at the same time on two successive clear evenings when the Moon is in the sky. The best time to try this is a few days after the date of new Moon. What do you notice about the Moon on the second night? Its sunlit area, or phase, looks just a little fuller--the crescent shape looks thicker. But what might surprise you is that on successive nights the Moon has also moved farther east (toward the left) in the sky.

We're all used to seeing the Moon glide westward as the hours go by during the course of a night. This motion is "apparent," not real. What's really happening is that we're being carried eastward by the turning of Earth, which makes the Sun, Moon, stars, and planets all seem to move westward across the sky.

But by going out at the same time each night, which sidesteps the effects of Earth's rotation, you'll see that the Moon moves eastward in the sky from one night to the next, accompanied by a change in its shape. What is this eastward motion? And does it really have some connection with lunar phases?

The answer to the first question is rather exciting, I think. That eastward movement demonstrates the Moon's motion in its orbit around our world. In other words, when you see this shift in the sky on consecutive nights, you're beholding the true movement of Earth's single large natural satellite as it sails smoothly but ponderously through space some 240,000 miles away.

The Ping-Pong Perspective

How about that second question: are the Moon's phases related to this orbital motion? The answer is yes, and there's a simple and fun observation that shows how they're connected. All you'll need is a Ping-Pong ball to simulate the Moon. (Actually, any small white sphere will do--a golf ball even has tiny dimples that could represent lunar craters!)

Head outside about an hour before sunset around the time of a first-quarter Moon. Find the Moon in the southern part of the sky, then hold the ball up at arm's length right beside it. You'll be struck by the fact that the ball shows exactly the same phase as the Moon. Both the ball and the Moon are being illuminated by the Sun from the same direction, and you see them as partly sunlit and partly in shadow, their bright and dark portions mimicking each other perfectly.

If the weather stays clear, repeat this observation on the next several afternoons. Each day the Moon's orbital motion has carried it farther east, and the sunlit portion of its disk has grown larger. Hold your ball up near the Moon, and you'll see that its "phase" has thickened too.

If you want a sneak preview of the Moon's appearance in the days to come, just move the ball in steps farther east. And if you move it all the way over so your arm points low in the eastern sky, the side of the ball that's facing you will be almost completely illuminated--nearly a "full Ball," so to speak. And, sure enough, a day or two before full Moon, the lunar orb hangs low in the eastern sky just before sunset and is almost completely illuminated.

Watching the Moon Move

It's impressive to sweep a Ping-Pong ball eastward across the sky, simulating the two weeks of lunar phases between new and full Moon in a matter of seconds. But there are ways to detect the Moon's orbital motion directly in the course of just an hour or less. The trick is to catch the Moon on a night when it appears quite close to a bright star or planet. Most thrilling is when the Moon starts out just to its west. Over time, orbital motion noticeably closes the already small gap between the Moon and the star or planet in a matter of an hour or minutes.

During these close encounters, the Moon moves relative to the distant starry background by a bit more than one lunar diameter per hour--a rather convenient coincidence! Since the lunar disk appears %[degrees] wide, this means that the Moon glides eastward roughly 12[degrees] in 24 hours--and it makes an entire 360[degrees] circuit of the heavens in just under a month.

From New Moon to Full Moon

Armed with a better understanding of the interplay of the Moon's phases and its orbital motion in the sky, let's try to follow a complete cycle of lunar phases. The most logical place to start is when the Moon is "new," that is, passing roughly between the Earth and Sun. At these times the Moon rises and sets with the Sun, and it's so near the Sun in the sky that it's completely lost in the bright glare. But there's another reason you can't see the new Moon: at such times the entire lunar hemisphere facing us is in shadow and therefore dark.

Since the Moon appears roughly 12[degrees] farther east each night, two nights later the Moon is about 24[degrees] east of the Sun and sets roughly two hours after sunset. That's enough separation to place the Moon just above the brightest part of the twilight glow and render it visible very low in the west or southwest. And what phase will the Moon then show? Having moved slightly to the side of our line of sight to the Sun, the lunar disk displays a thin sliver of sunlight along the edge that's pointing toward the Sun in your sky. Bring out your Ping-Pong ball, hold it out off to the left of the Sun before sunset, and the ball should show the same thin, bright crescent rimming the edge of an otherwise dark "night side."

Night by night, this sliver grows thicker (it's waxing) as the Moon moves farther from our line of sight to the Sun. After about a week the Moon reaches the point in its orbit that makes a 90[degrees] angle with Earth and the Sun. From our perspective the lunar disk appears half sunlit and half dark. We call this phase "first quarter" because the Moon has moved a fourth of the way around in its orbit since it was "new." And because it's 90[degrees] east of the Sun in our sky, a first-quarter Moon can always be found in the south at sunset, and it sets around the middle of the night.

A gibbous Moon is one that's more than half lit but less than fully lit. At each sunset after first-quarter phase, the Moon looks fatter and shifts farther eastward in the sky. In the hours just before sunset, you may see its fat oval fairly low in the southeast or east.

What happens when the Moon has traveled far enough east to be exactly opposite the Sun in the sky? As your Ping-Pong ball shows, the face aimed toward Earth is now completely lit by the Sun. It's a full Moon. And because it's 180[degrees] from the Sun, a full Moon rises around sunset, climbs highest in the sky around midnight, and sets around sunrise.

From Full Moon to New Moon

Inexorably, that brilliant orb keeps moving eastward. In the days after full Moon, it rises about one hour later each night. So if you see a big beautiful Moon along the horizon as you drive home from work some evening, it won't be visible at the same time the next evening because it's 12[degrees] farther east--it won't have risen yet. You won't see the Moon until it rises approximately one hour later.

And the sunlit portion of the disk keeps getting thinner (it's waning) as orbital motion carries the Moon farther away from a direct lineup with the Sun and Earth. You'll see a gibbous oval until about one week after full Moon, when it reaches 90[degrees] west of the Sun. It's now at last-quarter phase, because the Moon is starting the final fourth of its month-long journey around Earth. Unlike at first-quarter phase, when the illuminated portion is on the right, facing west, it's now on the left, facing the Sun that's still hidden below the horizon in the east. Last-quarter Moon rises around the middle of the night and is highest in the south at sunrise.

After last quarter, you'll glimpse the Moon as an ever-thinner crescent edging closer and closer to the Sun in the sky. Your last sight is a wire-thin fingernail of light peeking up low in the east just before morning twilight has grown too strong to see it. A day or two later the Moon, now lost in the solar glare, is again almost perfectly in line between the Sun and Earth. It has returned to new Moon, and another cycle of phases begins. A few days later the Moon, moving ever eastward in its orbit, emerges once more as a slender waxing crescent low in the west at dusk.

That's it--all the information you need to determine the where, when, and why of the Moon's phases. And what about those times when you don't have a Ping-Pong ball handy? I like to test myself by imagining where the Moon should be in the sky and picturing its phase. Try it yourself--and let me know how you do!

Fred Schaaf ( follows the Moon's waxings and wanings from his home in southern New Jersey.
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Author:Schaaf, Fred
Geographic Code:1USA
Date:Jan 1, 2016
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