20. Let us suppose the Earth’s Orbit to be a thin, even, solid plane;
cutting the Sun through the center, and extended out as far as the
Starry Heavens, where it will mark the great Circle called the
_Ecliptic_. This Circle we suppose to be divided into 12 equal parts,
called _Signs_; each Sign into 30 equal parts, called _Degrees_; each
Degree into 60 equal parts, called _Minutes_; and every Minute into 60
equal parts, called _Seconds_: so that a Second is the 60th part of a
Minute; a Minute the 60th part of a Degree; and a Degree the 360th part
of a Circle, or 30th part of a Sign. The Planes of the Orbits of all the
other Planets likewise cut the Sun in halves; but extended to the
Heavens, form Circles different from one another, and from the Ecliptic;
one half of each being on the north side, and the other on the south
side of it. Consequently the Orbit of each Planet crosses the Ecliptic
in two opposite points, which are called the Planet’s _Nodes_. These
Nodes are all in different parts of the Ecliptic; and therefore, if the
planetary Tracks remained visible in the Heavens, they would in some
measure resemble the different rutts of waggon-wheels crossing one
another in different parts, but never going far asunder. That Node, or
Intersection of the Orbit of any Planet with the Earth’s Orbit, from
which the Planet ascends northward above the Ecliptic, is called the
_Ascending Node_ of the Planet; and the other, which is directly
opposite thereto, is called it’s _Descending Node_. Saturn’s Ascending
Node is in 21 deg. 13 min. of Cancer ♋, Jupiter’s in 7 deg. 29 min. of
the same Sign, Mars’s in 17 deg. 17 min. of Taurus ♉, Venus’s in 13 deg.
59 min. of Gemini ♊, and Mercury’s in 14 deg. 43 min. of Taurus. Here we
consider the Earth’s Orbit as the standard, and the Orbits of all the
other Planets as oblique to it.
[Sidenote: The Planets Orbits, what.]
21. When we speak of the Planets Orbits, all that is meant is their
Paths through the open and unresisting Space in which they move; and are
kept in, by the attractive power of the Sun, and the projectile force
impressed upon them at first: between which power and force there is so
exact an adjustment, that without any solid Orbits to confine the
Planets, they keep their courses, and at the end of every revolution
find the points from whence they first set out, much more truly than can
be imitated in the best machines made by human art.
[Sidenote: Mercury.
Fig. I.
May be inhabited.