That orbed' maiden
With white fire laden
Whom mortals call the Moon.
- Shelley
Although people often refer to "the dark side of the Moon", there is no
such thing. Every part of the Moon receives sunlight, though only half is
lit at any one point. Because the Moon's rotational and orbital periods are
the same, we can only see one side of the Moon from Earth. So it is more
correct to refer to the "far side of the Moon."
Because the "horns" of a crescent moon are always facing away from the
setting or rising sun, they always point upward in the sky at dusk or dawn. It is fun to
watch for paintings and pictures which show an "impossible moon" with the
horns pointed downwards at night or in an early morning scene.
The Moon, Earth's only natural satellite, is relatively large, about
one-fourth the diameter the Earth, more than two-thirds of Mercury's
diameter, and more than three times the diameter of the largest asteroid
(Ceres).
After the sun, the full moon is the second brightest object in the sky.
The surface of the Moon has two hemispheres with rather
asymmetric properties; the near side of the Moon is substantially different
from the far side.
The near side is divided into light areas called the
lunar Highlands and darker areas called Maria. The Maria (literally, "seas";
the singular is Mare) are lower in altitude than the Highlands. The dark
material filling the Maria is actually dark, solidified lava from earlier
periods of lunar volcanism. Both the Maria and the Highlands exhibit large
craters that are the result of meteor impacts. However, there are many more
such impact craters in the Highlands.
The far side of the Moon (unseen from the Earth) has a
very different appearance than the near side. In particular, there are
almost no Maria on the far side, but plenty of impact craters.
Different parts of the surface of the Moon exhibit
different densities of cratering, which implies different ages: the maria
are younger than the highlands, because they have fewer craters.
The bulk density of the Moon is 3.4 g/cc, which is
comparable to that of (volcanic) basaltic lavas on the Earth. Hhowever, the
bulk density of the Earth is a much higher 5.5 g/cc because of the dense
iron/nickel core.
The Moon is coverered with a gently rolling layer of
powdery soil with scattered rocks that is called the regolith; it is made
from debris blasted out of the lunar craters by the meteor impacts that
created them.
One striking difference between the lunar surface
material and that of Earth concerns the most common kinds of rocks. On
Earth, the most common rocks are sedimentary, because of atmospheric and
water erosion of the surface. On the Moon, there is no atmosphere to speak
of and little or no water, and the most common kind of rock is igneous
("fire-formed rocks"). Geologically, the lunar surface material has the
following characteristics: The Maria are mostly composed of dark basalts,
which form from rapid cooling of molten rock from massive lava flows. The
Highlands rocks are largely Anorthosite, which is a kind of igneous rock
that forms when lava cools more slowly than in the case of basalts. This
implies that the rocks of the Maria and Highlands cooled at different rates
from the molten state and so were formed under different conditions.
Lunar Phases:
Questions
The monthly progression from new moon to full and then to new moon is the
most easily and widely recognized of the motions in the heavens. How can we
explain the phenomenon of lunar phases?
Where is the moon in relation to the sun when the moon is full?
Where is the moon in relation to the sun when the moon is new?
Where is the moon in relation to the sun when the moon is in either first or
third quarter phase?
Also, some more subtle questions to discuss are:
What is the shape of the line dividing light from dark (the terminator) on
the lunar surface?
Suppose the moon is in the 1st quarter phase. In what phase would the Earth
appear to an astronaut on the Moon? What about a full moon?
In what direction does the moon appear to move during the night?
In what direction does the moon appear to move from night to night?
If the new moon occurs when the moon lies between us and the sun while the full
moon occurs when we are between the moon and the sun, why do solar and lunar
eclipses not happen every month?
/ˈmunˌstrʌk/
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