Predicting Weather By The Moon - Xavier University Libraries

PERIGEES AND APOGEES
The rising or setting Moon looks large on the horizon
due to an illusion of perspective playing tricks on the eye.
Apart from that, most assume the Full Moon high in the
sky is always the same. A spectacular phenomena escapes
notice by the vast majority of people simply because the
eye and brain can’t compare the size and brightness of objects
observed on separate occasions. The Full Moon varies
dramatically in size and intensity, and hardly anybody
notices.
As the Moon travels anti-clockwise around the Earth,
orbiting the same way as the Earth as both go around the
Sun, it’s orbit is not a perfect circle. That would indeed be
a marvel. It does not remain a constant distance from the
Earth. This was discovered or rediscovered by Hipparchus,
the 2 nd Century Greek astronomer, so it is by no means
news. He realised that the Moon wanders in and out, towards
the Earth and away again, as if the Moon is on a fixed
spring, for the outer limit is relatively fixed and varies much
less than the inner.
This cycle is called the Apogee/Perigee, or the Apsidal
Cycle, and we say that the Moon’s orbit around the Earth is
elliptical, that is to say, off-centre, like a circus-clown’s
bicycle wheel. It has a substantial eccentricity, as major
Solar System bodies go, of 5.49%. In addition, the tidal
effect of the Sun’s gravitational field increases this eccentricity
when the orbit’s major axis lines up with the Sun-
Earth vector or, in other words, the Moon is Full or New.
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