© 1989, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112
Editor's Note: The 20th anniversary of the first manned landing on the Moon--when Neil Armstrong and Edwin Aldrin became the first members of our species to walk on another world--will be celebrated this July 20th (1989). To prepare for this epochal event, the next two issues of The Universe in the Classroom will be devoted to the Moon. In this issue, we'll take a look at the Moon in our sky--its phases and some ways in which students can enjoy (and learn from) its ever-changing appearance. Our next issue will consider the Moon as a world it its own right--its craters, plains, and mountains--and present a short history of lunar exploration.
Our planet's large natural satellite, the Moon, is the easiest (and cheapest!) astronomical object to observe. The only "scientific instrument" you'll need at first is a pair of eyes. The Moon is the only thing in the sky (other than the Sun) that doesn't look like just a pinpoint of light or an indistinct fuzzy patch as seen with the unaided eye. Even better, the way the Moon looks to us is continually changing; keeping track of its appearance from night to night (or day to day) is a fascinating and easy way to get acquainted with the rhythms of change in the sky.
The Moon's OrbitThe Moon orbits around the Earth, taking about a month to go all the way around our planet. Its orbit is very nearly circular; it stays about 380,000 kilometers away from us as it moves counterclockwise (as viewed from a northern hemisphere perspective). It also stays fairly close to the Earth's equatorial plane (an imaginary extension of Earth's equator out into space); the point on the Earth directly under the Moon is never more than 29° north or south of the equator.
Younger students can enjoy demonstrating this motion by "orbiting" around another student seated in the center of the classroom. Have the "Moon" student begin by facing the "Earth" student (the one seated in the center). Then have the "Moon" student "orbit" — circle around the student in the center. The "orbiting" student will find that if he carefully swivels his body 1/4 turn for each 1/4 "orbit" around the room, then he will always be facing inward as he completes a full circle. (His back will never be visible to the seated "Earth" student in the center.) This is why astronomers speak of the "nearside" and "farside" of the Moon.
Since the Moon
shines by reflecting sunlight, what it looks like to us at any particular time
depends on the angle at which the Sun's light is hitting the Moon. For
example, if sunlight is coming from the right, the right half of the Moon's
globe will be bright and the left will be dark. If sunlight is coming from behind
the Moon as we look at our satellite, then its farside will be illuminated,
and we will see a dark globe or "new moon."
FIGURE 1: The pictures are shown from a northern hemisphere perspective. Those of us who live in the northern hemisphere (outside the tropics) have to look generally south to see the Moon when it's highest in the sky, and we wee the right-hand side illuminated at first quarter phase, for example. People who live in temperate or high latitudes south of the equator must look toward the north; to them the first quarter moon has its left side illuminated.
The diagram is not to scale. In reality, the Moon is 1/4 the diameter of the Earth and its orbit's width is about 60 times the Earth's diameter.
Figure 1 shows the Moon during one orbit around the Earth from a vantage point far above Earth's North Pole. It also shows what the Moon would look like when it is highest in the sky at eight times during the month.
Let's look at these phases of the Moon one by one:
Strictly speaking, "full moon" occurs only for an instant: the time when the Moon is as nearly opposite the Sun on the sky as it will be during its current orbit. however, the Moon will look only imperceptibly different from full for a night or two around that time.
The Moon's orbit around the Earth is tilted by about 5 degrees relative to our planets orbit around the Sun. (If you think of the two orbits as hoops, the two hoops are tilted relative to each other.) This means that full moon doesn't always bring the Sun, Earth and Moon into perfect alignment. If it did, the Moon would move directly into the Earth's shadow, which extend straight back, away from the Sun, at full moon. This does happen occasionally though; the phenomenon is call a lunar eclipse.
Near the time of full phase, the Moon is opposite the Sun; thus it rises around sunset, is high in the sky around midnight, and sets around sunrise, providing light that's bright enough to read by all night long when the weather is clear. (We're not sure your opthamologist would actually recommend reading by the light of the Moon, but astronomers like to point out that it is possible.)
Interestingly enough, though, the Moon is actually one of the darker objects in the solar system. it reflects only 7% of the sunlight that strikes it, absorbing the rest. For comparison, the Earth reflects 37%, Venus 65%, and Mars 15%.
It is during this time that the delay of moonrise from one night to the next is most easily noticed. As the Moon moves farther toward the east from night to night the Earth has to turn a little farther toward the east each night for us to see it. On the average, the Moon rises about 50 minutes later each night, but this can vary significantly depending on the time of year and your latitude north or south of the equator.
A day or two after this phase, the Moon will be new again, and the cycle begins anew...
| 1 | 2 | next page >>
back to Teachers' Newsletter Main Page