The Saros

The cycle of 6,585.32 days or 18.03 solar years is the saros, which we initially discussed in Chapter 2 without detailing its origin. This is the period over which conjunctions and oppositions repeat, making an eclipse possible.

Although the contrasting astronomical year lengths all involve fractions, each discrete calendar year must contain a whole number of days. Consider the saros counted off against our calendar. If there are 4 leap years within it, that cycle represents 18 years and 11 days, but just 10 days if by chance there are 5 leap years.

The saros contains close to, but not precisely, 19 eclipse years. Nineteen of those years persist for 6,585.78 days, which is 0.46 days longer than the saros. This means that when the syzygy passages repeat after 6,585.32 days, the lunar node is not quite in the same place as it was one saros earlier, because there is still 0.46 of a day to go. We can work out how much that equates to in terms of celestial longitude by expressing it as a fraction of the eclipse year and multiplying by 360 degrees; the answer is about 0.48 degrees, which is just less than the angular diameter of the Moon.

The situation can be visualized more easily by reference to Figure A-10. In Figure A-9 we were looking at successive nodal transits, producing a longitude jump of 1.44 degrees. Now we are

Saros B A

Longitudes spaced by 0.48 deg l l l l

Eeliptic

FIGURE A-10. After a complete saros the Moon comes back to a node just 0.48 degrees away from where it was 18.03 years before. Unlike in Figure A-9, the second saros (labeled B) starts with the longitude being enhanced (the node has moved counterclockwise, towards the left).

considering the situation after a whole saros. During that time the node has circuited the Earth 19 times, but returns to a position just 0.48 degrees from where it began the saros; the longitude is actually enhanced rather than reduced by that amount. As Figure A-10 shows, because the Moon is of slightly larger angular extent than this longitude jump, the lunar disks just overlap in terms of their positions from one saros to the next.

We know that the Sun is of virtually the same angular size as the Moon. Does this bare overlap mean that an eclipse occurring at the start of one saros will result in a miss at the start of the next?

Regarding Figure A-10, one can see that although the lunar disk has shifted by 0.48 degrees in longitude, practically a whole diameter, because the Moon is crossing the ecliptic at such an oblique Telescopes Mastery

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