The wandering Moon
Q: Why does the moon appear to have a nineteen-year cycle of wandering? Lanney A., Sandia Park,
New Mexico
A: We tend to think of the Moon going about Earth and Earth orbiting the Sun in fairly simple terms. The
reality is so complicated that its mathematical solution contains more than 6,000 terms. Newton said thinking
about it gives him a headache. Your question gets into the interaction subtleties of Earth, Moon, Sun, and
planets.
![[NASA] On an 18.6 year-cycle, the Moon nods north and south in the sky.](2002-03-27-moon.jpg) [NASA] On an 18.6 year-cycle, the Moon nods north and south in the sky.
The Moon has an 18.6-year cycle of wandering because of the Sun's gravitational influence on its plane of orbit about Earth. (Actually the
Earth and Moon orbit each other about their common center of gravity, but we'll ignore this complexity.)
Moons orbiting close to their parents often do so in an equatorial plane. "Look at Jupiter's flock, obediently circling their master's middle
as if on leading-strings from his belt," says Guy Ottewell in his Astronomical Companion.
The Moon, however, is not close to Earth in the same sense that Jupiter's moons are "...but rather it is so far away that solar perturbations
are enormous and even effects of planets must be considered," says Lawrence Aller, astronomy professor emeritus at the University of
California at Los Angeles.
Consequently, the Moon doesn't orbit in a nice, simple equatorial plane about Earth. Instead it circles Earth in a plane tilted a small
amount (about five degrees) from the plane that Earth orbits the Sun in. The tug of the distant Sun and even planets produce a rotation in
the Moon's orbit. It precesses like a spinning top. This, in turn, produces the 18.6-year cycle of Moon wandering.
The orbit circles westward at a rate of 0.053 degrees a day. So, at this rate, it takes 6,793.5 days (18.6 years) to travel 360 degrees or a
complete cycle. The tiny change is too insignificant for us to notice with casual glances at the Moon. However, Hipparchus in the 2nd
century B.C. observed it and so did the Anasazi more than a thousand years ago here in the American Southwest.
What happens is-- a north/south nodding of the Moon's position. "At the peak of its cycle, the Moon rises and sets in the most northerly
direction at the summer solstice and at its most southerly position at the winter solstice," says Laurel Ladwig, planetarium manager in
Albuquerque, speaking of its effects in the Northern Hemisphere.
(Answered by April Holladay, science correspondent, Feb. 27, 2002)
Further Surfing:
ThinkQuest: The Earth, Sun, and Moon
John Walker: Inconstant Moon
Lodestar Astronomy Center
Solstice Project: Remarkable moon astronomy by Anasazi
The Astronomical Companion by Guy Ottewell.
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