Moon And Eclipses (On The) This satellite revolves round the earth in the space of twenty-seven days eight hours, in an orbit nearly coinciding with the plane of the earth's orbit, and accompanies us in our revolution round the sun Her motion, therefore, is of a complicated nature; for, as the earth advances in her orbit, whilst the moon goes round her, the moon proceeds in a sort of progressive circle.
The moon always presents the same face to us, by which it is evident that she turns hut once upon her axis while she performs a revolution round the earth ; so that the inhabitants of the moon have but one day and one night in the course of a lunar month. Since we always see the same hemisphere of the moon, the inhabitants of that hemisphere alone can see the earth. One half of the moon, therefore, enjoys our light every night, while the other half has constantly nights of darkness; and we appear to the inhabitants of the moon under all the changes, or phases, which the moon exhibits to us.
When the moon is in the same direction from us as the sun, we cannot see her, as her dark side is towards us; but her disappearance is of very short duration, and as she advances in her orbit, we perceive her under the form of a new moon. When she had gone through one-sixth of her orbit, one quarter of her enlightened hemisphere will be turned towards the earth, and she will then appear horned. When she has performed one quarter of her orbit, she shows us one half of her enlightened side. She next appears gibbous; and after that full. As she proceeds in her orbit she becomes again gibbous, and her enlightened hemisphere turns gradually away from us, til! she completes her orbit and disappears ; and then again resumes her form of a new moon.
When the moon is full, she is always in opposition to the sun - when a new moon, in conjunction with it. At each of these times the sun. the moon, and the earth are in the same right line; hut in the first case, the earth is between the sun and the moon; in the second, the moon is between the sun and the earth An eclipse can only take place when the sun, moon, and earth are in a straight line, or nearly so. When the moon passes between the sun and the earth, she intercepts his rays, or in other words, casts a shadow on the earth : this is an eclipse of the sun, and it continues whilst the shadow is passing over us. When, on the contrary, the earth is between the sun and the moon, it is we who intercept the sun's rays, and. east a shadow on the moon; she then disappears from our view, and is eclipsed.
Why, it may be asked, have we not a solar and a lunar eclipse every month? Because the planes of the orbits of the earth and moon do not exactly coincide, but cross or intersect each other; and the moon generally posses either on one side or the other, when she is in conjunction with, or in opposition to, the sun ; and, therefore, does not intercept the sun's rays, or produce an eclipse; for this can only take place when the earth and moon are in conjunction near those parts of their orbits which cross each other (called the nodes of their orbits), because it is then only that they are both in the same plane, and in a right line with the sun. A partial eclipse takes place when the moon, in passing by the earth, does not entirely escape her shadow. When the eclipse happens precisely at the nodes, they are not only total, hut last for some length of time.
When the sun is eclipsed, the total darkness is confined to one particular spot of the earth, as the moon's shadow is not large enough to cover the earth. The lunar eclipses, on the contrary, are visible from every part of the earth, where the moon is above the horizon.