The Moon emits no light of its own. It reflects the Sun's rays, and as a whole is a rather poor reflector of light. The side of the Moon that is turned toward the Sun is always bright while the side turned away from the Sun is dark. This fact, coupled with the Moon's orbital motion around the Earth, causes the Moon to show different areas of light. These changing shapes of the lighted portion of the Moon are called its phases. The amount of time required for the Moon to revolve once around the Earth is 29 1/2 days, during which time it completes one full cycle of its phases. The four distinct phases are (1) New Moon, (2) First Quarter Moon, (3) Full Moon, and (4) Last Quarter Moon. Since these four phases are observed during the 29 1/2 days, each phase occurs about 7 3/8 days apart. During the two weeks following the New Moon, the time when the Moon is not visible in our sky, the Moon is said to wax (increase in brightness) from New Moon to Full Moon. During the two weeks that follow the Full Moon, the Moon is said to wane (decrease in brightness) from Full Moon to New Moon.
Each of the four major phases of the Moon are separated from each other by an angle of 90o. As the Moon revolves around the Earth and passes through its various phases, each phase has a definite time of the day when it appears over a meridian of longitude on the Earth. The New Moon and the Sun rise, cross the meridian, and set at the same time. A First Quarter Moon is just that -- one-quarter of the way around the sky from the Sun. The First Quarter Moon will rise, cross the meridian, and set six hours after the Sun. A Full Moon rises in the east at the same time the Sun sets in the west. In other words the Moon is twelve hours behind the Sun. A Third Quarter Moon is 3/4 the way around the sky which means that it will trail the Sun by 3/4 of a day or 18 hours. The Third Quarter Moon will rise at midnight if the Sun rose at 6:00 a.m. (Activity: Phases of the Moon)
Figure 1 is a concrete illustration of the phases of the
Moon as it revolves around the Earth. When the Moon is at
position 1, we see only darkness because the unlighted or
nightside of the Moon is facing the Earth. A day after New Moon,
a faint rim of light begins to show. This faint rim of light is
known as a waxing cresent phase because it grows larger with each
passing day. By Position 3 the Moon will have moved in its orbit
so that we can see one-half of its lighted side. At this
position we say the Moon is in its First Quarter phase. As the
Moon moves from 3 through 4 and on toward Position 5, we see more
of its lighted face. This is called the waxing Gibbous phase.
At Position 5 we see all of the Moon's face that is possible.
When the Moon is at this position we say it is at Full phase.
After the Full phase as been reached, the lighted Moon once again
grows smaller. From 5 to 7 the Moon is passing through a waning
Gibbous phase. After the Third Quarter phase at Position 7, the
Moon passes into the waning Crescent phase. When the Moon gets
back to Position 1 again, the sequence starts over again.
As it orbits, the Moon keeps the same side facing the Earth,
which can be seen by watching it through a cycle of its phases.
You might think from this behavior that the Moon does not rotate,
but as it progresses through the phases we see the same craters
all the time. Thus, the Moon does turn on its axis once for
every trip it makes around the Earth. Although we see only one
side of the Moon, we are able to view about 59% of the surface
from here on Earth. This is due to the Moon's orbit and how it
is oriented toward the ecliptic. We know what the back side of
the Moon is like from photographs taken by lunar orbiting
spacecraft such as Apollo. The back side is much more rugged
than the Earth side. Only one area large enough to be called a
mare can be seen on the lunar far side.
Questions:
1. What is meant by "phases of the Moon?"
2. How many phases does the Moon pass through in one month? How many days separate each phase?
3. Describe the Moon as it waxes and wanes and include the phases it passes throguh during these periods.
4. During what time of day does each phase of the Moon appear over a meridian of the Earth? In what direction does the Moon rotate? In what direction does it revolve?
5. What is meant by New Moon? By Full Moon? How many days separate these two phases?
6. Why do we always see the same side of the Moon from Earth?
