"Magnus ab integro sæclorum nascitur ordo.
Jam redit et Virgo, redeunt Saturnia regna;
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Alter erit tum Tiphys, et altera quæ vehat Argo
Delectos heroas; erunt etiam altera bella,
Atque iterum ad Trojam magnus mittetur Achilles."
We have seen that the orbit of the earth is an ellipse, and that the sun is situated at what is called the focus, a point not in the middle of the ellipse, but rather towards one of its ends. Therefore, during the course of the year the distance of the earth from the sun varies. The sun, in consequence of this, is about 3,000,000 miles nearer to us in our northern winter than it is in our northern summer, a statement which sounds somewhat paradoxical. This variation in distance, large as it appears in figures, can, however, not be productive of much alteration in the amount of solar heat which we receive, for during the first week in January, when the distance is least, the sun only looks about one-eighteenth broader than at the commencement of July, when the distance is greatest. The great disparity in temperature between winter and summer depends, as we have seen, upon causes of quite another kind, and varies between such wide limits that the effects of this slight alteration in the distance of the sun from the earth may be neglected for practical purposes.
The Tides are caused by the gravitational pull of the sun and moon upon the water of the earth's surface. Of the two, the moon, being so much the nearer, exerts the stronger pull, and therefore may be regarded as the chief cause of the tides. This pull always draws that portion of the water, which happens to be right underneath the moon at the time, into a heap; and there is also a second heaping of water at the same moment at the contrary side of the earth, the reasons for which can be shown mathematically, but cannot be conveniently dealt with here.
As the earth rotates on its axis each portion of its surface passes beneath the moon, and is swelled up by this pull; the watery portions being, however, the only ones to yield visibly. A similar swelling up, as we have seen, takes place at the point exactly away from the moon. Thus each portion of our globe is borne by the rotation through two "tide-areas" every day, and this is the reason why there are two tides during every twenty-four hours.
The crest of the watery swelling is known as high tide. The journey of the moon around the earth takes about a month, and this brings her past each place in turn by about fifty minutes later each day, which is the reason why high tide is usually about twenty-five minutes later each time.
The moon is, however, not the sole cause of the tides, but the sun, as we have said, has a part in the matter also. When it is new moon the gravitational attractions of both sun and moon are clearly acting together from precisely the same direction, and, therefore, the tide will be pulled up higher than at other times. At full moon, too, the same thing happens; for, although the bodies are now acting from opposite directions, they do not neutralise each other's pulls as one might imagine, since the sun, in the same manner as the moon, produces a tide both under it and also at the opposite side of the earth. Thus both these tides are actually increased in height. The exceptionally high tides which we experience at new and full moons are known as Spring Tides, in contradistinction to the minimum high tides, which are known as Neap Tides.
The ancients appear to have had some idea of the cause of the tides. It is said that as early as 1000 B.C. the Chinese noticed that the moon exerted an influence upon the waters of the sea. The Greeks and Romans, too, had noticed the same thing; and Cæsar tells us that when he was embarking his troops for Britain the tide was high because the moon was full. Pliny went even further than this, in recognising a similar connection between the waters and the sun.
From casual observation one is inclined to suppose that the high tide always rises many feet. But that this is not the case is evidenced by the fact that the tides in the midst of the great oceans are only from three to four feet high. However, in the seas and straits around our Isles, for instance, the tides rise very many feet indeed, but this is merely owing to the extra heaping up which the large volumes of water undergo in forcing their passage through narrow channels.
As the earth, in rotating, is continually passing through these tide-areas, one might expect that the friction thus set up would tend to slow down the rotation itself. Such a slowing down, or "tidal drag," as it is called, is indeed continually going on; but the effects produced are so exceedingly minute that it will take many millions of years to make the rotation appreciably slower, and so to lengthen the day.
Recently it has been proved that the axis of the earth is subject to a very small displacement, or rather, "wobbling," in the course of a period of somewhat over a year. As a consequence of this, the pole shifts its place through a circle of, roughly, a few yards in width during the time in question. This movement is, perhaps, the combined result of two causes. One of these is the change of place during the year of large masses of material upon our earth; such as occurs, for instance, when ice and snow melt, or when atmospheric and ocean currents transport from place to place great bodies of air and water. The other cause is supposed to be the fact that the earth is not absolutely rigid, and so yields to certain strains upon it. In the course of investigation of this latter point the interesting conclusion has been reached by the famous American astronomer, Professor Simon Newcomb, that our globe as a whole is a little more rigid than steel.