(Frequently Asked Questions)
Note: This FAQ is intended for adults and to give teachers some background so that they can better teach lessons about the moon. The FAQ is not intended to be read by elementary-age students since many of the concepts are too abstract for them to understand yet. Adults, if you are confused by the answers below, don't feel bad: these concepts are difficult to explain/learn through words and diagrams! Send us a question and we'll keep trying!
1: Does the moon rotate on an axis like the earth?
2: Does the moon have day and night?
3: Why isn't the moon visible every night (or every day)? (see also #19)
4: Are the phases different in the northern and southern hemispheres?
5: Why don't the phases of the moon happen on the same day each month?
6: So exactly how long does it take the moon to orbit the Earth?
7: If I observe the moon at noon, and then again that evening, would the phase be the same?
8: What causes the moon to shine?
9: Wouldn't the earth block the light from the sun to the moon?
10: Could light from the sun bounce off the earth and hit the moon?
11: Is there a cute way to remember the phase terms? I am always mixing up waxing and waning. Also some people say "no moon" for new moon.
12: Would the earth appear to go through phases for an observer on the moon?
13: Does the moon rise and set at the same time each day?
14: Why isn't the quarter moon called a half moon?
15: If we are seeing a full moon here in the United States, would people in India see the full moon?
16: Why does the Harvest Moon look orange when it rises?
17: Do the times on moon calendars apply to my time zone?
18: If we were at the North Pole, when would we see the moon?
19: Which phases of the moon can we see during the day? (see also #3)
20: Does the moon have a North Pole?
21: Why is the crescent moon at sunset sometimes tilted like a backwards "C", and sometimes flat on its back like a "U"?
22: Why does the Moon wobble?
Answer: Yes it does. Many people have heard that the same side of the moon always faces the earth, and because of this, many think that the moon does not rotate. It does rotate, exactly once for each orbit around the earth. Imagine looking down on the Earth and Moon from above the Earth's north pole. To illustrate this, place two coins on a table-top, one for the earth and one for the moon. Choose a reference point on the "moon" coin and place it closest to the earth (e.g., Lincoln's head on a penny). Now move the "moon" in a circle around the "earth" coin but keep Lincoln's head pointing to the earth. You will see that to accomplish this, you must rotate the penny exactly once for every trip around the earth. See an animated version of the graphic you see to the right. The Moon keeps the same side facing toward the Earth because it is tidally locked.
Answer: Every side on the moon experiences day and night. Since the moon rotates on its axis once each month (see the previous question), any given location on the moon would see a "day" about two weeks long, followed by a "night" of the same length. Your students may make this connection themselves by realizing that the visible face of the moon is darkened (e.g., night) for about half of each month as the moon goes through its phases.
Look at the picture to the right. This amazing photograph was taken by NASA's Galileo spacecraft as it swung by Earth on its way to Jupiter. The photo is one of very few that exist that show the Earth and Moon together in the same picture. It's easy to tell which is the day and night side of Earth. The relationship is the same on the moon, which we're used to seeing in phases. There is a day side and a night side of the Moon too.
Answer: The moon is only visible during part of each month. Whether it's visible during the day or night depends on how "far" the phase is from full or new. The moon orbits the earth once (approximately) each month. As a result, sometimes the moon appears very close to the sun in the sky, and sometimes it is far away from the sun in the sky. When the moon is opposite the sun in the sky (which is when full moon happens), it will rise as the sun sets and set as the sun rises. Therefore, a full moon will be up all night long, but not during the day. The further before or after full moon (in days), the more the moon will be visible during daytime hours (when the sun is in the sky). However, as the moon gets very close to new moon (when the moon and the sun are closest together), it is very difficult or impossible to see in the daytime sky. [The answer to question #19 may also help.]
Answer: The phase of the moon would appear the same no matter where you are on the planet Earth. If an observer in Florida sees a full moon tonight, and observer in Wisconsin will also see a full moon tonight. However, if you traveled from one hemisphere to the other you might notice a difference. Here in the northern hemisphere, we say that when you see a phase that looks like a half-circle, and it's Light on the Left side, it's Last Quarter. The "L's" make a nice mnemonic for keeping First Quarter and Last Quarter straight. So, when it's light on the right it's First Quarter. However, look at the illustration. An observer in the southern hemisphere (Argentina) would see the moon upside-down compared to the way we are accustomed to seeing it (USA). (See also question #15, #17, and our Crescent Moon Around the World page.)
Answer: The phases of the moon do not happen on the same days each month because the moon's orbit around the Earth does not take exactly one month. Actually, there is no such thing as "exactly" a month because our months are different lengths, some with 30 days, some with 31, and even one with 28 or 29 days (depending on whether it's a leap year or not). Calendar makers many years ago originally tried to make our months relate exactly to the phases of the moon, but quickly became frustrated because they would then not be able to have 12 equal months for one year. Unfortunately, 12 full moons do not equal one 365 day year. It's not even close. Calendar makers eventually decided to stick with a solar year of 365 days (with a leap year every fourth year) and to make the months vary between 30, 31, and 28 days. The cycle of the phases, like from full moon to full moon, takes 29 and 1/2 days. Because of this, moon phases can happen anywhere during the months.
Answer: This explanation can be a bit confusing, so have patience. The moon takes 27 1/3 days to orbit the earth once, but the number of days between one full moon and the next full moon is 29 1/2 days. Why are these two "moonths" not the same? The answer has to do with the Earth and moon relationship to the sun. If you chart the moon's position every night for a full month, you could compare its position with a bright star, and then wait to see how long it takes for the moon to come back to its position relative to that star. The answer would be about 27 1/3 days. But during this time, the earth has moved nearly 1/12 of the way in its orbit around the sun. So to get from one full moon to the next would take about two days longer, 29 1/2 days, because the moon would have to travel a little further around the earth to get to "full" again.
Answer: Since it takes nearly an entire month for the moon to go through its full cycle of phases, the change that happens over a period of a few hours is very small. A careful observer with a telescope could note the change, but to somebody looking at the moon with the unaided eye, the phase will appear exactly the same within a period of several hours.
Answer: The moon does not shine by its own light, but by the reflected light of the sun. We see the moon because the sun is shining on it. The surface of the moon is actually very rocky and fairly dark (about the color of the asphalt on most of our city roads). But because it is so close to the earth, it provides enough light at night to cast shadows when it is full or nearly full.
Answer: Many people grow up with the idea that the phases of the moon are caused by the shadow of the earth. The earth does cast a shadow (as does any object in sunlight), but the moon usually passes above or below it. When the moon does pass through the earth's shadow, it is called a lunar eclipse (see illustration at right). When the moon passes into the shadow of the earth, it can at first glance mimic some of the phases, but the effect is in fact very different. A lunar eclipse, when the moon IS in the earth's shadow, can only happen when the moon is full (on the opposite side of the earth from the sun). Click on the illustration to the right to see an animation of a lunar eclipse (41k).
By the way, the shadow of the moon can also hit the earth. When this happens, observers on earth will see either a partial solar eclipse or a total solar eclipse (if they're lucky enough to be in the right place at the right time).
Answer: Reflected light from the earth does hit the moon. This is most noticeable when the moon is in a thin crescent phase. Look at the thin crescent moon at night, and you may notice a dim, ghost-like circle of the full moon. The part that is very dim and ghost-like is shining by sunlight reflecting off the earth, and then off the moon, and back to your eyes. This is called "earthshine" and is relatively common. See the illustration on the right. Since most people observe the crescent moon in the evening just after sunset, this is sometimes called "the new moon in the old moon's arms". The crescent is the young or nearly "new" moon, the earthshine illuminated part is the "old" part. To observe this, look on a Moon calendar for the next new moon, and go out and look for the Moon after sunset for a few days after the new moon.
Answer: A few come to mind. There may be more, but this is what I use: If you are waxing something (a car), you are putting wax on. Since you are adding wax to the car, the car is getting bigger. Therefore, for a waxing moon, the phase is getting larger each night. Another one, a rhyme, goes like this: 'If the light is on the right, it is going to get bright,' meaning that if the lighted part of the moon is on the right, it's waxing, or getting bigger. "No moon" is a pretty good way to remember that at the phase we call the new moon, you can't see the moon at all; however, remember that the moon is still there!
Answer: Yes it would. The phases of the Earth would be the exact opposite of those of the moon. When an observer on earth sees a full moon, an observer on the moon would see a "new earth." That is, from the moon, the earth would seem very close to the sun in the sky, and therefore, all but invisible. As the phase of the moon waxes through crescent, first quarter, and gibbous, the phases of the earth would be waning though gibbous, last quarter, and crescent. You can check out the phase as seen from the space probe Galileo above in question #2.
Answer: No. The moon orbits around the earth about once each month (see questions 5 and 6). Each day, the moon will rise (and set) an average of 50 minutes later than it did the day (or night) before. To find the rise and set times for the moon today, go to TimeAndDate.com's Moonrise and Moonset Calculator and enter your city.
Answer: The terms first quarter and last quarter refer not to the moon's apparent shape but to the points in time within the lunar month. We recognize four main phases, and therefore divide the lunar cycle into 4 parts, or quarters. Starting with new moon, then first quarter, full, then last quarter, then back to new. For each of the 4 main phases, the moon has orbited one quarter of the way around the earth.
Answer: Someone on the opposite side of the earth would see the full moon 12 hours earlier or later than us. Let's say that people in India are seeing a Full Moon at midnight right now. It would be noon here, and since we can't see through the earth, we wouldn't be able to see the moon. We would just have to wait 12 hours until it's midnight here and our part of the earth has rotated around so that we are facing the right direction out in space to see the moon.
Technically, it's only a "Full Moon" for a brief moment when the moon is in the exact opposite direction of the sun. So, most of the time, when we say we are seeing a full moon, we are officially seeing a nearly full (gibbous) moon. (See also question #4 regarding northern and southern hemispheres, question #17 regarding time zones, and our Crescent Moon Around the World page.)
Answer: Actually, it's not just during the Harvest Moon that this happens. You can see this on any full or nearly full moon as the moon is rising. And it's for the same reason that sunsets are red and orange. The sun is shining all of the colors of the rainbow on to the moon, and the moon is reflecting all of those colors pretty evenly.
[See graphics to the right. Click on each image to get a larger image.]
When the moon is low, the reflected sunlight has to travel through more air than when the moon is high in the sky. Colors at the blue and violet end of the rainbow (spectrum) are easily scattered or bent (refracted) by the atmosphere. (That's why the sky is blue: you're seeing blue sunlight scattered in every direction.) So it's the colors at the red and orange end of the spectrum that pass through the atmosphere and into your eyes.
Answer: If you are referring to a paper wall calendar that lists a time for a certain phase, that is the exact moment that the phase occurs. For example, if your calendar says "Full Moon 2:25 PM", that means that at that moment, the Sun and Moon will be 180 degrees apart in space. But that time listed is for a specific time zone: look on the calendar to see which time zone those times listed are for. It's usually the eastern time zone. Online moon calendars tend to use your location to set the calendar for your time zone.
Answer: (short answer) Wow, that's a great question, but a difficult one. The simple, rough answer is that the moon would be up in the sky for about 2 weeks at a time, and then it wouldn't be up in the sky for the next 2 weeks. The long answer, with diagrams
[By the way, if you are reading this page because you are trying to figure out if a photo called "Moon at the Pole.jpg" is authentic, it is NOT. Explanation here.]
Answer: Actually, you can see all of them except for the phases new and full. So, the phases you can see during the day are: waxing crescent, first quarter, waxing gibbous, waning gibbous, last quarter, and waning crescent. For the moon to be visible during the day, it must be up in the sky at the same time as the sun, but not so close to the sun in the sky that you can't see it. The full moon rises at sunset, is up all night, and sets at sunrise, so you can't see a full moon in the daytime. And for a few days closest to new moon, the moon is too close to the sun in the sky, and the phase is too thin for you to see. These changes in phases, and the changes in when you can see the moon in the sky are caused by the fact that the moon orbits around the Earth: so the angle between the moon and the sun in the sky is always changing. [The answer to question #3 may also help.]
|The Moon's north and south
(image created using Starry Night)
Answer: Yes, and no. Yes, the moon has a north pole (geographic north) based on its rotational axis: and no, the moon does not have a north MAGNETIC pole like the Earth does. The geographic north pole is the point in the northern hemisphere where the axis of rotation meets the surface of the moon. It's the same as the Earth's, at 90? north latitude. The Earth's (dipolar) magnetic field, which includes the north magnetic pole, is generated internally by a dynamo created as molten iron flows in the outer core. But the moon's core doesn't appear to be capable of producing a global magnetic field today. The only magnetic fields are localized, and seem to be caused by permanent magnetization of the lunar crust.
Short Answer: This is a tough question and one that even the experts make mistakes on. It's similar to the cause of the seasons but more complicated. The short answer is that it's caused by the fact that the Earth is tilted 23.5 degrees, and that the Earth orbits around the sun. In Madison, Wisconsin (USA), and at similar latitudes, this causes the waxing crescent moon to look like a backward "C" in September, and a "U" in March. [See the images to the right.] This difference has been noted throughout history and has become the subject of folklore and superstitions, such as referring to these as a "wet moon" and a "dry moon", with misguided weather and climate connections. Don't worry too much about which is "wet" and which is "dry", because you can find references each way. Click here to read the long answer, with diagrams
Short Answer: I'm assuming you mean that even though the Moon is tidally locked (see question #1), we see more than 50% of the Moon. This happens for a couple of reasons. First, even though the Moon's rotational (spin) period is the same as its orbital period, the two don't always match up. The spin rate, for our purposes here, is constant. But because the Moon's orbit is elliptical, the Moon's orbital speed varies — fastest while closest, and slowest while farthest. So sometimes the spin is "ahead" and we see a little more of one side, and sometimes the spin is a little "behind", and we see a little more of the other side. Here's a full explanation with great graphics.