Sailing by Dave Franzel

Author:Dave Franzel
Language: eng
Format: epub
ISBN: 9781493029815
Publisher: Lyons Press
Published: 2017-11-10T05:00:00+00:00

Figure 6.1 Moon’s gravitational effects cause high tide bulges at X and Z.

In the situation shown in Figure 6.1, the moon pulls harder on point “X” than on the center of the earth, resulting in a bulge at point “X,” and it pulls harder on the center of the earth than on point “Z,” resulting in a bulge at point “Z.” So at “X” and “Z” it is high tide. At “Y” and “W” it is low tide. Notice that as the earth rotates you pass through “X,” “Y,” “Z,” and “W,” or high, low, high, low, at six-hour intervals. After 24 hours, when you are back at “X,” the moon has traveled about another th of its orbit, so high tide occurs roughly 50 minutes later each day.

This simplified explanation does not take into account land masses (which impede the advance of this “tidal wave”) or local topographical conditions, which combine to help create variations such as a 35-foot mean tidal range in parts of Nova Scotia as compared with a negligible tide in the Bahamas and parts of Scandinavia.

Two factors that affect the tidal range on a daily basis, however, are the phase of the moon and the distance of the moon from the earth (see Figure 6.2).

The sun, which is much farther from the earth than the moon is, has a lesser effect on tides. It does exert some influence, however, so that when the moon is full or new and the sun and moon line up with the earth, their tidal effects combine to create more extreme tides—higher highs and lower lows. These extreme conditions are called spring tides (which have nothing to do with the season of the same name), and they occur twice per month.

When the moon is in its first or third quarter, the tidal effect of the sun reduces that of the moon, resulting in tides that are less extreme than average. These are called neap tides.

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