Phases of the Moon

Goals of the Lab

To observe and understand the phases of the Moon.
To be able to predict the phases of the moon.
To understand the cause of lunar eclipses.

Requirements: A web browser with a Flash plugin.

Part I: How Much Sunlight does the Moon Recieve?

As you have most likely noticed, the appearance of the Moon appears to change throughout the month. The Moon doesn't actually change throughout the month, itsappearance from Earth changes. Many misconceptions surround the reasons for phases of the Moon. What causes these phases of the Moon? These cyclical phases are a result of the portion of the Moon lit by the Sun as well as the Moon's orbit around the Earth.

To better understand the phases of the moon, we begin by examining which portion of the moon receives sunlight. Sunlight shines on the side of the Earth, or Moon, that faces the Sun.

Question 1:How much of the moon is illuminated by the Sun? Does your answer depend on where the moon is relative to the Earth?

Part II: How does the movement of the Moon affect the phases of the Moon?

The Moon shines as a result of reflected sunlight. As the Moon orbits the Earth, the fraction of its illuminated side visible from the Earth changes. The following animation demonstrates how much of the illuminated Moon can be seen from the Earth depending on where the moon is in its clyclic orbit around the Earth.

Question 2:Describe the similarities and differences in appearance of the 1st and 3rd quarter moon to an observer on Earth.
Question 3:Do you ever observe the Moon during the day? Could you ever observe the full Moon during the day? Explain why or why not.

Question 4:If you can see a full Moon at your position on Earth, what type of Moon will you see if you go to exactly the opposite side of the world on the same day?

Part III: Phases of the Moon

The following animation allows you to see the Moon moving around the Earth. Play with the animation and then answer the following questions.

Question 5:In three or four sentences, explain why we observe the Moon going through difference phases.

Question 6:We always see the same side of the Moon. Explain this observation.

Question 7:How many degrees does the Moon move in the sky from night to night? Use the fact that the Moon moves around the Earth in an approximately circular every 27.3 days. How many degrees does the Moon appear to move in an hour?
Question 8:Briefly describe the apparent motion of the Moon against the background stars hour by hour. Briefly describe the apparent motion of the Moon day by day. Does the Moon rise and set at the same time every day?

Lunar Eclipses

From the previous diagrams, one would guess that there would be at least one lunar eclipse every month. However, lunar eclipses occur approximately twice a year. The following diagram gives shows you why this isn't the case.

The plane of the Moon's orbit about the Earth is tilted by about 5° relative to the plane of the Earth's orbit around the Sun. The "line of nodes" is the intersection of thses two planes.

Question 8: Briefly describe the conditions under which we would observe a lunar eclipse. Include where the Earth, Moon and Sun would be during an eclipse.