ASTRO 801
Planets, Stars, Galaxies, and the Universe

The Phases of the Moon

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Additional Reading at www.astronomynotes.com:


The changing Moon is a familiar sight to all of us, since it is the second brightest object in the sky after the Sun. The movie of the Moon below was created by taking images of the Moon day after day for a month and merging the individual frames into a sequence. In this movie, you can witness all of the apparent changes in the Moon as seen from Earth in a short clip.

Short clip of a movie showing the apparent changes in the Moon as seen from Earth over the course of a complete cycle of phases
Changes in the Moon as seen from Earth

In this movie, you see a few different changes in the appearance of the Moon:

  • It always keeps the same face pointed towards the Earth.
  • It goes from completely dark to completely illuminated and back again.
  • The apparent size changes a bit.
  • It appears to wobble a bit.

We will talk about the reasons for the first three points, but will leave the last one for more advanced astronomy courses (if you are really curious, I can recommend references about the libration of the Moon).

Let’s start with the phases. First, we need to again talk about the layout of the Sun, Earth, and Moon system. We already have learned that the Earth is orbiting the Sun, and it is rotating on its tilted axis as it orbits. At the same time, the Moon is orbiting the Earth, and as it orbits around the Earth, it is rotating, too. Just like the Earth, the half of the Moon that is pointed towards the Sun is illuminated, and the half of the Moon that points away from the Sun is dark. So, the simple explanation for the phases of the Moon is that, at any one time, half of the Moon is light and half is dark, and the appearance of the half of the Moon that we see changes as it orbits the Earth. For a listing of the individual phases and a description of their appearance, see "Phases of the Moon and Percent of the Moon Illuminated."

Try This!

For this lesson, we are going to refer heavily to a set of interactive modules available from the University of Nebraska-Lincoln.

  1. Go to the table of contents for the interactive modules: Astronomy Education at the University of Nebraska-Lincoln.
  2. From the table of contents, select "Lunar Cycles," which should bring up a new window with tabs near the bottom that say "Animations," "Images," and "Outlines."
  3. We are going to look at a few images and animations in order to understand the phases of the Moon. To see the full Sun, Earth, Moon system in action, click on Animations, and then select "Three views simulator."
  4. When you click on "Run animation," you should see that the Earth begins to rotate, the Earth begins to orbit the Sun, and the Moon begins to orbit the Earth. The Moon is also rotating, but that is less obvious (we will discuss this more later). Please keep this animation in mind when we discuss the phases—remember that all of these phenomena happen simultaneously (Earth's rotation and orbit, Moon's rotation and orbit).
  5. You can summarize the phases of the Moon and how and when they appear using a single image. To see this image, in the "Lunar Cycles" pane, close the Animations tab and click on the Images tab.
  6. Select the top left image, which says "Lunar Cycles" when you mouse over it.

    This is an image that requires some interpretation, but if you can figure it out, you will have mastered an understanding of the phases of the Moon. The Sun is far off to the left on the diagram, so sunlight is illuminating the left side of both the Earth and the Moon. The point on the Earth immediately underneath the Sun experiences noon, and if you imagine the stationary Earth in the diagram rotating counterclockwise, that point will experience sunset when the Earth has rotated 1/4 of the way around, then midnight, then sunrise, then back to noon. Those times are labeled.

    In the same diagram, the Moon is shown in eight different locations along its orbit around the Earth. For example, the full Moon occurs when the Sun, Earth, and Moon are in a line in that order (labeled as position 4). The sunlight that is hitting the Moon illuminates the half of the Moon that points towards the Earth. So, when the Moon is full, its entire illuminated half is pointed directly at the Earth.

    The next question that you can answer from this diagram is: What time is the full Moon visible on Earth? To answer this question, we can integrate our knowledge about the rotation of the Earth. Picture yourself on the daylight part of the Earth pointing directly at the Sun. Remember, it is noon for you when you are on the part of the Earth pointing directly at the Sun, which is when the Sun is transiting your meridian. Six hours later, when the Earth has rotated one quarter of the way around, you would have to look to the western horizon to see the Sun, and to the eastern horizon to see the Full Moon. So, at sunset (about 6:00 PM), the full Moon is rising. Six hours later, the Earth has rotated an additional one quarter of the way around. Now, the Moon is directly in front of you (that is, this time it is transiting your meridian). So, the full Moon transits at midnight. Six hours later, at about 6:00 AM, the Moon will now be on your western horizon (setting), and the Sun will be on the eastern horizon (rising).

    The New Moon phase occurs when the Sun, Moon, and Earth are in a line in that order (unlabeled, but it is position 8). In this case, the unilluminated side of the Moon faces the Earth. Thus, during New Moon, we do not see the Moon in our sky at all. Using the logic from the paragraph above, even though we can't directly see the New Moon, we know that the New Moon transits at 12 PM (noon), sets at sunset (about 6 PM), and rises at sunrise (6 AM).

    The other phases fall in between these two extremes. For example, at First Quarter, the side of the Moon facing the Earth is half illuminated and half dark. It will rise at noon, transit at 6 PM, and set at midnight.
  7. If you want to watch this entire process play out, you can return to the Animations tab on the Lunar Cycles module, and you can choose the "Lunar Phases Simulator." When you run that animation, you will see the Moon orbit the Earth, you will see how its appearance changes depending on its location along its orbit, and how it rises, transits, and sets at different times depending on its location along its orbit.

There are two other similar animations that you might want to compare while you are studying the Phases of the Moon. They are:

  1. Teachers' Domain phases of the Moon
  2. University of Illinois phases of the Moon applet

Let's end this discussion of the Moon by returning to its rotation. If the Moon rotates, why does it always show the same face to the Earth? Shouldn’t we see the face of the Moon slowly changing as it rotates? For example, think about observing the Earth from the point of view of the Sun; during the course of 24 hours, you will see North America rotate out of your view and then back again. We will discuss this in more detail in a later lesson, but the short answer is that the Moon’s rotation rate is matched to its orbital rate. That is, the Moon takes the same amount of time to rotate as it takes to orbit. Because of this, it keeps the same face pointed towards the Earth at all times.

The last point to make about the phases is the time it takes for the Moon to complete one complete cycle of phases. We know that the length of our day is tied to the rotation rate of the Earth and the length of our year is tied to the orbital period of the Earth around the Sun, so what about the Moon? Well, it takes approximately 29.5 days for the Moon to complete one set of phases, or roughly one month.