NODES, in astronomy, the two points where the
orbit of a planet intersects the ecliptic.
Plate CCIV. Such are the two points C and D; of which the node
fig. 4. no 1. C, where the planet ascends northward above the
plane of the ecliptic, is called the ascending node, or
the dragon's head, and is marked thus . The other
node D, where the planet descends to the south, is
called the descending node, or the dragon's tail, mark-
ed thus .
The line CD, wherein the two circles C E D F and
C G D H intersect, is called the line of nodes. It ap-
pears from observation, that the line of the nodes of
all the planets, constantly changes its place, and shifts
its situation from east to west, contrary to the order
of the signs; and that the line of the moon's nodes,
by a retrograde motion, finishes its circulation in the
compass of 19 years; after which time, either of the
nodes having receded from any point of the ecliptic,
returns to the same again; and when the moon is in
the node, she is also seen in the ecliptic. If the line
of nodes were immovable, that is, if it had no other
motion than that whereby it is carried round the sun,
it would always look to the same point of the ecliptic,
or would keep parallel to itself, as the axis of the
earth does.
From what hath been said, it is evident, that the
moon can never be observed precisely in the ecliptic,
but twice in every period; that is, when she enters
the nodes. When she is at her greatest distance from
the nodes; viz. in the points E, F, she is said to be
in her limits.
The moon must be in or near one of the nodes, when
there is an eclipse of the sun or moon.
To make the foregoing account of the motion of
moon's nodes still clearer, let the plane of no 2. ibid.
represent that of the ecliptic, S the sun, T the centre
of the earth, L the moon in her orbit D N d n. N n
is the line of the nodes passing between the quadrature
Q, and the moon's place L, in her last quarter. Let
now L P, or any part L S, represent the excess of
the sun's action at T; and this being resolved into the
force L R, perpendicular to the plane of the moon's
orbit, and P R parallel to it, it is the former only
that has any effect to alter the position of the orbit,
and in this it is wholly exerted. Its effect is twofold:
1. It diminishes its inclination by a motion which we
may conceive as performed round the diameter D d, to
which L T is perpendicular. 2. Being compounded
with the moon's tangential motion at L, it gives it
an intermediate direction L s, through which, and the
centre, a plane being drawn, must meet the ecliptic
nearer the conjunction C than before.