Astronomy for Everybody/Part 3/Chapter 5

The reader is doubtless aware that an eclipse of the moon is caused by that body entering the shadow of the earth, and that an eclipse of the sun is caused by the moon passing between us and the sun. Taking this knowledge for granted, we shall explain the more interesting features of these phenomena and the laws of their recurrence.

The first question to be considered is: Why is there not an eclipse of the moon at every full moon, since the earth's shadow must always be in its place opposite the

sun? The answer is that the moon commonly passes either above or below the shadow of the earth, and so fails to be eclipsed. This, again, arises from the fact that the orbit of the moon has a small inclination, about five degrees, to the plane of the ecliptic, in which the earth moves, and in which the centre of the shadow always lies. Returning to our former thought of the ecliptic being ​marked out on the celestial sphere, let us suppose that we also mark out the orbit of the moon during the course of its monthly period. We should then find the orbit of the moon crossing that of the sun in two opposite points, at the very small angle of five degrees. These points of crossing are called nodes. At one node the moon passes from below, or south of the ecliptic, to the north of it. This is called the ascending node. At the other the moon passes from north to south of the ecliptic. This is called the descending node. The terms ascending and descending are applied to the node, because to us in the northern hemisphere, the north side of the ecliptic and equator seem to be above the south side.

At the points halfway between the nodes the centre of the moon is above the ecliptic by about one twelfth its distance from us, that is, by about twenty thousand miles. The sun being larger than the earth, the shadow of the latter gradually grows smaller away from the earth. At the distance of the moon its diameter is about three fourths that of the earth, that is about six thousand miles. Its centre being in the plane of the ecliptic, it extends only about three thousand miles above and below that plane. Hence it is that the moon will pass through it only when near the nodes.

The line joining the sun and moon of course turns round as the earth moves around the sun. It therefore crosses the moon's nodes twice in the course of a year. That is to say if we supose the nodes to be marked in the ​sky, the ascending node at one point, and the descending node at the opposite point, then the sun will appear to us to pass each of these points in the course of a year. While the sun is passing one node the shadow of the earth will seem to be passing the other. It is only near these two times of the year that an eclipse of the sun or moon can occur. We may therefore call them eclipse seasons. They commonly last about a month; that is to say it is generally about a month from the time when the sun gets near enough to a node to allow of an eclipse until the time when it is too far past for an eclipse to occur. In 1901 the seasons were May and November.

If the moon's node stayed in the same place in the sky, eclipses would occur only some time during these two months. But, owing to the attraction of the sun on the earth and moon, the position of the nodes is continually changing in a direction opposite that of the motion of the two bodies. Each node makes a complete revolution around the celestial sphere in eighteen years and seven months. Hence in this same period the eclipse seasons will course all through the year. On an average they occur about nineteen days earlier every year than they did the year before. Thus it happens that in 1903 one season occurs in March and April and the other season in September and October. The change will keep going on until, in the year 1910, the season which in 1901 was in May will have gotten back to November, while the November one will have gotten back to May, each having passed through all the intermediate months, and the two ​having changed places. By 1919 each will have made an entire revolution through the year.

Let us imagine ourselves to be looking at the sun and earth from the moon when the latter is about to enter the earth's shadow. The earth, looking much larger than the sun, will be seen to approach it, and at length will begin to impinge on its disk and cut off a part of its light. The region within which this will occur is called the penumbra, and it is shown outside the shadow in the figure. So long as the moon is only in this region, an

ordinary observer would not notice any diminution in its light, although such a diminution could be detected by exact photometric measurements. The moon is not said to be eclipsed until it begins to enter into the actual shadow, where the whole direct light of the sun is cut off.

If we watch the moon when an eclipse is about to begin, we shall see a small portion of her eastern edge gradually grow dim and finally disappear. As the moon advances in her orbit, more and more of her face thus disappears from view by entering into the shadow. If, however, we look very carefully, we shall see that the part ​immersed in the shadow has not entirely disappeared, but shines with a very faint light. If the whole body of the moon enters into the shadow, the eclipse is said to be total; if only a portion of her body dips into the shadow, it is called partial. If the eclipse is total, the light which illuminates the eclipsed moon will be very plainly seen, because it is not drowned out by the dazzling light of the uneclipsed portion. This light is of a dingy red colour, and arises from the refraction of the earth's atmosphere, which was described in a former chapter. In consequence of this, those rays of the sun which just graze the earth, or pass within a short distance of its surface, are bent out of their course and thrown into the shadow by refraction. Thus they fill the shadow and fall on the moon. The red colour is due to the same cause that makes the sun appear red at sunset, namely, the absorption of the green and blue rays by the atmosphere, which lets the red rays pass.

Two or three eclipses of the moon occur every year, of which one, at least, is nearly always total. But, of course, the eclipse will be visible only in that hemisphere of the earth on which the moon is shining at the time.

When the moon is eclipsed an observer on that body would see an eclipse of the sun by the earth. The cause of the phenomenon we have described would then be plain enough to him. The apparent size of the earth would be much larger than that of the moon as we see it. Its diameter would be between three and four times that of the sun. At first this immense body would be invisible when it approached the sun. What the observer would see would be the cutting off of the light of the sun by the ​advancing but invisible earth. When the latter had nearly covered the sun, its whole outline would be shown to him by a red light surrounding it, caused by the refraction of the earth's atmosphere. Finally, when the last trace of true sunlight had disappeared, nothing would be visible but this ring of bright red light having inside of it the black but otherwise invisible body of the earth.

The circumstances of an eclipse of the moon are quite different from those of a solar eclipse, to be described in the next chapter. It can aways be seen at the same instant over the whole hemisphere of the earth on which the moon is shining at the time. A curious phenomenon occurs when the moon rises totally eclipsed. Then we may see it on one horizon, say the eastern one, while the sun is still visible on the western horizon. The explanation of this seeming paradox is that both bodies are really below the horizon, but are so elevated by refraction that we can see them at the same time.