Causes of the Tides
Gravitational pull or attraction of the Sun and the Moon is the single greatest cause of the tides.
It is easy to see that there is a tidal bulge beneath the moon as the ocean water is pulled towards the moon by gravity. The problem is in understanding why there is also a high tide on the opposite side of the earth at the same time. The cause of the second tidal bulge is the centrifugal force caused by the revolution of the Earth-Moon system.
On the side of the earth near the Moon, the gravitational force is greater than the centrifugal force, on the opposite side; the centrifugal force is greater than the gravitational force of the Moon. In this way, tidal bulges develop on moon facing side and the opposite side of the Earth. The former is primary flow tide and the latter as secondary flow tide.
In the same way, the Earth-Sun system also creates tide on the earth. But the strength of the gravitational attraction varies directly as the masses of the Sun and the Moon and inversely as the square of their distances from the Earth. Obviously, the distance between two objects is more important than their masses. As the Moon is far nearer to the Earth than the Sun, the gravitational attraction of the Moon is about 2.5 times greater than that of the Sun.
Spring tides: These tides relate to a twice-monthly tide of greater amplitude. Such type of tides occurs when the Moon, the Sun and the Earth remain in the same straight line. It may be in conjunction during new moon or opposition during full moon so that effects of gravitational pull of the Moon and the Sun are complementary. Normally, spring tides are approximately 20% greater than the mean tides.
Neap tides: These tides, 20% lower than mean tides, is marked by low amplitude occurring approximately at the time of the first quater and the third quarter of the Moon. This is produced when, the Sun and the Moon lie at right-angles with the Earth as the apex (i.e. in quadrature), so that tide-producing forces interfere with each other. This reduces the tidal range of Neap tides.
Time interval between tides: If the moon would have been stationary in relation to the Earth, Earth’s rotation once in about 24 hours would have caused any place to face the Moon once is 24 hours and two flow tides and two ebb tides in a day. But the Moon travels in its orbit in the same direction as the Earth’s rotation. In fact, the Moon revolves round the Earth in about 28 days. Thus by the time the Earth completes one rotation, the moon has travelled 13° (360° / 28 = 13°) approximately from its previous position in respect to a place on the earth. To cover a path of 13° in its orbit the Earth takes another 52 minutes (13 x 4 = 52). Thus a place on the Earth faces the moon after an interval of 24 h 52 min. The interval of time between a primary and a secondary flow tides becomes half of time interval between two primary flow tides i.e. 12 h 26 min. The duration of flow tide and ebb tides remains about 6 h 13 min.
