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Introduction | Process | References



Phases refer to the different looks we get from the heavenly bodies over time. The phases of the moon, as seen from the earth are the most widely known.


You can see different pictures of those phases by following the next link:

Phases of the Moon

Why do we see them?

What does it take for the moon to change its looks?, let's take a look at different explanations constructed by people all over the world.


Religion has been influenced by the stars, the moon playing an important role in the beliefs of different peoples. The following links show you the influence of the moon through legends, sculpture, and architecture


Native American


Scientific Explanation

When we observe the moon, it sometimes appears round and full. It sometimes appears to be a small crescent. And sometimes it seems to disappear altogether. This is not because the moon itself is changing size or shape, it is only our view of it that changes.

What causes us to see different views of the moon?

When we look at the moon we can see only that part which is illuminated by the sun. But depending on the relative positions of the earth the moon and the sun, only a portion of the illuminated part may be facing the earth. Thus, how much of the moon we see depends on the relative positions of the earth, moon and sun.

Let us consider the following diagram:


What do you think we would see then? click here for the answer.

How about the next diagram?:


What do you think we would see then? click here for the answer

Beyond the moon

The Planets

Now that we have examined the phases of the moon, we might wonder whether we will see phases of other heavenly bodies, such as:



Can we see a Mars Crescent from earth? This question is asked and answered in an interesting paper by John Lamb, which we use as the basis for this explanation: We saw above that the appearance of the moon from Earth depends on the relative position of the Earth, Moon and Sun. This is true for Mars also. Let us call the angle between the sun, Mars and Earth, angle SME. It turns out that the size of the portion of Mars which appears dark is directly proportional to this angle. (For an explanation of this fact click here). That means that the bigger angle SME is, the greater the portion of mars that appears dark. And the smaller angle SME is, the smaller the portion of Mars which appears dark. If Earth, the Sun and Mars are all in a line, then angle SME is zero, and all of Mars appears light. As the Earth and Mars move out of this alignment angle SME will increase for a while and so the amount of Mars which appears dark will grow. But how much? How dark will it get? See below to find out!

According to this explanation, when the angle Sun-Mars-Earth (SME) is maximized, we'll get to see the darkest mars we can see from earth. The next several figures and movies illustrate this fact. The figures are not drawn to scale in order to better illustrate the different phases of Mars.

The next figure illustrates angle SME at a specific point in time. If you click on the figure, you will see an animation that will show you how the dark area varies proportionally with SME.


The following snapshots, show you precisely the conditions under which we get to see the lightest and the darkest views of mars. Notice angle SME is maximized when angle SEM is approximately a right angle (90 degrees). This is called quadrature. Click on the first snapshot and you will see another animation that will show you how Mars becomes brighter as the angle SME decreases in size.


Did you notice in the previous animation the conditions under which Mars looks completely illuminated from earth? Take a look at the next picture to refresh your memory


The Amphion system provides a way to generate this type of figures and animations through the use of specialized editors. For a brief description of the process used to create those pictures and animations click here . More detailed explanations about the functionality of Amphion can be found in the Edu-Amphion home page . Amphion's lyre is the entry point for a text-based browser for Amphion specifications. More details on the information sources used to build this specific site and links to other sites with similar content can be found in the References page.

Last modified: Aug. 22, 2008 by Allen Dutra.

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