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onto the Earth, then the moon is displaced in its orbit
closest to the Earth's poles, 28.5° (23.5° + 5°) north
and south of the equator. The moon's gravitational
attraction on the Earth is slightly greater when the
moon is displaced poleward. In 9.3 years' time, the
moon will move to the opposite side of the sun's
equator. Relative to the Earth, the moon will be
closer to the equator, or in its minimum position
(Figure 2.17b). Although it is only 3.7 per cent the
daily effect, the gravitational effect of the M N tide has
two to three years to act on any standing, planetary
wave phenomena.
Standing atmospheric waves, such as Rossby waves
embedded in the jet stream (Figure 2.4), are amplified
by the M N lunar tide. An 18-20 year periodicity
appears in sea level, air pressure, and temperature
records at locations beneath the jet stream from Japan
to Scandinavia. Detailed examination of the occur-
rence of drought on the United States Great Plains
since 1800 shows a strong link between drought and
the 18.6-year M N lunar cycle. A 20-year cycle is also
evident in rainfall over the Yangtze River Basin of
China, coinciding with the American pattern and the
Indian flood record. In the southern hemisphere, a
20-year periodicity appears in summer rainfall in
The 18.6-year M N lunar cycle
(Tyson et al., 1975; Currie, 1981, 1984; Wang & Zhao, 1981)
The 18.6-year lunar tidal cycle (M N ) represents a
fluctuation in the orbit of the moon. The moon's axis of
orbit forms a 5° angle with the sun's equator as shown
in Figure 2.16; however, with each orbit the moon does
not return to the same location relative to the sun.
Instead, it moves a bit further in its orbit. The process
is analogous to a plate spinning on a table. The plate
may always be spinning at the same angle relative to
the table, but the high point of the plate does not occur
at the same point. Instead, it moves around in the
direction of the spin. The moon's orbit does the same
thing, such that 9.3 years later the high point of the
orbit is at the opposite end of the solar equator, and 9.3
years after that it returns to its original position. Thus,
there is an 18.6-year perturbation in the orbit of the
moon. This perturbation appears trivial until you
consider the moon's orbit in relation to the Earth
(Figure 2.17). Relative to the Earth, the solar equator
moves seasonally, reaching a maximum of 23.46°N of
the equator on 22 June and a minimum of 23.46°S of
the Earth's equator on 22 December. If the moon-sun
orbital configuration in Figure 2.16a is superimposed
Moon's orbit relative to sun
A
Moon's position
June 22
Sun ' s equator
5˚ angle
Moon's position
next orbit June 23
Sun's equator
Precession in moon's orbit
B
Moon's position
9.3 years later
Successive positions relative to
Sun's equator
Sun ' s equator
5˚ angle
Schematic diagrams showing the precession of the Moon's orbit relative to the Sun's equator for A) lunar maxima and B) lunar minima.
Fig. 2.16
 
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