f the movements of the
Earth and the Moon, so far as regards the bearing of those movements on
the question of eclipses. The Earth moves in a plane which is called the
"Plane of the Ecliptic," and correspondingly, the Sun has an _apparent_
annual motion in the same plane. The Moon moving in a different plane,
inclined to the first mentioned one to the extent of rather more than
5 deg., the Moon's orbit will evidently intersect the ecliptic in two
places. These places of intersection are called "Nodes," and the line
which may be imagined to join these Nodes is called the "Line of Nodes."
When the Moon is crossing the ecliptic from the S. to the N. side
thereof, the Moon is said to be passing through its "Ascending Node"
([Symbol: Ascending node]); the converse of this will be the Moon
passing back again from the N. side of the ecliptic to the S. side,
which is the "Descending Node" ([Symbol: Descending node]). Such changes
of position, with the terms designating them, apply not only to the Moon
in its movement round the Earth, but to all the planets and comets
circulating round the Sun; and also to satellites circulating round
certain of the planets, but with these matters we have no concern now.

FOOTNOTES:

[Footnote 1: D. Lardner, _Handbook of Astronomy_, 3rd ed., p. 288.]

[Footnote 2: But not one of them was visible at Greenwich.]

[Footnote 3: Latin _Annulus_, a ring.]

CHAPTER III.

THE "SAROS" AND THE PERIODICITY OF ECLIPSES.

To bring about an eclipse of the Sun, two things must combine: (1) the
Moon must be at or near one of its Nodes; and (2), this must be at a
time when the Moon is also in "Conjunction" with the Sun. Now the Moon
is in Conjunction with the Sun (= "New Moon") 12 or 13 times in a year,
but the Sun only passes through the Nodes of the Moon's orbit twice a
year. Hence an eclipse of the Sun does not and cannot occur at every New
Moon, but only occasionally. An _exact_ coincidence of Earth, Moon, and
Sun, in a straight line at a Node is not necessary to ensure an eclipse
of the Sun. So long as the Moon is within about 181/2 deg. of its Node, with a
latitude of not more than 1 deg. 34', an eclipse _may_ take place. If,
however, the distance is less than 151/4 deg. and the latitude less than
1 deg. 23' an eclipse _must_ take place, though between these limits[4] the
occurrence of an eclipse is uncertain and depends on what are ca