The Story of Eclipses

by George F. Chambers

CHAPTER XV.

ECLIPSES OF THE MOON—GENERAL PRINCIPLES.

In dealing with eclipses generally, but with more especial reference to eclipses of the Sun, in a previous chapter, it was unavoidable to mix up in some degree eclipses of the Moon with those of the Sun. There are, however, distinctions between the two phenomena which make it convenient to separate them as much as possible. Eclipses of the Moon are, like those of the Sun, divisible into “partial” and “total” eclipses, but those words have a different application in regard to eclipses of the Moon from what they have when eclipses of the Sun are in question. A little thought will soon make it clear why this should be the case. A partial eclipse of the Sun results from the visible body of the Sun being in part concealed from us by the solid body of the Moon, and so in a total eclipse there is total concealment of the one object by the other.

But when we come to deal with partial and total eclipses of the Moon, the situation, is materially different. The Moon becomes invisible by passing into the dark shadow cast by the Earth into space.

Fig. 13 will make this clear without the necessity of much verbal explanation. S represents the Sun, E the Earth, and mn the orbit of the Moon. It is obvious that whilst the Moon is moving from m to n it becomes immersed in the Earth’s shadow. But before actually reaching the shadow the Moon passes through a point in its orbit at which it begins to lose the full light of the Sun. This is the entrance into the “penumbra” (or “Partial shade”). Similarly, after the eclipse, when the Moon has emerged from the full shadow it does not all at once come into full sunshine, but again passes through the stage of penumbral illumination,[112] and under such circumstances (to speak in the style of Old “Oireland”) the invisible Moon is very often not invisible, and the part partially eclipsed is often not eclipsed, and when the Moon is totally eclipsed it is frequently still visible. Of course the general idea involved in all cases of a body passing into the shadow of another body is that the body which so passes disappears, because all direct light is cut off from it. In the case, however, of a lunar eclipse this state of things is not always literally accomplished, and very often some residual light reaches the Moon (of course from the Sun) with the result that traces of the Moon may often be discerned. The laws which govern this matter are very ill-understood. The fact remains that if we examine a series of reports of observed eclipses of the Moon extending over many centuries (and records exist which enable us to do this) we shall find that in some instances when the Moon was “totally” eclipsed in the technical sense of that word, it was still perfectly visible, whilst during other eclipses it absolutely and entirely disappeared from view. Such eclipses are sometimes spoken of as “black” eclipses of the Moon, but the phrase is not a happy one. Many instances of both kinds will be found mentioned in the chapter on historical lunar eclipses.[113]

The different conditions of eclipses of the Moon are illustrated by Fig. 14 which must be studied with the aid of the remarks made in a former chapter concerning the apparent movements of the Sun and Moon and their nodal passages. Suffice it to state here that in Fig. 14 AB represents the ecliptic, and CD the Moon’s path. The three black circles are imaginary sections of the Earth’s shadow as cast when the Earth is in three successive positions in the ecliptic. If when the Earth’s shadow is near A the Moon should be at E, and in Conjunction with the Earth the Moon will escape eclipse; if the Conjunction takes place with both the Earth’s shadow and the Moon a little further forward, say at F, the Moon will be partially obscured; but if the Moon is at or very near its node, as at G, it will be wholly involved in the Earth’s shadow and a total eclipse will be the result. In the case contemplated at G in the diagram, the Moon is concentrically placed with respect to the shadow, but the eclipse will equally be total even though the two bodies are not concentrically disposed, so long as the Moon is wholly within the cone of the Earth’s shadow.[114]

Just as in the case of the Sun so with the Moon there are certain limits on the ecliptic within which eclipses of the Moon may take place, other (narrower) limits within which they must take place, and again other limits beyond which they cannot take place. Reverting to what has been said on a previous page[115] with respect to these matters when an eclipse of the Sun is in question it is only necessary to substitute for the word “Conjunction,” the word “Opposition”; and for 18½° and 15¼° of longitude the figures 12½° and 9¼°. The limits in latitude will be 1° 3′ and 0° 52′ instead of 1° 34′ and 1° 23′. These substitutions made, the general ideas and facts stated with regard to the conditions of an eclipse of the Sun will apply also to the one of the Moon.

It is to be noted that whereas eclipses of the Sun always begin on the W. side of the Sun, eclipses of the Moon begin on the E. side of the Moon. This difference arises from the fact that the Sun’s movement in the ecliptic is only apparent (it being the Earth which really moves), whilst the Moon’s movement is real.