In starcraft, when the Earth passes across the Sun, and the Moon is hidden in its shadow, we say that it is an eclipse of the Moon; or when the Moon passes across the Sun and covers it we say that there has been an eclipse of the Sun.
How eclipses of the Sun and of the Moon are caused can be shown by a very simple experiment and you ought to make it.
An Eclipse of the Moon.—First let us suppose it is the Moon which is eclipsed by the Earth. All that is needed to show how this is done is a lighted lamp on a table and an apple with a knitting needle through its center. Let the flame of the lamp represent the Sun, your head the Earth and the apple the Moon.
Fig. 119.—Eclipse of the Moon by the Earth
(Experiment).
Hold the apple by the knitting needle in a line between your eyes and the flame and turn round just as you did in the experiment showing the phases of the Moon. When you have turned halfway round the apple will be in the shadow of your head, when the apple is eclipsed, as shown in [Fig. 119].
This is just what happens when the Earth gets between the Sun and the Moon and all these bodies are in a straight line, as shown in [Fig. 120]; the Moon will then be in the shadow of the Earth, and thus it is that the Moon is eclipsed.
Fig. 120.—Moon Eclipsed by the Earth
(Diagram).
If the Sun, Earth and Moon were always in the same plane with each other, every time the Earth passed between the Sun and the Moon the latter would be plunged into the Earth’s shadow and the Moon would be eclipsed. But the Moon does not revolve round the Earth so that it and the Earth make a straight line with the Sun every revolution for the reason that the Moon’s orbit is tilted a little and hence it is only once in a while that all of these bodies get in a straight line with each other, and when these times do happen then eclipses are produced.