Tides

The side of the Earth that faces the Moon experiences a larger gravitational pull, due to the Moon's closer proximity, than other parts of the Earth. This force causes the Earth itself to bulge slightly toward the Moon. This bulge is termed an Earth tide. Since water is free to move horizontally, the oceans tend to flow over the Earth's surface and collect in a secondary bulge on top of the Earth tide; this water bulge is termed an ocean tide.

At the same time, an Earth tide and an ocean tide form on the opposite side of the Earth, directly away from the Moon. This second bulge forms as follows (focusing on the ocean tide alone, for clarity): The Moon and the Earth, like all pairs of bodies orbiting each other in space, actually orbit around their common center of mass (that is, the point where, if their individual centers were attached to opposite ends of a rigid stick, the stick could be suspended from a string and remain in balance).

In the case of the Earth-Moon system, the common center of mass happens to be inside Earth, about 1068 miles (663 km) beneath the surface along a line connecting the center of the Earth to the center of the Moon. As the Earth and Moon revolve around this point like dancers spinning with linked hands, all points on both bodies experience a centrifugal force. This centrifugal force has the same magnitude and direction at every point on and in the Earth (i.e., away from the Moon parallel to a line connecting the center of the Earth to the center of the Moon). Where Earth's surface is at any angle other than 90° to the line connecting the center of the Earth to the center of the Moon, water experiences a horizontal component of this centrifugal force. On the half of Earth's surface facing away from the Moon, this horizontal force overcomes the pull of the Moon's gravity and causes water to flow over the Earth's surface to a point on the side of the Earth directly opposite the Moon-facing tidal bulge. A second tidal bulge thus form on the side of the Earth facing directly away from the Moon. This bulge is slightly smaller than the Moon-facing bulge because the imbalance between the Moon's gravitation and centrifugal force is smaller at this point. (The Moon is closer to the Moon-facing bulge, making its gravitation stronger there, whereas the centrifugal force considered here is the same everywhere on the Earth.) The larger, Moon-facing tide is termed the direct tide; the tide on the opposite side of the Earth is termed the opposite tide.

These two tidal bulges—one Moon-facing or direct, the other on the opposite side of the Earth—are the high tides. Because the Earth is spherical, these bulges are actually arcs, meeting at the poles to form a globe-girdling ring or belt of high tide aligned with the Moon. (Centrifugal force and the Moon's gravity cancel exactly at the poles, so the high tide is, in this simplified model, highest at the equator and diminishes to zero toward the poles.) Movement of water to this high-tide belt causes a complementary belt of low water to form around the Earth at 90° to the line connecting the centers of the Earth and Moon. This belt produces the phenomenon known as low tide.

The high-tide belt always lies along the line connecting the centers of the Earth and Moon; however, as the Earth rotates daily on its axis, land areas approach this belt, pass through it, and leave it behind. Thus, from the point of view of an observer fixed to the surface of the rotating Earth, the ocean tides are continually sloshing up against some coastlines and draining away from others. As a result, most coastal areas experience two high tides and two low tides each day. One high tide corresponds to the high-tide arc facing the Moon, and the other to the high-tide arc facing away from the Moon.

The Sun forms similar tidal bulges in the Earth and its oceans, one set due to gravitation and the other to centrifugal force. However, the Sun's tidal effect is slightly less than one half that of the Moon. (It is both more massive than the Moon and more distant; distance wins.) As the Moon orbits the Earth every 28 days, it twice comes into alignment with the Earth and Sun—once when it is directly between the Earth and the Sun (i.e., when observers on Earth see the shadowed side of the Moon) and once when the Earth is directly between itself and the Sun (i.e., when observers on Earth see the illuminated or "full" side of the Moon). When the Moon and Sun are aligned, their tidal forces add up to produce a maximum tidal change. These maximal tides are termed spring tides because the waters of the ocean "spring up" higher (and sink lower) at these times. When the Moon and Sun are at right angles to each other (i.e., when the Moon is half-illuminated as seen from Earth), the solar and lunar tidal bulges do not add, and the least dramatic tides of the month are observed. These are termed neap tides.