At times when the crescent is bright the whole dark disk of the Moon can be seen glowing dimly with a reddish, copper color, and this is called the old Moon in the new Moon’s arms. This copper-colored glow is the Earth-shine on the Moon, that is, the sunlight on the Earth, which is reflected to the Moon and back again to us.
On one side of the Moon you may be able to see a dark oval spot which is marked Grimaldi on the maps of the Moon. Grimaldi is a great plain, having nearly 14,000 square miles in it, with mountains flanking it on the sides. This is another good eyesight test, for it takes a mighty sharp eye to see it without a glass.
These and a dozen other interesting things on the Moon can be seen without a telescope.
The Motions of the Moon.—The Moon turns round on its axis once every month and it also revolves round the Earth once every month, so that the Moon’s day and year are of the same length just like Mercury and Venus, and this is the reason that one side of the Moon is always turned toward the Earth, as you will see if you look at her through a glass.
Fig. 107.—The Experiment Showing How One Revolution of the
Moon round the Earth Makes it Turn Once round its Axis.
A simple experiment will show the cause of this: Place an apple, which we will call the Earth, on the bottom of an inverted glass on a table, and draw a chalk circle a foot in diameter around it. Next, take a tablespoon to represent the Moon, and hold it upright with the point of its bowl on the chalk line and with the bowl turned toward the apple, as shown in [Fig. 107]. Now, draw the spoon round the circle, turning it in your fingers so that the bowl is always toward the apple.
It is easy to see that in order for the bowl of the spoon to be turned toward the apple during all of its travels round the chalk circle the spoon must also turn once round on its own axis, and this is the reason we always see the same side of the Moon.
The Moon’s Phases.—Since one side of the Moon is always turned toward the Earth, it is clear that this is the only side we can ever see, and, further, we are only able to see as much of this side as is made bright by the Sun’s light.
