Geography · Ch 19 — Movements of Ocean Water
Tides based on the Sun, Moon and the Earth Positions
Tides based on the Sun, Moon and the Earth Positions
The height of a high tide is not constant. It changes noticeably depending on where the sun and moon stand relative to the earth. This is the core idea behind classifying tides as spring tides and neap tides.
Spring tides
When the sun, the moon and the earth line up in a straight line, the tide rises higher than usual. These are called spring tides. They occur twice a month — once during the full moon period and once during the new moon period. In both positions, the gravitational pulls of the sun and moon act along the same line, reinforcing each other, and that is why the tidal height is greater.
Neap tides
There is normally a gap of about seven days between a spring tide and a neap tide. At this stage, the sun and the moon are at right angles to each other relative to the earth. Their forces then tend to counteract one another. The moon's attraction is more than twice as strong as the sun's, but during a neap tide this pull is weakened by the sun's opposing gravitational force. The result is a lower tidal range than normal.
Effect of the moon's distance from the earth
The moon's orbit is not a perfect circle, so its distance from the earth changes. Once a month, when the moon comes closest to the earth, a position called perigee, unusually high and unusually low tides occur. At this time the tidal range is greater than normal. About two weeks later, when the moon is farthest from the earth, a position called apogee, the moon's gravitational force is limited and the tidal ranges fall below their average heights.
Effect of the earth's distance from the sun …
The height of rising water (high tide) varies appreciably depending upon the position of sun and moon with respect to the earth. Spring tides and neap tides come under this category.
Spring tides: The position of both the sun and the moon in relation to the earth has direct bearing on tide height. When the sun, the moon and the earth are in a straight line, the height of the tide will be higher. These are called spring tides and they occur twice a month, one on full moon period and another during new moon period.
Neap tides: Normally, there is a seven day interval between the spring tides and neap tides. At this time the sun and moon are at right angles to each other and the forces of the sun and moon tend to counteract one another. The Moon's attraction, though more than twice as strong as the sun's, is diminished by the counteracting force of the sun's gravitational pull.
Once in a month, when the moon's orbit is closest to the earth (perigee), unusually high and low tides occur. During this time the tidal range is greater than normal. Two weeks later, when the moon is farthest from earth (apogee), the moon's gravitational force is limited and the tidal ranges are less than their average heights. …