In the Center, No 1. represents the SUN, supported by it’s Axis
inclining almost 8 Degrees from the Axis of the Ecliptic; and turning
round in 25-1/4 days on its Axis, of which the North Pole inclines
toward the 8th Degree of Pisces in the great Ecliptic (No. 11.) whereon
the Months and Days are engraven over the Signs and Degrees in which the
Sun appears, as seen from the Earth, on the different days of the year.
[Sidenote: Mercury.]
The nearest Planet (No. 2) to the Sun is _Mercury_, which goes round him
in 87 days 23 hours, or 87-23/24 diurnal rotations of the Earth; but has
no Motion round its Axis in the Machine, because the time of its diurnal
Motion in the Heavens is not known to us.
[Sidenote: Venus.]
The next Planet in order is _Venus_ (No. 3) which performs her annual
Course in 224 days 17 hours; and turns round her Axis in 24 days 8
hours, or in 24-1/3 diurnal rotations of the Earth. Her Axis inclines 75
Degrees from the Axis of the Ecliptic, and her North Pole inclines
towards the 20th Degree of Aquarius, according to the observations of
_Bianchini_. She shews all the Phenomena described from the 30th to the
44th Article in Chap. I.
[Sidenote: The Earth.]
Next without the Orbit of Venus is the _Earth_ (No. 4) which turns round
its Axis, to any fixed point at a great distance, in 23 hours 56 minutes
4 seconds of mean solar time (221 & _seq._) but from the Sun to the Sun
again in 24 hours of the same time. No. 6 is a sidereal Dial-Plate under
the Earth; and No. 7 a solar Dial-Plate on the cover of the Machine. The
Index of the former shews sidereal, and of the latter, solar time; and
hence, the former Index gains one entire revolution on the latter every
year, as 365 solar or natural days contain 366 sidereal days, or
apparent revolutions of the Stars. In the time that the Earth makes
365-1/4 diurnal rotations on its Axis, it goes once round the Sun in the
Plane of the Ecliptic; and always keeps opposite to a moving Index (No.
10) which shews the Sun’s daily change of place, and also the days of
the months.
The Earth is half covered with a black cap for dividing the apparently
enlightened half next the Sun, from the other half, which when turned
away from him is in the dark. The edge of the cap represents _the Circle
bounding Light and Darkness_, and shews at what time the Sun rises and
sets to all places throughout the year. The Earth’s Axis inclines 23-1/2
Degrees from the Axis of the Ecliptic, the North Pole inclines toward
the beginning of Cancer; and keeps its parallelism throughout its annual
Course § 48, 202; so that in Summer the northern parts of the Earth
incline towards the Sun, and in the Winter from him: by which means, the
different lengths of days and nights, and the cause of the various
seasons, are demonstrated to sight.
There is a broad Horizon, to the upper side of which is fixed a Meridian
Semi-circle in the North and South Points, graduated on both sides from
the Horizon to 90° in the Zenith, or vertical Point. The edge of the
Horizon is graduated from the East and West to the South and North
Points, and within these Divisions are the Points of the Compass. On the
lower side of this thin Horizon Plate stand out four small Wires, to
which is fixed a Twilight Circle 18 Degrees from the graduated side of
the Horizon all round. This Horizon may be put upon the Earth (when the
cap is taken away) and rectified to the Latitude of any place: and then,
by a small Wire called _the Solar Ray_, which may be put on so as to
proceed directly from the Sun’s Center towards the Earth’s, but to come
no farther than almost to touch the Horizon, the beginning of Twilight,
time of Sun-rising, with his Amplitude, Meridian Altitude, time of
Setting, Amplitude, and end of Twilight, are shewn for every day of the
year, at _that_ place to which the Horizon is rectified.
[Sidenote: The Moon.]
The Moon (No. 5) goes round the Earth, from between it and any fixed
point at a great distance, in 27 days 7 hours 43 minutes, or through all
the Signs and Degrees of her Orbit; which is called _her Periodical
Revolution_; but she goes round from the Sun to the Sun again, or from
Change to Change, in 29 days 12 hours 45 minutes, which is _her
Synodical Revolution_; and in that time she exhibits all the Phases
already described § 255.
When the above-mentioned Horizon is rectified to the Latitude of any
given place, the times of the Moon’s rising and setting, together with
her Amplitude, are shewn to that place as well as the Sun’s; and all the
various Phenomena of the Harvest Moon § 273 & _seq._ made obvious to
sight.
[Sidenote: The Nodes.]
The Moon’s Orbit (No. 9.) is inclined to the Ecliptic, (No. 11.) one
half being above, and the other below it. The Nodes, or Points at 0 and
0 lie in the Plane of the Ecliptic, as described § 317, 318, and shift
backward through all it’s Signs and Degrees in 18-2/3 years. The Degrees
of the Moon’s Latitude, to the highest at _NL_ (North Latitude) and
lowest at _SL_ (South Latitude) are engraven both ways from her Nodes at
0 and 0; and, as the Moon rises and falls in her Orbit according to its
inclination, her Latitude and Distance from her Nodes are shewn for
every day; having first rectified her Orbit so as to set the Nodes to
their proper places in the Ecliptic: and then, as they come about at
different, and almost opposite times of the year § 319, and then point
towards the Sun, all the Eclipses may be shewn for hundreds of years
(without any new rectification) by turning the Machinery backward for
time past, or forward for time to come. At 17 Degrees distance from each
Node, on both Sides, is engraved a small Sun; and at 12 Degrees
distance, a small Moon; which shew the limits of solar and lunar
Eclipses § 317: and when, at any change, the Moon falls between either
of these Suns and the Node, the Sun will be eclipsed on the day pointed
to by the annual Index (No. 10,) and as the Moon has then North or South
Latitude, one may easily judge whether that Eclipse will be visible in
the Northern or Southern Hemisphere; especially as the Earth’s Axis
inclines towards the Sun or from him at that time. And when, at any
Full, the Moon falls between either of the little Moon’s and Node, she
will be eclipsed, and the annual Index shews the day of that Eclipse.
There is a Circle of 29-1/2 equal parts (No. 8.) on the cover of the
Machine, on which an Index shews the days of the Moon’s age.
[Sidenote: PLATE IX. Fig. X.]
There are two Semi-circles fixed to an elliptical Ring, which being put
like a cap upon the Earth, and the forked part _F_ upon the Moon, shews
the Tides as the Earth turns round within them, and they are led round
it by the Moon. When the different Places come to the Semi-circle
_AaEbB_, they have Tides of Flood; and when they come to the Semicircle
_CED_ they have Tides of Ebb § 304, 305; the Index on the hour Circle
(No. 7.) shewing the times of these Phenomena.
There is a jointed Wire, of which one end being put into a hole in the
upright stem that holds the Earth’s cap, and the Wire laid into a small
forked piece which may be occasionally put upon Venus or Mercury, shews
the direct and retrograde Motions of these two Planets, with their
stationary Times and Places as seen from the Earth.
The whole Machinery is turned by a winch or handle (No. 12,) and is so
easily moved that a clock might turn it without any danger of stopping.
To give a Plate of the wheel-work of this Machine, would answer no
purpose, because many of the wheels lie so behind others as to hide them
from sight in any view whatsoever.
[Sidenote: Another ORRERY.