Sand-dunes are composed of drift sand thrown up by the waves of the sea, and blown, when dry, to some distance inland, until it is stopped by large stones, tree roots or other obstacles. It gradually accumulates around these, until the heaps become very large, often forming dunes or sand-hills.
What do We Mean by an “Eclipse”?
Any good dictionary will tell us that an eclipse is an interception or obscuration of the light of the sun, moon or other heavenly body by the intervention of another and non-luminous heavenly body. Stars and planets may suffer eclipse, but the principal eclipses are those of the sun and the moon.
An eclipse of the moon is an obscuration of the light of the moon occasioned by the interposition of the earth between the sun and the moon; consequently all eclipses of the moon happen at full moon; for it is only when the moon is on that side of the earth which is turned away from the sun, and directly opposite, that it can come within the earth’s shadow. Further, the moon must at that time be in the same plane as the earth’s shadow; that is, the plane of the ecliptic in which the latter always moves. But as the moon’s orbit makes an angle of more than five degrees with the plane of the ecliptic, it frequently happens that though the moon is in opposition it does not come within the shadow of the earth.
Diagrams Illustrating the Theory of Eclipses.
The theory of lunar eclipses will be understood from [Fig. 1], where S represents the sun, E the earth, and M the moon. If the sun were a point of light there would be a sharply outlined shadow or umbra only, but since the luminous surface is so large, there is always a region in which the light of the sun is only partially cut off by the earth, which region is known as the penumbra (P P). Hence during a lunar eclipse the moon first enters the penumbra, then is totally eclipsed by the umbra, then emerges through the penumbra again.
An eclipse of the sun is an occultation of the whole or part of the face of the sun occasioned by an interposition of the moon between the earth and the sun; thus all eclipses of the sun happen at the time of new moon.