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 The combined effects of orbital eccentricity and the Sun’s tides result in a substantial difference in the apparent size and brightness of the Moon at Perigee and Apogee. Apogee is generally about 404,510 km(252,700 miles) from the centre of the Moon to the centre of the Earth. Compare that to a typical Perigee of 359,000 km(221,500 miles). So from when the Moon is most distant from the Earth, to when it is at its closest point, there is a difference of about 50,211km (31,200 miles) The changes in distance are thus quite considerable, and the Moon’s apparent diameter at Apogee is only nine tenths of the same value at Perigee. The difference is sufficient to add 20% to a high sea tide when the Moon is at Perigee, for this is when the Moon’s gravitational attraction is at its strongest. But the difference is not marked enough to be noticeable with the naked eye. If this off-center orbit of the Moon were drawn to a scale of, say, three inches in diameter, so that it could be fitted onto a page of this book, it would look circular unless carefully measured. Although extreme values for Perigee and Apogee distance occur when Perigee or Apogee passage occurs close 94
Perigees and Apogees to New or Full Moon, long-term extremes are in the months near to Earth’s perihelion passage (closest approach to the Sun, when the Sun’s tidal effects are strongest) in the first few days of January. The Moon’s perceived size is up to 25% larger at a Full-Moon Perigee compared to Apogee. Most people don’t notice the difference because they see the Moon in a sky that offers no reference by which angular extent may be judged. To observe the difference, you have to either make a scale to measure the Moon, or else photograph the Moon at Perigee and Apogee and compare the pictures. When Apollo 11 Commander Neil Armstrong walked on the Moon on that historic day of Monday July 21 st , 1969 this date had been taken into account by the NASA planning team months before. The day of the Perigee for that month was July 28 th , leaving them with a few days spare. The Moon was sitting at 357,925km away. Why wait for a Perigee? To save fuel. Why stack on board more weight than you have to? Especially 31,000 miles worth. The fact that it was July was significant too. Perigees have an order, with one closest and others at different orders of closeness. July’s was no ordinary Perigee, but the closest equal for the year, saving the project an extra 10,000kms of travelling distance than if they had chosen any other month than July or June in 1969. When we gaze at the Moon on a still Full Moon evening we think of the Moon as being very bright but it is really no brighter than asphalt. Next time you see the Full Moon rising just after sunset, try to position yourself so 95
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Perigees and Apogees<br />
to New or Full <strong>Moon</strong>, long-term extremes are in the months<br />
near to Earth’s perihelion passage (closest approach to the<br />
Sun, when the Sun’s tidal effects are strongest) in the first<br />
few days of January. <strong>The</strong> <strong>Moon</strong>’s perceived size is up to<br />
25% larger at a Full-<strong>Moon</strong> Perigee compared to Apogee.<br />
Most people don’t notice the difference because they see<br />
the <strong>Moon</strong> in a sky that offers no reference by which angular<br />
extent may be judged. To observe the difference, you<br />
have to either make a scale to measure the <strong>Moon</strong>, or else<br />
photograph the <strong>Moon</strong> at Perigee and Apogee and compare<br />
the pictures.<br />
When Apollo 11 Commander Neil Armstrong<br />
walked on the <strong>Moon</strong> on that historic day of Monday July<br />
21 st , 1969 this date had been taken into account by the<br />
NASA planning team months before. <strong>The</strong> day of the Perigee<br />
for that month was July 28 th , leaving them with a few<br />
days spare. <strong>The</strong> <strong>Moon</strong> was sitting at 357,925km away. Why<br />
wait for a Perigee? To save fuel. Why stack on board more<br />
weight than you have to? Especially 31,000 miles worth.<br />
<strong>The</strong> fact that it was July was significant too. Perigees<br />
have an order, with one closest and others at different orders<br />
of closeness. July’s was no ordinary Perigee, but the<br />
closest equal for the year, saving the project an extra<br />
10,000kms of travelling distance than if they had chosen<br />
any other month than July or June in 1969.<br />
When we gaze at the <strong>Moon</strong> on a still Full <strong>Moon</strong><br />
evening we think of the <strong>Moon</strong> as being very bright but it is<br />
really no brighter than asphalt. Next time you see the Full<br />
<strong>Moon</strong> rising just after sunset, try to position yourself so<br />
95
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