Predictability and Eclipse Warnings

Eclipses can only occur when the Sun and Moon are near the nodes on the lunar orbit, so that eclipses begin to occur when the Sun is within the eclipse limits, and the Moon is new (for a solar eclipse) or full (for a lunar eclipse). Recall (from Table 2.5) that the Moon's nodal or draconic period is 27d2122, compared with its sidereal period, 27d3217. The difference is caused by the regression or westward movement of the line of nodes, which amounts to 360° in 18.6 years. The mean motion of the Sun is ~ (360°/365d2422) = 0.9856°/d, so that in 27d2122 it travels 26°82; because twice the minor solar ecliptic limit is 30°. 7, at least one partial solar eclipse must occur whenever the Sun is near a node—a condition that occurs twice a year, ~173d apart. But, suppose an eclipse were to occur near a node; would another eclipse then occur 173 days later? No! In 173d, the Moon would complete 173/29.530589 = 5.86 synodic months (of mean length 29d530589). The remaining part of the cycle,16 0.14P, requires another four days to complete; i.e., for the Moon to catch up to the Sun. Therefore, even though the 173d interval is known as an eclipse season, and twice this interval, 346d62 is an eclipse year, neither is an eclipse interval!

15 The reliability we mention here is for the purpose of acceleration determinations;for more general and historical purposes, eclipses other than those ranked "A" or even "B" may be useful.

Table 5.3. Solar node passage and eclipse intervals.

Eclipse season

Lunations

Intervals

Difference

0d

0 0

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