Phases of the Moon is near the top of the list of concepts that students seriously misunderstand. Most teachers run into problems trying to explain the Moon's phases to youngsters. Many students have trouble imagining how the Moon, half lit by the light of the Sun, can show phases as a month of time passes. Visualizing in three dimensions the Earth's unique perspective relative to the Moon and Sun and visualizing the perspective from outside the same system of bodies often proves difficult to students.
This demonstration addresses overcoming that difficulty through the use of two videos clips. The first is a demonstration of the phases of the Moon as seen from the Earth. A dark room is required. The video camera represents "Earth," or someone on Earth looking skyward. For stability the "Moon" is attached to the Earth using two yardsticks glued together and painted black. The Moon is a 9" plastic, air-filled ball, painted with a matt gray spray paint. The "Sun" is a lamp with a high wattage bulb (e.g., 400 Watts or more; shade removed) and is located a good distance from the Earth and Moon (e.g., ~15 feet). All other lights are out. A black backdrop is used to improve contrast.
At the same time, a second video camera films the Earth/Moon system from a very large distance (e.g., > 20 feet) and at a right angle with respect to an imaginary line between the Earth and Sun. This perspective embodies the second video clip. Keep in mind that all through the demonstration the Moon is physically half lit at all times. It is our unique perspective that gives us the ever-changing appearance of the Moon's illuminated surface.
At the start of the demonstration, the Moon is completely blocking the Sun. (This is demonstrating a total solar eclipse, which is fairly rare for any given location on Earth.) Usually the Moon passes above or below the Sun as viewed from Earth. All of the sunlight is shining on the far side, opposite the side that they are viewing as from Earth. This phase is called "new moon."
The Moon moves counterclockwise to the left, about 45 degrees (1/8) of the way around the lunar orbit. Observe the sunlight on the Moon now. Students should see the right hand edge illuminated as a "crescent." The crescent will start out very thin and fatten up as the Moon moves farther away from the Sun. (Note: although the Moon is closer to the Sun during new and crescent phases, it is still 400 times closer to Earth; i.e., the Sun is VERY far away in reality.)
When the Moon is at 90 degrees from its position at new moon, the students will see the right half of the Moon illuminated. This phase is called "first quarter." Remember that one half of the sphere is illuminated fully at all times (except during lunar eclipses), but the illuminated portion that we observe changes as the Moon changes position.
As the Moon continues to move counter-clockwise past first quarter, the Moon goes into its "gibbous" phase (more than half but less than fully illuminated) which grows as the Moon moves towards 180 degrees.
When the Moon is positioned directly opposite the Sun, as viewed from Earth, we see the half of the moon that is fully illuminated. This phase is called "full moon." Of course only half of the Moon is illuminated. It has taken the Moon about two weeks to move from new to full. This growth in illumination is known as "waxing."
With the Moon at full, the Moon's counterclockwise motion continues. Students will observe the reverse of the Moon's phases seen so far with the left portion of the Moon illuminated. From full to new, the Moon is "waning" and leading the Sun.
As the gibbous phase diminishes, the Moon will reach the 270 degrees position. This is "third" or "last quarter." It is followed by a thinning crescent and a return to new moon. The phase cycle takes 29.53 days.
Be sure to observe the real Moon! The view students get from this demonstration is the same for both this activity and for observations of the real sky. To help out, most newspapers give the Moon phases along with the weather data. For additional comparison, we provide you with an animation on the actual phases of the Moon and an animated diagram of the lunar orbit, which illustrates the Moon's location corresponding to the phase shown.
For an excellent web page reference on phases of the Moon, visit:
Get your Red/Blue glasses ready to view selected images of the moon in 3-D!
(Portions of this article are adapted from:
Note: The referenced demonstrations require QuickTime which is available for both the Macintosh and Windows platforms.