318. The point _X_ where the Moon’s Orbit crosses the Ecliptic is called
_the Ascending Node_, because the Moon ascends from it above the
Ecliptic: and the opposite point of intersection _V_ is called _the
Descending Node_, because the Moon descends from it below the Ecliptic.
When the Moon is at _Y_ in the highest point of her Orbit, she is in her
greatest _North Latitude_; and when she is at _W_ in the lowest point of
her Orbit, she is in her greatest _South Latitude_.
[Sidenote: The Nodes have a retrograde motion.
Fig. I.
Which brings on the Eclipses sooner every year than they would
be if the Nodes had not such a motion.]
319. If the line of the Nodes, like the Earth’s Axis, was carried
parallel to itself round the Sun, there would be just half a year
between the conjunctions of the Sun and Nodes. But the Nodes shift
backward, or contrary to the Earth’s annual motion, 19-1/3 degrees every
year; and therefore the same Node comes round to the Sun 19 days sooner
every year than on the year before. Consequently, from the time that the
ascending Node _X_ (when the Earth is at _E_) passes by the Sun as seen
from the Earth, it is only 173 days (not half a year) till the
descending Node _V_ passes by him. Therefore, in whatever time of the
year we have Eclipses of the Luminaries about either Node, we may be
sure that in 173 days afterward we shall have Eclipses about the other
Node. And when at any time of the year the line of the Nodes is in the
situation _VGX_, at the same time next year it will be in the situation
_rGs_; the ascending Node having gone backward, that is, contrary to the
order of Signs from _X_ to _s_, and the descending Node from _V_ to _r_;
each 19-1/3 degrees. At this rate the Nodes shift through all the Signs
and degrees of the Ecliptic in 18 years and 225 days; in which time
there would always be a regular period of Eclipses, if any compleat
number of Lunations were finished without a fraction. But this never
happens, for if the Sun and Moon should start from a conjunction with
either of the Nodes in any point of the Ecliptic, whilst the same Node
is going round to that point again the Earth performs 18 annual
revolutions about the Sun and 222 Degrees (or 7 Signs 12 Degrees) over;
and the Moon 230 Lunations or Courses from Change to Change and 85
Degrees (or 2 Signs 25 Degrees) over; so that the Sun will be 138
Degrees from the same Node when it comes round, and the Moon 85 Degrees
from the Sun. Hence, the period of Eclipses and revolution of the Nodes
are completed in different times.
[Sidenote: A period of Eclipses.