How the Moon Causes Tides.—The cause of tides is the attrac tion of the moon and of the sun. The reason why they occur at regular intervals is the rotation of the earth and the revolution of the moon around the earth. Gravitation tends to bring any two particles of matter together, and the tendency is stronger the nearer the particles. But the movements of the earth and the moon in their orbits keep the two bodies apart even though their relative distances vary continually. Nevertheless the moon's gram itation is able to distort the surface of the ocean. A water sur face always places itself at right angles to the pull of gravitation. Since the moon as well as the earth exerts a gravitational pull, the surface of the ocean or of any other body of water must place itself at right angles to the combined strong pull of the earth and weak pull of the moon. But the strength and direction of the moon's gravitational pull keep changing, because the earth's rota tion, as well as the moon's own revolution around the earth, introduce constant and regular variations. Suppose that the surface of a section of the ocean were a vast sheet of curved glass. The varying direction of the moon's gravitation, or more specifically the so-called horizontal component of that pull, which is the part that causes the tide, may be thought of as tipping the sheet first one way and then another. Thus one side is raised a little while the opposite side is depressed and the central portions remain stationary. A complicated series of warpings like this is the primary cause of the tides. The size of the areas which act as units depends on the depth and configuration of the oceans. Moreover, when a so called " stationary " wave of the kind here described has once been started it progresses outward like any other wave. The result is an extremely complicated series of tidal waves moving in all directions according to the part of the ocean which one happens to observe. Often a tide lags many hours behind the condition of the moon which caused it, and in deep bays there may even be two tides at the same time. For example, as a tidal wave pro gresses up Chespeake Bay from Old Point Comfort the shallow ness of the bay hinders it so much that by the time it reaches the head of the bay north of Baltimore, a second tide has entered the lower part of the bay.
How the Sun Modifies the Tides.—The sun causes tides like those of the moon, but in most places not so high: The usual way in which they become apparent is by increasing or decreasing the lunar tides, as appears in Figs. 12 and 13. When moon, earth, and sun are in a straight line at full moon or new moon (Fig. 12) the two tides combine so that the high tides are higher than usual and the low tides lower. These are spring tides. When the sun and moon are at right angles to one another as seen from the earth (Fig. 13), they partially counteract one another so that neap tides neither rise so high nor fall so low as ordinary tides. In Figs. 12 and 13 it should be noted that the high tides are shown 90 degrees from the sun or moon which cause them, thus allowing for a certain lag which is usually in evidence. At ports where the harbor bars are just pass able at high tide, a ship may have to wait some days if it happens to arrive at neap tide. The exact time of occurrence of either spring or neap tides varies from place to place, and in some regions may be as much as five days before or after the combination of lunar and solar activity which causes it.
The Construction of Tide Tables.—The chief practical applica tion of our knowledge of how the moon and sun influence tides lies in the construction of tide and current tables. These depend not only on the relative positions of the sun and moon but on the variations in the height of these bodies above the horizon at noon in different seasons. These cause such complex relations that they require laborious calculations which are sometimes performed by means of mechanical devices; these sum up all the different effects and determine for years in advance how high the normal tide will be in any given place at any given time. The tides at any
given place can only be predicted after observations have been made for at least a month, and they have to be separately computed for each port. The tides at nearby places can be roughly deduced from those at the principal ports. The alterations in the usual course of the tides because of storms and winds, however, cannot readily be predicted. At London, for example, a storm with east winds has been known to make the tide five feet higher than was predicted.
How Tides Improve Harbors.—Tides have an important effect upon harbors. Many ship channels such as those of New York, Boston, and Liverpool are kept from silting up by the tidal currents which scour them out daily. In many cases where it has not been worth while to dredge channels the tide enables ships to enter har bors which would otherwise be inaccessible. Off the mouth of most rivers there is a narrow zone where the sediment brought by the river is largely deposited, and forms bars. The depth over the bar is just. enough to allow the water from the river to pass over it at all times. Where there are tides, the depth at low water is the same as it would be at all times if there were no tides, while at high water the depth is correspondingly greater. Thus harbors like Bangkok in Siam, and Liverpool in its natural state, which would not be deep enough if there were no tides, admit ocean liners because of the depth at high tide.
Revolution of the around the Sun.—Thus far we have been studying the effect of the earth's rotation. Now, we are to consider the earth in its varying positions in its path around the sun. The earth not only rotates on its own axis, but revolves around the sun in an enormous and practically circular path at a distance of about 93,000,000 miles from that body. Fig. 14 represents the size that this path, or orbit, would have if the sun were the size of the little dot in the center. The earth is so small that on this scale twenty earths would be needed side by side to equal the thickness of the thinnest part of the line representing the orbit.
How to Show the Earth's Changing Attitude toward the The earth's revolution around the sun would make little difference to mankind if the axis on which the earth rotates were vertical to the plane of the orbit around the sun. As a matter of fact, however, the axis is tilted, and hence the earth's revolution causes seasons. The tilting of the axis may be understood from Fig. 14. Here the plane of the orbit coincides with the page. Let the earth's axis be repre sented by a pin around which we will imagine that there is a tiny rotating sphere representing the earth, and lying in the plane of the page. Set the pin perpendicular to the page at any point on the circle in Fig. 14. Now carry the pin around the circle or orbit, keeping it perpendicular to the page all the time. Wherever the pin may be, the relation of the earth to the central sun remains the same. That is, some part of the equatorial region of our imaginary earth always faces the sun, and neither pole has any special advantage. Now tip the pin so that its head points toward a certain point in the ceiling on the farther side of the room. Set the pin at various places on the circle with its head always pointing toward this same point in the ceiling, which corresponds to the North Star. On one side of the circle the northern hemisphere will incline toward the sun while the southern hemisphere will incline away from the sun. On the other side of the orbit the conditions will be reversed, for the northern hemisphere will incline away from the sun, while the southern will incline toward it.