The Physical Moon and its History


The Moon can move within 218,000 miles (351,000 km) of the earth's surface. This proximity to the earth produces reactions in the earth's atmosphere, seas and the earth's surface. The most observable reaction occurs with the ocean's tides. The tides are due to the gravitational attraction of the Sun and Moon. The Earth's great oceans are pulled regularly to and fro by the Moon, as well as the fluids in our very own bodies. High and low tides occur in a roughly 6 hour cycle, with two high and two low tides a day. This 6 hour cycle is caused by the gravitational tugging of the moon upon the earth.

The Moon's gravity pulls the Earth's water surface into two bulges, one on the side facing the Moon and the other on the opposite side. The water on the side of the Earth closest to the Moon is pulled, by the Moon's gravitational force, much stronger than the mass of the Earth. The water on the side furthest from the Moon has a weaker pull than the gravitational force of the Earth. As the earth rotates under this ocean bulge, it causes a high tide. The height of the tide is determined by the shape of the coastline and of the nearby continental shelf.

A change in the gravitational force across the body of the earth produces two bulges, causing us to experience two high tides, and two low tides each day. The tides make the earth slow down its rotation around its axis, and when the angular force of the earth decreases the moon compensates by increasing its angular momentum.

The effect of the Sun's pull also causes similar effects. Twice a month, at new and full Moon, the Sun and Moon pull in line so that the tidal bulge occurs on both sides of the earth. One bulge occurs toward the Moon and the other away from the Moon. The tides that we see at this time are the result of both pulls. Their tidal effects combine to produce tides higher than normal called Spring Tides, because they "spring up".

When the solar low tide and lunar low tides occur simultaneously , it produces what is known as a Neap Tide. This occurs at first or last quarter, when the Moon is pulling at right angles to the sun. The tidal effects work against each other and produce tides of a smaller range, called "neap", or scanty tides. As the earth rotates under these two bulges, points on its surface experience a cycle of low tides twice a day. The actual height of the tide is partly determined by the shape of the coastline and depth of water.

Tidal Information



Observational Data Interactive Navigation

NOAA CO-OPS provides an interactive map to view oceanographic data for tides and currents. Zoom into specific regions, from the East, West and Gulf coasts to Alaska, the Pacific, Great Lakes and the Caribean.

Religious rites and festivals have surrounded the Moon's rhythmic cycle with many predictions based on this cycle. Fishing predictions for both fresh and saltwater anglers are based partly on the Moon's rising and setting times and phases of the moon. If the fishing is in a saltwater area then predictions are amplified by tidal flows. The Old Farmers Almanac has provided planting guides, and other information based on phases of the Moon since 1792.

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