Activity:
Moon Phases: Rising and Setting

Introduction

Did you know that if you know the phase of the Moon you can easily and accurately predict when it will rise and set? It just takes a bit of careful thought to work it out. In this activity you will work it out for yourself.

The phases of the Moon are shown below. Your task is to figure out when each one rises and sets by thinking carefully about the geometry of the Moon’s position in its orbit, relative to the Sun, as it circles around the Earth.

New 1	Waxing Crescent 2	First Quarter 3	Waxing Gibbous 4	Full 5	Waning Gibbous 6	Third Quarter 7	Waning Crescent 8	New 1

In the diagram below the Moon’s orbit of the Earth is depicted as if viewed from above the Earth’s North Pole. The Earth is shaded to show where it is night (the gray part) and where it is day (the unshaded part). Four times are noted around the edges of the Earth: Sunrise, Noon, Sunset, and Midnight. Note that the order they are shown in reflect the fact that the Earth rotates counter-clockwise when viewed from above the North Pole. The Moon orbits the Earth in the same direction: counter-clockwise. The cycle of the phases of the Moon takes 29 ½ days so roughly speaking there is a week between New Moon and First Quarter, another week passes before the Full Moon, the Third Quarter follows in another week and 7 days later the Moon is New again.

The New Moon rises at the same time as the Sun: sunrise. It also sets with the Sun: sunset. Each day, as the Moon orbits the Earth, the Moon rises a bit later. It takes the entire 29 ½ days for it to come around and rise at the same time as the Sun again. To help you to figure out the other rise-set times imagine that you are standing on the surface of the Earth in the diagram below at the Sunrise line. Mentally, draw a line tanget to the circle that represents the Earth: this is your horizon. A line drawn from your position on the surface of the Earth that is perpendicular to the horizon is straight up toward the zenith. Notice that the Sun is 90° to the overhead direction: it is on your horizon. That is why that position represents Sunrise. Finally, notice that the Moon is in the same direction as the Sun at this time: right on your Eastern horizon.

You can tell the East from the West by thinking about the direction of the rotation of the Earth. East is the side of the horizon line that is the same as the direction of the rotation of the Earth. West is the side of the horizon line that is the opposite from the direction of the rotation of the Earth. As you put your mental self at different positions on the Earth (different times of day) your horizon moves around with you.

At any given point around the circle representing the Earth the point directly overhead is the place where you would see objects in the sky transit. Remember, to transit means to cross the Celestial Meridian: the imaginary line running from North to South through the Zenith.

1. Shade the Moon in each of the eight positions shown at left. Remember, you are looking at the Moon from above its orbit around the Earth.
2. Label each numbered position with the correct phase of the Moon.
3. What time, relative to the Sun, does the New Moon rise? (Hint: it is in the same direction as the Sun)
4. What time, relative to the Sun, does the New Moon set?
5. What time, relative to the Sun, does a Waxing Crescent Moon rise? When does it set?

6. What time, relative to the Sun, does a First Quarter Moon rise? When does it set?
7. What time, relative to the Sun, does a Waxing Gibbous Moon rise? When does it set?
8. What time, relative to the Sun, does a Full Moon rise? When does it set?

9. What time, relative to the Sun, does a Waning Gibbous Moon rise? When does it set?
10. What time, relative to the Sun, does a Third Quarter Moon rise? When does it set?
11. What time, relative to the Sun, does a Waning Crescent Moon rise? When does it set?

12. In the boxes below draw the position and phase of the New Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture. The box on the left is the Eastern horizon. The box in the middle is the sky overhead: East in on the left and West is on the right; you are looking South. The box on the right is the Western horizon.

13. In the boxes below draw the position and phase of the Waxing Crescent Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

14. In the boxes below draw the position and phase of the First Quarter Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

15. In the boxes below draw the position and phase of the Waxing Gibbous Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

16. In the boxes below draw the position and phase of the Full Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

17. In the boxes below draw the position and phase of the Waning Gibbous Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

18. In the boxes below draw the position and phase of the Third Quarter Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

19. In the boxes below draw the position and phase of the Waning Crescent Moon at three times of day: (1) Moonrise, (2) Moon Transit, and (3) Moonset. Label each box for the time when the Moon is visible in those locations. Include the position of the Sun in each picture.

20. The picture below shows the Waxing Crescent Moon just before Sunset near the Western horizon. What is wrong with the picture? Explain. Draw a correct version of the picture.