Predicting Weather By The Moon - Xavier University Libraries
Predicting Weather By The Moon - Xavier University Libraries Predicting Weather By The Moon - Xavier University Libraries
Weather By The Moon Over the course of time this can have a noticeable effect. Astronomers trying to compare ancient solar eclipse records with their predictions found that they were off by a significant amount. But when they took the slowing down of the Earth’s rotation into account, their predictions agreed with the solar eclipse records. Also growth rings in ancient corals about 400 hundred million years old show that the day was only 22 hours long then, and that there were over 400 days in a year. Gravity acts both ways. Eventually the Earth’s rotation will slow down to where it keeps only one face toward the Moon. The Earth has also been creating tidal bulges on the Moon and has slowed it’s rotation down so much that now it only rotates once every orbital period. For that reason the Moon now keeps one face always toward the Earth. Ocean tides occur with a dominant period (the time between successive high or low tides) of 12 hours and 25 minutes. High tide comes twice a day; once under the Moon (or somewhat behind it, because friction with the solid Earth delays the water), and once when the Moon is on the opposite side of the Earth. The Sun also has a tidal pull on the Earth, rather less than half that of the Moon. So at or just after the New and Full Moons of each month, when Sun, Moon and Earth are in line, the tide is amplified into a ‘spring’ tide(nothing to do with the season called spring) There is a tidal swelling on both sides of the Earth. So whether spring tide is at New of Full Moon, if there is one floodtide under the noonday Sun, the other is at mid- 74
Tides night. As high tides are produced by the heaping action resulting from the horizontal flow of water toward two areas of the Earth representing positions of maximum attraction of combined Moon and Sun, low tides are created by a compensating maximum withdrawal of water from areas around the Earth midway between these two humps. Normally there are two tides a day, called semi-diurnal. When there is only one tide per day, this is called diurnal tendency, and is caused by lunar declination, 28.5° from the equatorial plane. Twice a sidereal month (just under 15 days) during Full and New Moon the tides are the strongest tides, the spring tides. At 1st and Last Quarter of the Moon’s phases the Sun and Moon pull at right angles to one another, and the tides are weakest, the neap tides. When the Moon is near; that is, in Perigee, its tidal pull is greater. Perigees will be discussed a little more fully in the next chapter. Perigees are greater when they coincide with New of Full Moon. At these times of coincidence, we expect the highest tides of the year - those of greatest amplitude - and lowest low tides. Perigean spring tides may cause coastal flooding, especially if, as is usually the case, they happen to be accompanied by storms. This is because the Moon in Perigee acts in a more pronounced way on the atmospheric tide as well. The tidal day is 24 hours 50 minutes, longer than 24 hours because of the lunar orbit. The Earth-Moon tidal coupling lengthens the day; this lengthening is due to tidal friction. 75
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Tides<br />
night.<br />
As high tides are produced by the heaping action resulting<br />
from the horizontal flow of water toward two areas<br />
of the Earth representing positions of maximum attraction<br />
of combined <strong>Moon</strong> and Sun, low tides are created by a compensating<br />
maximum withdrawal of water from areas around<br />
the Earth midway between these two humps.<br />
Normally there are two tides a day, called semi-diurnal.<br />
When there is only one tide per day, this is called diurnal<br />
tendency, and is caused by lunar declination, 28.5° from<br />
the equatorial plane. Twice a sidereal month (just under 15<br />
days) during Full and New <strong>Moon</strong> the tides are the strongest<br />
tides, the spring tides. At 1st and Last Quarter of the<br />
<strong>Moon</strong>’s phases the Sun and <strong>Moon</strong> pull at right angles to<br />
one another, and the tides are weakest, the neap tides.<br />
When the <strong>Moon</strong> is near; that is, in Perigee, its tidal<br />
pull is greater. Perigees will be discussed a little more fully<br />
in the next chapter. Perigees are greater when they coincide<br />
with New of Full <strong>Moon</strong>. At these times of coincidence,<br />
we expect the highest tides of the year - those of greatest<br />
amplitude - and lowest low tides. Perigean spring tides may<br />
cause coastal flooding, especially if, as is usually the case,<br />
they happen to be accompanied by storms. This is because<br />
the <strong>Moon</strong> in Perigee acts in a more pronounced way on the<br />
atmospheric tide as well.<br />
<strong>The</strong> tidal day is 24 hours 50 minutes, longer than 24<br />
hours because of the lunar orbit. <strong>The</strong> Earth-<strong>Moon</strong> tidal<br />
coupling lengthens the day; this lengthening is due to tidal<br />
friction.<br />
75
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