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Author: Scott Miller, Pennsylvania State University
Editor: Grace L. Deming, University Of Maryland
The goals of this module: After completing this exercise, you should be able to:
Understand why we can see the Moon.
Explain why the Moon appears to go through a cycle of phases.
Determine the order of phases in which the Moon will appear.
Determine the rise and set times of the Moon based on its phase.
In this module you will explore:
The orbit of the Moon around the Earth
How the relative positioning of the Earth, Moon and Sun causes the lunar phases
The rise and set times of the Moon based on the position of the Moon in its orbit
Why you are doing it: Next to the Sun, the Moon is the most obvious object in the sky, yet people don't fully understand its changing appearance. People naturally assume that while the Sun is up during the daytime, the Moon is up at night, but this is not always the case. People also misunderstand exactly why the Moon's appearance changes over time. This activity will help you understand why the position of the Moon in the sky changes and why the lunar phases occur.
3.2Background
The Earth and the Moon
If you've ever been out on a clear night when the full Moon is up, you may have noticed that it appears bright enough to illuminate the night, causing objects to cast shadows. It is a common misconception, though, that the Moon is only observable during the night. As you will find out in this activity, it is sometimes observable during the day.
Have you noticed that the Moon's appearance changes over time? We say that the Moon goes through a cycle of phases; its apparent shape changing regularly over time. What we see starts out as a thin, crescent-shaped sliver of the Moon, but grows over the course of days until it becomes half-full, more than half full (or gibbous) and then completely full, only to reverse the cycle until it disappears from our view once again.
In this activity you will investigate why it is we are able to see the Moon, why it appears to change in shape, and how its phase determines when it is visible in the sky? We say that the Moon goes through a cycle of phases; its apparent shape changing regularly over time. What we see starts out as a thin, crescent-shaped sliver of the Moon, but grows over the course of days until it becomes half-full, more than half full (or gibbous) and then completely full, only to reverse the cycle until it disappears from our view once again.
3.3Moonlight
Here is a view of the Full Moon at 9 PM at night. The Sun set roughly 3 hours ago.
Can you see the Sun? No, it is below the Earth's horizon.
Can you see the Moon? Yes, it is above the Earth's horizon.
But why can you see the Moon? In the second view you can observe what happens without the sun.
Question
1.
Does the Moon give off light of its own?
A.
B.
C.
D.
E.
3
Try Again.
Correct. The only reason we see the Moon is because light from the Sun reflects off it.
Incorrect. The only reason we see the Moon is because light from the Sun reflects off it.
Summary
As you can see without the Sun, we don't see the Moon. We see the Moon in the sky because it reflects light from the Sun. The Moon does not give off any visible light of its own.
3.4Moon Illumination
Looking at the Earth and Moon from outer space, we see that both are illuminated by the Sun. Actually the Moon is much farther away from the Earth than shown in the figures on this page. Remember, it would really take 30 Earths placed side-by-side to reach the Moon.
Question Sequence
Question
2.
How much of the Moon's surface is illuminated by the Sun?
A.
B.
C.
D.
E.
3
Look at the figure. What portion of the Moon is in sunlight? What portion is in shadow? Try again.
Correct. Exactly half of the Moon is illuminated by the Sun, the half facing the Sun. In fact, half of the Moon is always illuminated by the Sun, and half of the Moon always faces the Earth, but these two halves don't always coincide. During the Full Moon phase as shown above, they do. The half facing Earth is the half which is illuminated.
Incorrect. Exactly half of the Moon is illuminated by the Sun, the half facing the Sun. In fact, half of the Moon is always illuminated by the Sun, and half of the Moon always faces the Earth, but these two halves don't always coincide. During the Full Moon phase as shown above, they do. The half facing Earth is the half which is illuminated.
Question
3.
Now let's move the Moon to the other side of the Earth.
What fraction of the Moon facing the Earth is illuminated now?
A.
B.
C.
D.
E.
3
Look carefully at the picture above. The left side of the Moon faces the Earth. How much of it is illuminated by the Sun? Try again.
Correct. None of the side facing Earth is illuminated. At this point in the Moon's orbit, the illuminated side is facing away from the Earth. We call this phase of the Moon the New Moon phase. At this point of the Moon's orbit, we can't see any portion of the Moon's illuminated surface from Earth.
Incorrect. None of the side facing Earth is illuminated. At this point in the Moon's orbit, the illuminated side is facing away from the Earth. We call this phase of the Moon the New Moon phase. At this point of the Moon's orbit, we can't see any portion of the Moon's illuminated surface from Earth.
3.5Lunar Phases
The animation shows how the Moon orbits around the Earth, and the amount of time that it takes is shown by the "day counter" at the bottom of the animation frame. The Moon is shown in various positions throughout its orbit. The appearance of the Moon as seen from Earth is shown in the upper right along with the name of the phase. Of course, the Moon really isn't as close to the Earth as shown in this animation. The Moon's distance from Earth is roughly 30 Earth diameters, or approximately 384,000 km.
Question Sequence
Question
4.
Roughly how long does it take the Moon to complete one full cycle of phases?
A.
B.
C.
D.
3
Look carefully at the animation again and pay attention to the "day counter" which shows how long a lunar cycle of phases takes. Try again.
Correct. The Moon takes roughly 4 weeks, or about 1 month to go from New Moon to New Moon again.
Incorrect. The Moon takes roughly 4 weeks, or about 1 month to go from New Moon to New Moon again.
Question
5.
Look at the first 5 phases of the Moon, from New Moon to Full Moon. How does the appearance of the Moon change from one phase to the next?
A.
B.
C.
3
Look carefully at the orientation of the lit and unlit portions. Try again.
Correct. The Moon is becoming a little fuller each night, or the portion of the Moon which we can see becomes more illuminated.
Incorrect. The Moon is becoming a little fuller each night, or the portion of the Moon which we can see becomes more illuminated.
Question
6.
We say that when the amount of illumination which we can see becomes greater each night that the Moon is waxing. When the amount of viewable illumination decreases from one night to the next, we say that the Moon is waning. This happens as the Moon goes from a Full Moon to a New Moon.
There are two crescent phases, a waxing crescent and a waning crescent. If you saw the Moon in the sky how could you tell which crescent you are observing?
A.
B.
C.
3
Look carefully at the animation again and pay attention to the "day counter" which shows how long a lunar cycle of phases takes. Try again.
Correct. The waxing moons are illuminated on the right side, and the waning moons are illuminated on the left side, but only if you live in the Northern Hemisphere! It turns out that people living in the Southern Hemisphere observe the Moon from a different perspective, and so during the waxing phases the illumination occurs on the left side of the Moon, while the right side is illuminated during the waning phases.
Incorrect. The waxing moons are illuminated on the right side, and the waning moons are illuminated on the left side, but only if you live in the Northern Hemisphere! It turns out that people living in the Southern Hemisphere observe the Moon from a different perspective, and so during the waxing phases the illumination occurs on the left side of the Moon, while the right side is illuminated during the waning phases.
3.6Lunar Phases - Continue
Question
7.
3.7Moonrise and Moonset
Have you ever noticed the Moon in the sky during the daytime? It turns out that different phases of the Moon will rise and set at different times.
The animation shows a person (stick figure) standing on Earth. The arrows mark the observer's eastern and western horizons. As the animation begins the Sun and Moon are in the same direction and highest in the sky.
What lunar phase is it? It's a New Moon phase (so, actually, you wouldn't see it).
What time of day is it? It's noon, since the person's head points toward the Sun.
Click the Start button and watch the Earth rotate.
The Earth has rotated through one-fourth of a day.
What time is it for the person now? It's 6 PM, because one-fourth of a day is equal to 6 hours and in the previous view it was noon.
In which direction would the observer look to see the Sun setting? The Sun is in the same direction as the New Moon. The Sun sets towards the west. We have indicated the direction towards the western horizon with an arrow labeled 'W'. So the Sun is setting on the western horizon at 6 pm and so is the Moon.
Click the Continue button to watch the Earth continue to rotate until sunrise (6 AM).
At this point, if you could see the New Moon phase, the Moon would be in the direction labeled east by the arrow or on the eastern horizon. It would be rising in the east at the same time as the Sun. We are now going to add in the motion of the Moon around the Earth as the Earth rotates.
Click Continue to watch the Moon orbit the Earth while the Earth spins.
Question
8.
One day later, where is the Moon relative to the Sun?
A.
B.
C.
3
Try Again.
Correct. The Moon rises on the eastern horizon, just like the Sun. Has the person on Earth seen the Moon on his eastern horizon yet? Is the eastern horizon arrow pointed directly toward the Moon? Try again.
Correct. The Moon is a little farther over in its orbit, so it is not yet viewable at sunrise. It is still below the eastern horizon for the observer. It will take about an hour before the Earth has rotated enough so that the Moon will be on the eastern horizon. As you can see, from day to day, due to the Moon's motion around the Earth while the Earth rotates, it will appear to rise roughly 1 hour later each day.
What about moonset? Click on Continue and watch the Moon approach the western horizon.
Incorrect. The Moon is a little farther over in its orbit, so it is not yet viewable at sunrise. It is still below the eastern horizon for the observer. It will take about an hour before the Earth has rotated enough so that the Moon will be on the eastern horizon. As you can see, from day to day, due to the Moon's motion around the Earth while the Earth rotates, it will appear to rise roughly 1 hour later each day.
What about moonset? Click on Continue and watch the Moon approach the western horizon.
Summary
When does it set? It will take another hour for the Moon to be on the western horizon, or at approximately 7 pm. The Moon sets roughly 1 hour later each day.
3.8Moonrise and Moonset - Continue
From the previous animation you have determined that the New Moon rises and sets with the Sun.
Fill in the times the New Moon rose and set in Question 9 below.
You were also shown that the Moon moves a little bit each day in its orbit around the Earth. This results in the Moon rising and setting a little bit later each day.
The figure below shows the Full Moon phase and the orientation of the observer when the Full Moon rises. In which direction is the Sun? What is the observer's time?
Fill in the time the Full Moon rises and sets in Question 9 below and then add the times for the quarter moons.
Question Sequence
Question
9.
Complete the chart below with the rise and set times of the Moon. Assume that sunrise occurs at 6 AM and sunset occurs at 6 PM.
Moonrise
Moonset
New Moon
1st Quarter
Full Moon
3rd Quarter
Table
3
Remember we keep time by the Sun so think about where the Sun is as the Moon rises. Setting times are always 12 hours later.
Correct. The Moon rises on the eastern horizon, just like the Sun. Has the person on Earth seen the Moon on his eastern horizon yet? Is the eastern horizon arrow pointed directly toward the Moon? Try again.
Correct.
Incorrect.
Question
10.
When would a waning gibbous Moon rise?
A.
B.
C.
D.
3
When does the waning gibbous phase occur? Between which phases? Try again.
Correct. The waning gibbous phase occurs between the Full Moon and the Third Quarter phases, and so rises between 6 PM and midnight. If the waning gibbous phase rises around 9 PM, then it sets around 9 AM.
Incorrect. The waning gibbous phase occurs between the Full Moon and the Third Quarter phases, and so rises between 6 PM and midnight. If the waning gibbous phase rises around 9 PM, then it sets around 9 AM.
3.9Quick Check Quiz
Indepth Activity: Lunar Phases
Question
11.
We see the Moon because:
A.
B.
C.
D.
E.
Correct. The Moon does not give off light of its own. We only see it because it reflects sunlight.
Incorrect. The Moon does not give off light of its own. We only see it because it reflects sunlight.
Question
12.
A waxing crescent moon occurs:
A.
B.
C.
D.
E.
Correct. Lunar phases occur due to the relative positions of the Earth, Moon & Sun. They are not caused by the Earth casting shadows on the Moon.
Incorrect. Lunar phases occur due to the relative positions of the Earth, Moon & Sun. They are not caused by the Earth casting shadows on the Moon.
Question
13.
A full moon occurs when:
A.
B.
C.
D.
E.
Correct. Correct. Half of the Moon is always illuminated by the Sun. When that half is also facing us, we see a full Moon.
Incorrect. Correct. Half of the Moon is always illuminated by the Sun. When that half is also facing us, we see a full Moon.
Question
14.
On a particular day, the Moon rises in the waxing gibbous phase. What phase is it when it sets?
A.
B.
C.
D.
E.
Correct. It takes the Moon roughly 1 month to go through a complete cycle of phases. In half of a day, its phase will not change. It will look the same.
Incorrect. It takes the Moon roughly 1 month to go through a complete cycle of phases. In half of a day, its phase will not change. It will look the same.
Question
15.
If the Moon is in the first quarter phase today, what phase will it be one week from now?
A.
B.
C.
D.
E.
Correct. It takes the Moon roughly 1 month to go through a complete cycle of phases. Therefore, in one week it will go through about one-fourth of its cycle. One-fourth of the cycle after the first quarter phase is the full Moon phase.
Incorrect. It takes the Moon roughly 1 month to go through a complete cycle of phases. Therefore, in one week it will go through about one-fourth of its cycle. One-fourth of the cycle after the first quarter phase is the full Moon phase.
Question
16.
At what time does a waning crescent Moon set?
A.
B.
C.
D.
E.
F.
Correct. The waning crescent Moon occurs between Last Quarter and New Moon, so it sets shortly before the Sun sets, around 3 PM.
Incorrect. The waning crescent Moon occurs between Last Quarter and New Moon, so it sets shortly before the Sun sets, around 3 PM.
Question
17.
You are meeting a friend for a late dinner at 9 PM. While waiting for him, you notice the Moon setting on the western horizon. What phase is it?
A.
B.
C.
C.
D.
Correct. If the Moon is near the western horizon, then it must be setting. If it is setting at 9 PM, then it is setting shortly after the Sun set, and therefore must be a phase close to the New Moon (which sets at the same time as the Sun). The waxing crescent phase sets shortly after the Sun sets.
Incorrect. If the Moon is near the western horizon, then it must be setting. If it is setting at 9 PM, then it is setting shortly after the Sun set, and therefore must be a phase close to the New Moon (which sets at the same time as the Sun). The waxing crescent phase sets shortly after the Sun sets.
Question
18.
You look at the Moon and notice that more than half of the side facing the Earth is illuminated, and that it is illuminated on the left-hand side. Because of this, you know that it is a...
A.
B.
C.
D.
Correct. A gibbous phase occurs when more than half of the Moon is illuminated. If the left half of the Moon is illuminated, then it is waning. Therefore, it is a waning gibbous Moon.
Incorrect. A gibbous phase occurs when more than half of the Moon is illuminated. If the left half of the Moon is illuminated, then it is waning. Therefore, it is a waning gibbous Moon.
Question
19.
Which of the following lunar phases is never seen above the horizon at 7 PM?
A.
B.
C.
D.
E.
Correct. A person standing on the Earth at 6 PM would see a new Moon setting (during a new Moon phase), a full Moon rising (during a full Moon phase), and any phase in between (in other words, any of the waxing phases). Therefore an hour later at 7 PM the person could see all of the waxing phases (except the new Moon) and the first waning phase – waning gibbous. All the other waning phases are never seen at 7 PM.
Incorrect. A person standing on the Earth at 6 PM would see a new Moon setting (during a new Moon phase), a full Moon rising (during a full Moon phase), and any phase in between (in other words, any of the waxing phases). Therefore an hour later at 7 PM the person could see all of the waxing phases (except the new Moon) and the first waning phase – waning gibbous. All the other waning phases are never seen at 7 PM.
Question
20.
If the Moon sets at 7 PM tonight, it will set around __________ tomorrow night.
A.
B.
C.
D.
E.
Correct. The Moon sets roughly 1 hour later each night, so if it set at 7 PM tonight, it will set around 8 PM tomorrow night.
Incorrect. The Moon sets roughly 1 hour later each night, so if it set at 7 PM tonight, it will set around 8 PM tomorrow night.
