Even the most casual sky observer will notice the changing phases of the Moon. However, the reason for the Moon’s phases during a 29 ½ day period seems to be almost all but forgotten. When I’ve asked Boy and Girl Scout groups, adults of all ages, including college students, I’ve gotten some real imaginative answers. So let me get back to some basic astronomy principles and illuminate you about lunar phases.
The Sun’s path through the sky is called the ecliptic, and it also represents the plane of our solar system. You’ll find that the planets very closely follow this imaginary path as well. The Moon, however, moves above, below and across the ecliptic in its orbit about the Earth. The Moon crosses the ecliptic twice each lunar month. Occasionally when the Sun, Earth and Moon line up when the Moon crosses the ecliptic we observe lunar and solar eclipses.
One of the answers I often get to my lunar phase question is that the phase is caused by the shadow of the Earth. Not an exceptionally bad answer. At least they are thinking! For indeed during a lunar eclipse the Moon does slide through the Earth’s shadow and we do see that shadow sweep across the lunar surface. However, the Moon’s monthly cycle of phases occurs because the angle between the Earth, Sun, and Moon is constantly changing, which in turn changes our viewing perspective. Oh, and one important fact to remember is that one-half of the Moon is always bathed in sunlight.
The following is perhaps one of the best videos I have found to-date which demonstrates and explains the Moon’s phases: http://www.youtube.com/watch?v=0vXWXqGmPCk I think it will help you to more easily understand the lunar phase cycle and will most certainly assist you with completing the Phases of the Moon booklet during the lab.
We’ll start with the New Moon. When the Moon is between the Earth and the Sun, but passes above or below the solar disk, it cannot be seen. The side facing the earth is not lit, but the back-side or far-side of the Moon is therefore fully illuminated. (You may know that our desolate neighbor has become locked into an orbit whereas we always see only one side of it. The Moon’s rotational period is equal to its period of revolution. And you may at one time have heard the side we do not see referred to as the “dark side” of the Moon. Unfortunately that is a poor term to use. The “dark side” is the far side of the Moon that we never see from the Earth. However, it does receive as much sunlight as the near side.)
Only as the Moon moves away from the vicinity of the Sun and appears in the western sky after sunset will you see a slender lunar crescent. The Sun, Earth, and Moon (SEM) angle is small, and so is the illuminated area we see. (Again, keep in mind that one-half of the Moon is fully illuminated, but due to our viewing perspective/angle we only see a portion of the lit area.) At this phase, called a Waxing (increasing) Crescent, you’ll see the complete outline of the Moon in addition to the illuminated crescent. This view is often referred to as the “old Moon in the new Moon's arms.”
The un-illuminated surface will even show some minor detail. It’s an effect called “Earthshine.” You’re seeing sunlight reflected onto the Moon from the Earth. Why is there so much reflected light? If you could observe the Earth from the lunar surface at this time you’d see an almost Full Earth! And that’s another point to remember. Whatever the phase the Moon is in when viewed from the Earth, the Earth as viewed form the Moon would be the opposite or complimentary phase. In addition, the horns or cusps of the crescent Moon point away from the sunset or sunrise point.
As the month progresses the Moon moves towards the eastern sky and the SEM angle increases. So does our view of the sunlit portion of the lunar surface. (Be sure to check out the website animation.) The crescent grows larger and larger until after seven days the Moon reaches First Quarter (the Moon is one-quarter of the way around the Earth; or 90 degrees from the Sun). The First Quarter Moon will be at its highest point in the sky at sunset. At that time we see one half of the lunar surface facing the Earth illuminated on the right hand side. In effect we are seeing one quarter of the entire lunar surface lit because of our perspective. One half of the Moon is still illuminated…the other quarter is around and beyond the right hand edge/limb of the Moon. Watch the progression of the terminator (the dividing line between the lit and unlit portion of the lunar surface – between New Moon and Full Moon it is the sunrise point).
The lunar month continues as the Moon moves further to the east. As the SEM angle grows larger the lit portion of the Moon grows as well. Between First Quarter and Full Moon, which takes another seven days, the phase is Waxing Gibbous. At Full Moon the moon is opposite the Sun (180 degrees) in the sky with the Earth in the middle of this celestial configuration. The surface of the Moon facing the Earth is fully illuminated. The far side of the Moon would be in total darkness. The Sun is setting when the Full Moon rises. (Though the Earth is between the Moon and the Sun, the Moon’s path passes above or below the Earth’s shadow most months. Otherwise we’d experience a lunar eclipse monthly.) At this time the Moon is halfway around our sky in its orbit.
After Full Moon the SEM angle begins to get smaller again as the Moon swings around the Earth and gets closer to the Sun in our sky once again. The sunlit portion of the lunar surface starts to grow smaller. From Full to Last or Third Quarter, which takes another seven days, the Moon is said to be in Waning (decreasing) Gibbous phase. Every night you can watch the terminator move from the right edge of the Moon towards the left. Last Quarter has been attained when the left hand side of the lunar surface is half illuminated. At that time the Moon is three-quarters of the way around the sky and is now once again 90 degrees from the Sun. It resembles a mirrored image of the First Quarter phase. The Moon will rise around midnight, and at sunrise it will be at its highest point in the sky.
As the Moon progresses in its orbit about the Earth the SEM angle continues to decrease and the terminator (which became the sunset point on the sunset point on the lunar surface just after Full Moon) advances across the lunar surface. This phase between Last Quarter and New Moon is called Waning Crescent. You'll soon see a slender crescent in the early morning sky just before sunrise. Earthshine will again be very noticeable. A day or so later the Moon passes between the Earth and the Sun and we once again experience a New Moon and the cycle starts all over.
Enjoy watching our Moon as its appearance changes from night to night. It’s just a phase it’s been going through for billions of years!
Keep your eyes to the skies.