lled the
"horizontal parallaxes" and the "apparent semi-diameters" of the two
bodies at the moment of conjunction, in other words, on the nearness or
"far-offness" of the bodies in question. Another complication is
introduced into these matters by reason of the fact that the Nodes of
the Moon's orbit do not occupy a fixed position, but have an annual
retrograde motion of about 191/4 deg., in virtue of which a complete
revolution of the Nodes round the ecliptic is accomplished in 18 years
218-7/8 days (= 18.5997 years).

The backward movement of the Moon's Nodes combined with the apparent
motion of the Sun in the ecliptic causes the Moon in its monthly course
round the Earth to complete a revolution with respect to its Nodes in a
less time (27.2 days) than it takes to get back to Conjunction with the
Sun (29.5 days); and a curious consequence, as we shall see directly,
flows from these facts and from one other fact. The other fact is to the
Sun starting coincident with one of the Moon's Nodes, returns on the
Ecliptic to the same Node in 346.6 days. The first named period of 27.2
days is called the "_Nodical_ Revolution of the Moon" or "Draconic
Month," the other period of 29.5 days is called the "_Synodical_
Revolution of the Moon." Now the curious consequence of these figures
being what they are is that 242 Draconic Months, 223 Lunations, and 19
Returns of the Sun to one and the same Node of the Moon's orbit, are all
accomplished in the same time within 11 hours. Thus (ignoring
refinements of decimals):--

Days Days. Years. Days. Hours.

242 times 27.2 = 6585.36 = 18 10 81/2
223 times 29.5 = 6585.32 = 18 10 73/4
19 times 346.6 = 6585.78 = 18 10 183/4

The interpretation to be put upon these coincidences is this: that
supposing Sun and Moon to start together from a Node they would, after
the lapse of 6585 days and a fraction, be found again together very near
the same Node. During the interval there would have been 223 New and
Full Moons. The exact time required for 223 Lunations is such that in
the case supposed the 223rd conjunction of the two bodies would happen a
little before they reached the Node; their distance therefrom would be
28' of arc. And the final fact is that eclipses recur in almost, though
not quite, the same regular order every 6585-1/3 days, or more exactly, 18
years, 10 days, 7 hours, 42 minutes.[5] This is the celebrated Chaldean