[Sidenote: _Ecliptic._]
[Sidenote: _Equinoctial._]
[Sidenote: _Solstitial Points._]
[Sidenote: _Colures._]
[Sidenote: _Equinoctial Colure._]
[Sidenote: _Solstitial Colure._]
7. The _Ecliptic_ is that great circle in whose plane the Earth
performs its annual motion round the Sun; or, in which the Sun seems
to move round the Earth, once in a year. This circle makes an angle
with the equinoctial of 23 degrees 29 minutes, and intersects it in
two opposite points, which are called the _Equinoctial Points_; and
the two points in the ecliptic that are at the greatest distance from
the equinoctial points, are called the _Solstitial Points_. The two
meridians passing through those points, are, by way of eminence, called
_Colures_; whereof that which passeth thro’ the equinoctial points, is
called the _Equinoctial Colure_; and that which is at right angles to
it, passing through the Solstitial Points, is called the _Solstitial
Colure_.
[Sidenote: The Ecliptic divided into signs.]
The ecliptic is divided into 12 equal parts, called _Signs_, each sign
being 30 degrees, beginning from one of the equinoctial points, and
numbered from West to East; the names and characters of the twelve
signs are as follows, _viz._
_Aries_, _Taurus_, _Gemini_, _Cancer_, _Leo_, _Virgo_,
1. ♈ 2. ♉ 3. ♊ 4. ♋ 5. ♌ 6. ♍
_Libra_, _Scorpio_, _Sagittarius_, _Capricornus_, _Aquaries_,
7. ♎ 8. ♏ 9. ♐ 10. ♑ 11. ♒
_Pisces_.
12. ♓
[Sidenote: _Northern Signs._]
The first six of these are called the _Northern Signs_, and possess
that half of the ecliptic which is to the Northward of the equator;
beginning with the first point of ♈, and ending with the last point of
♍.
[Sidenote: _Southern Signs._]
The latter six are called the _Southern Signs_, because they possess
the Southern half of the ecliptic; beginning at the first point of ♎,
and ending with the last point of ♓.
The division of the ecliptic into signs, and the names of the colures,
are particularly expressed upon the globes.
The signs of the ecliptic took their names from 12 constellations
mentioned in the Introduction to be situated in the Heavens near those
places. It is to be observed, that the signs are not to be confounded
with the constellations of the same name: For the _Sign of Aries_, is
not the same with the _Constellation_ Aries; the latter is a system of
Stars digested into the figure of a _Ram_, but the sign of _Aries_ is
only 30 degrees of the ecliptic, counted from the equinoctial point ♈,
(which is reckoned the first point in the ecliptic) to the beginning
of _Taurus_: Or, it is sometimes taken for all that space upon the
Celestial Globe contained between the two circles passing through the
first points of ♈ and ♉. What has been here said of _Aries_, is to be
noted of all the rest of the signs.
The constellations above-mentioned were formerly situated within
the signs which now bear their names; but by a slow motion of the
equinoctial points, being one degree in 72 years, the constellation
_Aries_ has now got into the sign ♉, and so of the rest. So that
_Pisces_ is now got into the Sign of ♈; this slow motion in the Heavens
is called the _Precession of the Equinoctial Points_.
[Sidenote: _Poles of the Ecliptic._]
The _Poles of the Ecliptic_ are both situated in the Solstitial Colure,
at 23 degrees, 29 minutes distance from the Pole of the world; and they
take their denomination from the Hemisphere wherein they are placed,
_viz._ that which lies in the (Northern/Southern) Hemisphere, is called
the (North/South) Pole of the ecliptic. The arctic and antarctic
circles, are described by the Poles of the ecliptic in the diurnal
motion of the Earth round its axis, whence it seems these two circles
are called _Polar_.
[Sidenote: _Circles of Longitude._]
8. All great circles passing through the Poles of the ecliptic, and
consequently intersecting it at right angles, are called _Circles of
Longitude_: So that,
[Sidenote: _Longitude of any Point in the Heavens._]
[Sidenote: _Place of a Star._]
9. The _Longitude_ of any _Point_ in the Heavens (as a _Star_ or
_Planet_, _&c._) is an arch of the ecliptic contained between the
circle of longitude passing thro’ that point, and the equinoctial
point ♈. And that degree of any sign which lies under the circle of
longitude, passing thro’ any Star or Planet, is called the _Place_ of
that Star or Planet.
_Note_, The _Sun_ never goes out of the ecliptic, and it is not usual
to say the Sun’s longitude, but we commonly express it the _Sun’s
Place_, which is that sign, degree, minute, _&c._, of the ecliptic,
which he at any time passes.
10. All circles conceived to be drawn parallel to the ecliptic, are
called _Parallels of Latitude_: So that,
[Sidenote: _Latitude of a Star, &c._]
11. The _Latitude_ of any point in the Heavens, (as a fixed Star,
_&c._) is an arch of the circle of longitude, in passing thro’ that
point, and intercepted betwixt it, and the ecliptic; or, the latitude
is the distance from the ecliptic; and if the said point be to the
Northward of the ecliptic, it is called North Latitude; but if it be to
the Southward, is called South Latitude.
Upon the _Terrestrial Globe_, none of the circles of longitude are
described; and upon the _Celestial_, they are commonly drawn thro’
the beginning of every _Sign_; but they are all supplied upon both
Globes, by fastening a thin plate of brass over one of the Poles of the
ecliptic, and so as to be moved to any degree thereof at pleasure. The
parallels of latitude are also supplied by the graduations upon the
said plate, as shall be shewn in a proper place.
We have now done with all those circles that are fixed, and such as
are drawn upon the Globes themselves; we next proceed to the moveable
circles.