438. The _Improved Celestial Globe_. On the North Pole of the Axis,
above the Hour Circle, is fixed an Arch _MKH_ of 23-1/2 Degrees; and at
the end _H_ is fixed an upright pin _HG_, which stands directly over the
North Pole of the Ecliptic, and perpendicular to that part of the
surface of the Globe. On this pin are two moveable Collets at _D_ and
_H_, to which are fixed the quadrantal Wires _N_ and _O_, having two
little Balls on their ends for the Sun and Moon, as in the Figure. The
Collet _D_ is fixed to the circular Plate _F_ whereon the 29-1/2 days of
the Moon’s age are engraven, beginning just under the Sun’s Wire _N_;
and as this Wire is moved round the Globe, the Plate _F_ turns round
with it. These Wires are easily turned if the Screw _G_ be slackened;
and when they are set to their proper places, the Screw serves to fix
them there so, as in turning the Ball of the Globe, the Wires with the
Sun and Moon go round with it; and these two little Balls rise and set
at the same times, and on the same points of the Horizon, for the day to
which they are rectified, as the Sun and Moon do in the Heavens.
Because the Moon keeps not her course in the Ecliptic (as the Sun
appears to do) but has a Declination of 5-1/3 Degrees on each side from
it in every Lunation § 317, her Ball may be screwed as many Degrees to
either side of the Ecliptic as her Latitude or Declination from the
Ecliptic amounts to at any given time; and for this purpose _S_ is a
small piece of pasteboard, of which the curved edge _S_ is to be set
upon the Globe at right Angles to the Ecliptic, and the dark line over
_S_ to stand upright upon it. From this line, on the convex edge, are
drawn the 5-1/3 Degrees of the Moon’s Latitude on both sides of the
Ecliptic; and when this piece is set upright on the Globe, it’s
graduated edge reaches to the Moon on the Wire _O_, by which means she
is easily adjusted to her Latitude found by an Ephemeris.
The Horizon is supported by two semicircular Arches, because Pillars
would stop the progress of the Balls when they go below the Horizon in
an oblique sphere.
[Sidenote: To rectify it.]
_To rectify the Globe._ Elevate the Pole to the Latitude of the Place;
then bring the Sun’s place in the Ecliptic for the given day to the
brasen Meridian, and set the Hour Index to XII at noon, that is, to the
upper XII on the Hour Circle; keeping the Globe in that situation,
slacken the Screw _G_, and set the Sun directly over his place on the
Meridian; which done, set the Moon’s Wire under the number that
expresses her age for that day on the Plate _F_, and she will then stand
over her place in the Ecliptic, and shew what Constellation she is in.
Lastly, fasten the Screw _G_, and laying the curved edge of the
pasteboard _S_ over the Ecliptic below the Moon, adjust the Moon to her
Latitude over the graduated edge of the pasteboard; and the Globe will
be rectified.
[Sidenote: It’s use.]
Having thus rectified the Globe, turn it round, and observe on what
points of the Horizon the Sun and Moon Balls rise and set, for these
agree with the points of the Compass on which the Sun and Moon rise and
set in the Heavens on the given day; and the Hour Index shews the times
of their rising and setting; and likewise the time of the Moon’s passing
over the Meridian.
This simple Apparatus shews all the varieties that can happen in the
rising and setting of the Sun and Moon; and makes the forementioned
Phenomena of the Harvest Moon (Chap. xvi.) plain to the Eye. It is also
very useful in reading Lectures on the Globes, because a large company
can see this Sun and Moon going round, rising above and setting below
the Horizon at different times, according to the seasons of the year;
and making their appulses to different fixed Stars. But, in the usual
way, where there is only the places of the Sun and Moon in the Ecliptic
to keep the Eye upon, they are easily lost sight of, unless covered with
Patches.
[Sidenote: The PLANETARY GLOBE.