5. The Moon: stepping stone to the planets
The Moon's face
When a full Moon rises or sets, it is a captivating sight. It looks huge, dwarfing everything in the foreground. But appearances can be deceiving. The Moon is no bigger when it is close to the horizon than when it is high in the sky. Its changing size is an illusion caused by comparing the Moon to other objects when it is viewed along the ground.
This so-called Moon illusion arises from the way that the brain deals with apparent distance, not size. When people view the Moon near the horizon, there are large foreground objects, such as trees, buildings and hills, for comparison, so the Moon looks very far away and huge. When the Moon is overhead, alone in an otherwise empty sky, there are no other objects to gauge its distance; the Moon then appears to be closer and we think it is smaller than at the horizon.
The Moon is the only planetary body that can be distinguished with the unaided eye as a globe, and even without a telescope you can tell that its surface is not uniform. Its face contains large, irregular features of light and dark material, familiarly known as "The Man in the Moon".
The Moon's rough terrain is mostly confined to the brighter regions that Galileo called terrrae, Latin for "lands"; they are now known as the highlands because they are higher than the dark regions. Galileo also discovered that the dark patches are smooth and level, resembling seas seen from a distance. He called them maria, the Latin word for "seas"; mare, pronounced "MAHrey", is the singular for "sea". We now know there is no water on the Moon there. The dark maria cover about 17 percent of the lunar surface. When spacecraft were sent past the Moon to look at its averted face, they found that the far side contains very few maria.
Chemical examination of rock samples returned from the Moon has shown that the maria are ancient volcanic outflows composed of dark lava. This material flowed out from inside the Moon to fill large impact basins that were formed at about the same time as the lunar highlands. One of them, the Imbrium Basin that contains Mare Imbrium, now forms the "eyesocket" in the face of the "Man on the Moon"; it has a diameter of 1.5 million meters.
Craters form one of the most striking features of the Moon's landscape. The word crater is derived from the Greek word for "cup or bowl", and it is a good description of the bowl-shaped depressions. They are just beyond the limit of visibility with the unaided eye, but a pair of binoculars will reveal a few of the larger ones. When seen through a telescope, the bright highlands are resolved into an enormous number of overlapping craters that have been visible to generations of telescopic observers.
At around the time of full Moon, a pair of binoculars will also show bright streaks that radiate from several craters like the spokes of a wheel. These are the lunar rays, and they were produced by the debris of crater-formation. Some of the rays go more than one-quarter of the way around the Moon.
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Copyright 2010, Professor Kenneth R. Lang, Tufts University