The duration between two new Moons, a lunar month, is of about one calendar month (29.53 days).
The duration for the passage of the Moon between two ascending knots for which there is a new Moon is a little less than a year. Indeed, the knots have a backwards drifting movement in the ecliptic plane of about 1.5 (o) degree per month. One can thus observe two eclipses each year : one of the ascending and the other of the descending knot. One talks about two seasons of eclipses.
Because of this movement of the knots in the ecliptic plane, the eclipses of a same season do not occur at the same time of year. One has to wait 18.6 years for one season of eclipse to come back to its starting month in the year.
This duration corresponds to the mutation period of the year which is superimposed by the precession movement.
Another remarkable cycle already detected by the Babylonians is the "SAROS".
This 18-year, eleven days and 1/3 is the duration after which an eclipse repeats itself along circumstances practically identical which correspond to 223 lunar years.
Indeed, 223 Moon revolutions, from new Moon to new Moon, total 6,585.321 days while 242 knot revolutions from ascending to ascending knots, last 6,585.357 days.
The multiples 223 and 242 are the only ones for which the difference is so small.
So it becomes possible to classify the solar eclipses which occur during a Saros. This classification is called a long series of eclipses.
One of these long series contains from 69 to 86 solar eclipses which are all spaced out by 18 years, eleven days and 1/3. These series extend over 12 to 16 centuries. about 40 long series superimpose themselves because an eclipse occurs about every six months.
The successive eclipses of a same long series occur each time 120 degrees more to the west (which corresponds to the rotation of the Earth during 1/3 of a day). They also slowly shift from the North Pole to the South Pole for a continuation in which the eclipses happen at the ascending knot at from the south to the north in the other case.
The 11th of August 1999 eclipse is the 21st eclipse belonging to the long series N# 145 that started on the 4th of January 1639 at the North Pole and that will end on the 17th of April 3009 at the South Pole. This long series will have counted 77 eclipses.
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