Tidal Bulges: The Moon’s Effect
It is easier to understand how tides on Earth are created if we consider an ideal Earth and an ideal ocean. The ideal Earth has two tidal bulges, one toward the Moon and one away from the Moon (called the lunar bulges).The ideal ocean has a uniform depth, with no friction between the seawater and the sea floor. Newton made these same simplifications when he first explained Earth’s tides. If the Moon is stationary and aligned with the ideal Earth’s equator, the maximum bulge will occur on the equator on opposite sides of Earth. If you were standing on the equator, you would experience two high tides each day. The time between high tides, which is the tidal period, would be 12 hours. If you moved to any latitude north or south of the equator, you would experience the same tidal period, but the high tides would be less high, because you would be at a lower point on the bulge. In most places on Earth, however, high tides occur every 12 hours 25 minutes because tides depend on the lunar day, not the solar day. The lunar day (also called a tidal day) is measured from the time the Moon is on the meridian of an observer— that is, directly overhead—to the next time the Moon is on that meridian and is 24 hours 50 minutes.3The solar day is measured from the time the Sun is on the meridian of an observer to the next time the Sun is on that meridian and is 24 hours.
Here the tides reduce to zero amplitude a day or so after the Moon’s declination is zero, increasing to their largest values when the Moon is at its greatest declination, either north or south of the equator.
The tidal ranges on the relatively shallow continental shelves are usually larger than those of the oceans. However, very small tidal ranges are observed in some shallow areas, often accompanied by curious distortions of the normal tidal patterns.