Chapter 3. Lunar Phases

3.1 Introduction

The goals of this module: After completing this exercise, you should be able to:

1. Understand why we can see the Moon.
2. Explain why the Moon appears to go through a cycle of phases.
3. Determine the order of phases in which the Moon will appear.
4. Determine the rise and set times of the Moon based on its phase.

In this module you will explore:

1. The orbit of the Moon around the Earth
2. How the relative positioning of the Earth, Moon and Sun causes the lunar phases
3. 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.

3.2 Background

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.

3.3 Moonlight

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.

3.4 Moon 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 3.2

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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 3.3

Now let's move the Moon to the other side of the Earth.

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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.

3.5 Lunar 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.

3.6 Lunar Phases - Continue

Question 3.7

3.7 Moonrise 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.

3.8 Moonrise 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.

3.9 Quick Check Quiz

Indepth Activity: Lunar Phases