46 PART 1^ |^ EXPLORING THE SKY
Although there are historical reasons to doubt that Th ales
actually predicted the eclipse, the important point is that he could
have done it. If he had had records of past eclipses of the sun vis-
ible from the area, he could have discovered that they tended to
recur with a period of 54 years 1 month (three saros cycles).
Indeed, he could have predicted the eclipse without ever under-
standing what the sun and moon were or how they moved.
Total and annular solar eclipses Lunar eclipses1990
J FMAMJ J A SOND91
92
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95
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992000
01
02
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092010
Eclipse
seasons■ Figure 3-15
A calendar of eclipse seasons. Each year the eclipse seasons begin about 19
days earlier. Any new moon or full moon that occurs during an eclipse season
results in an eclipse. Only total and annular eclipses are shown here.
1988
2024 March 18
April 81970
March 72006
March 29■ Figure 3-16
The saros cycle at work. The total solar eclipse of March 7, 1970, recurred
after 18 years 11^13 days over the Pacifi c Ocean. After another interval of 18
years 11^13 days, the same eclipse was visible from Asia and Africa. After a
similar interval, the eclipse will again be visible from the United States.SCIENTIFIC ARGUMENT
Why can’t two successive full moons be totally eclipsed?
Most people suppose that eclipses occur at random or in some pat-
tern so complex you need a big computer to make predictions. In
fact, like many natural events, eclipse prediction can be reduced to
a series of simple steps.
A lunar eclipse can happen only when the sun is near one node
and the moon crosses Earth’s shadow at the other node. A lunar
eclipse season is only 22 days long, and any full moon in that time
will be eclipsed. However, the moon takes 29.5 days to go from
one full moon to the next. If one full moon is totally eclipsed, the
next full moon 29.5 days later will occur long after the end of the
eclipse season, and there can be no second eclipse.
Now use your knowledge of the cycles of the sun and moon to
revise your argument. How can the sun be eclipsed by two suc-
cessive new moons?