Chapter 18
A Family of Planets
The Moon
Phases of the Moon
The revolution of the Moon around the Earth makes the Moon appear as if it is
changing shape in the sky. This is caused by the different angles from which we see
the bright part of the Moon's surface. These are called "phases" of the Moon.

Of course, the Moon doesn't generate any light itself; it just reflects the light of the
Sun. The Moon passes through four major shapes during a cycle that repeats itself
every 29.5 days. The phases always follow one another in the same order.  
What you see when you look at the moon depends on its location in relationship to the sun and Earth.
We see a different fraction of sunlight being reflected from the moon to Earth
The four major moon phases are
"New" , "1st Quarter" , "Full" and
"Last or 3rd Quarter". These
phases have to do with the
relative positions of the sun, the
moon and the earth in the
moon's 29 day monthly orbit of
the earth.
Moon Phase Descriptions...
Although this cycle is a continuous process, there are eight distinct, traditionally recognized stages,
called phases. The phases designate both the degree to which the Moon is illuminated and the
geometric appearance of the illuminated part. These phases of the Moon, in the sequence of their
occurrence (starting from New Moon), are listed below  

(1) New Moon - When the Moon is roughly in the same direction as the Sun, its illuminated half is facing
away from the Earth, and therefore the part that faces us is all dark: we have the new moon. When in this
phase, the Moon and the Sun rise and set at about the same time.

(2) Waxing Crescent Moon - As the Moon moves around the Earth, we get to see more and more of
the illuminated half, and we say the Moon is waxing. At first we get a sliver of it, which grows as days go
by. This phase is called the crescent moon.

(3) First Quarter Moon - A week after the new moon, when the Moon has completed about a quarter of
its turn around the Earth, we can see half of the illuminated part; that is, a quarter of the Moon. This is
the first quarter phase.

(4) Waxing Gibbous Moon - During the next week, we keep seeing more and more of the illuminated
part of the Moon, and it is now called waxing gibbous (gibbous means "humped").

(5) Full Moon - Two weeks after the new moon, the moon is now halfway through its revolution, and now
the illuminated half coincides with the one facing the Earth, so that we can see a full disk: we have a full
moon. As mentioned above, at this time the Moon rises at the time the Sun sets, and it sets when the
Sun rises. If the Moon happens to align exactly with the Earth and Sun, then we get a lunar eclipse.

(6) Waning Gibbous Moon - From now on, until it becomes new again, the illuminated part of the Moon
that we can see decreases, and we say it's waning. The first week after full, it is called waning gibbous.

(7) Third Quarter Moon- Three weeks after new, we again can see half of the illuminated part. This is
usually called last quarter.

(8) Waning Crescent Moon - Finally, during the fourth week, the Moon is reduced to a thin sliver from
us, sometimes called waning crescent.

A while after four weeks (29.5 days, more precisely) the illuminated half of the Moon again faces away
from us, and we come back to the beginning of the cycle: a new moon. Sometimes, when the Moon is
almost new, it is possible to dimly see its darkened disk. The light from the Sun cannot reach this part of
the Moon directly; but at this time the Earth (as viewed from the Moon) is at its full and very bright, and
what we see is light reflected from the Earth, that then bounces back at us from the Moon. It's a long trip
for this light: from the Sun to the Earth, to the Moon, and back to the Earth.  

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