CHAPTER 3 | CYCLES OF THE MOON 45
day longer than 18 years 11 days. When the eclipse happens
again, Earth will have rotated one-third of a turn farther east, and
the eclipse will occur one-third of the way westward around
Earth (■ Figure 3-16). Th at means that after three saros cycles—a
period of 54 years 1 month—the same eclipse occurs in the same
part of Earth.
One of the most famous predictors of eclipses was the Greek
philosopher Th ales of Miletus (about 640–546 bc), who suppos-
edly learned of the saros cycle from the Chaldeans. No one
knows which eclipse Th ales predicted, but some scholars suspect
the eclipse of May 28, 585 bc. In any case, the eclipse occurred
at the height of a battle between the Lydians and the Medes, and
the mysterious darkness in midafternoon so startled the two
armies that they concluded a truce.(sometimes referred to simply as the saros). After one saros cycle
of 18 years 11^13 days, the pattern of eclipses repeats. In fact, saros
comes from a Greek word that means “repetition.”
One saros contains 6585.321 days, which is equal to 223
lunar months. Th erefore, after one saros cycle, the moon is back
to the same phase it had when the cycle began. But one saros is
also equal to 19 eclipse years. After one saros cycle, the sun has
returned to the same place it occupied with respect to the nodes
of the moon’s orbit when the cycle began. If an eclipse occurs on
a given day, then 18 years 11^13 days later the sun, the moon, and
the nodes of the moon’s orbit return to nearly the same relation-
ship, and the eclipse occurs all over again.
Although the eclipse repeats almost exactly, it is not visible
from the same place on Earth. Th e saros cycle is one-third of a
Unfavorable for eclipseEarth, moon, and shadows drawn to scaleUnfavorable for eclipseSunFavorable for eclipse5 ° inclination of
plane of moon’s orbit Line of nodesN'NN'N
N'NN'NPlane of moon’s orbit
Plane of Earth’s orbitFavorable for eclipseFull FullFull
FullNew NewNewNewLine of nodes
points toward sunLine of nodes
Line of nodes points toward sunFull
moonNew
New moon shadow moon
passes north of Earth;
Full moon passes south no eclipse
of Earth’s shadow;
no eclipse■ Figure 3-14
The moon’s orbit is tipped a bit over 5° to Earth’s orbit. The nodes N and N’ are the points where the moon passes through the plane of Earth’s orbit. If the
line of nodes does not point at the sun, the long narrow shadows miss, and there are no eclipses at new moon and full moon. At those parts of Earth’s orbit
where the line of nodes points toward the sun, eclipses are possible at new moon and full moon.