Editing Armillary sphere (section)

== Description and use ==
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{{Hatnote|This section, edited from the ''[[Encyclopedia Britannica]]'' first edition, refers to labels in the diagram below. (Open it in a second window on your screen for easy magnified reference.)}}
[[Image:EB1711 Armillary Sphere.png|thumb|upright|left|Armillary sphere diagram]]

The exterior parts of this machine are a [[Wiktionary:compages|compages]] [or framework] of brass rings, which represent the principal circles of the heavens:

# The equinoctial ''A'', which is divided into 360 degrees (beginning at its intersection with the ecliptic in [[Aries (constellation)|Aries]]) for showing the sun's [[right ascension]] in degrees; and also into 24 hours, for showing its right ascension in time.
# The ecliptic ''B'', which is divided into 12 signs, and each sign into 30 degrees, and also into the months and days of the year, in such a manner that the degree or point of the ecliptic on which the sun appears, on any given day, stands over that day in the circle of months.
# The [[tropic of Cancer]] ''C'', touching the ecliptic at the beginning of [[Cancer (constellation)|Cancer]] in ''e'', and the [[tropic of Capricorn]] ''D'', touching the ecliptic at the beginning of [[Capricorn (constellation)|Capricorn]] in ''f''; each circle 23{{frac|1|2}} degrees from the equinoctial circle.
# The [[Arctic Circle]] ''E'', and the [[Antarctic Circle]] ''F'', each circle 23{{frac|1|2}} degrees from its respective pole at ''N'' and ''S''.
# The [[equinoctial colure]] ''G'', passing through the north and south poles of the heavens at ''N'' and ''S'', and through the equinoctial points in Aries and Libra, in the ecliptic.
# The [[solstitial colure]] ''H'', passing through the poles of the heavens, and through the solstitial points in Cancer and Capricorn, in the ecliptic. Each quarter of the equinoctial [[colure]] is divided into 90 degrees, from the equinoctial to the poles of the world, for showing the [[declination]] of the sun, moon, and stars; and each quarter of the solstitial colure, from the ecliptic as ''e'' and ''f'', to its poles ''b'' and ''d'', for showing the [[Celestial latitude|latitude]] of the stars.

In the north pole of the ecliptic is a nut ''b'', to which is fixed one end of the quadrantal wire.  To the other end is a small sun ''Y'', which is carried around the ecliptic ''B''—''B'', by turning the nut. In the south pole of the ecliptic is a pin ''d'', on which another quadrantal wire is situated, with a small moon ''Ζ'' upon it, which may be moved around by hand. A mechanism causes the moon to move in an orbit which crosses the ecliptic at an angle of 5{{frac|1|3}} degrees, to opposite points called the [[lunar nodes]], and allows for shifting these points backward in the ecliptic, as the lunar nodes shift in the heavens.

Within these circular rings is a small terrestrial globe ''I'', fixed on an axis ''K'', which extends from the north and south poles of the globe at ''n'' and ''s'', to those of the celestial sphere at ''N'' and ''S''. On this axis the flat celestial meridian ''L'' is fixed, which may be set directly over the meridian of any place on the globe, so as to keep over the same meridian upon it. This flat meridian is graduated the same way as the brass meridian of the common globe, and its use is much the same.

To this globe is fitted the movable horizon ''M'', so as to turn upon the two strong wires proceeding from its east and west points to the globe and entering the globe at the opposite points off its equator, which is a movable brass ring set into the globe in a groove all around its equator. The globe may be turned by hand within this ring, so as to place any given meridian upon it, directly under the celestial meridian ''L''. The horizon is divided into 360 degrees all around its outermost edge, within which are the points of the [[compass]], for showing the amplitude of the sun and the moon, both in degrees and points. The celestial meridian ''L'' passes through two notches in the north and south points of the horizon, as in a common globe: if the globe is turned around, the horizon and meridian turn with it. At the south pole of the sphere is a circle of 25 hours, fixed to the rings. On the axis is an index which goes around that circle, if the globe is turned around its axis.

[[File:Clock Tower from Su Song's Book desmear.JPG|thumb|upright|The original diagram of Chinese scientist [[Su Song]]'s book of 1092 showing the inner workings of his [[clocktower]]; a [[Escapement|mechanically rotated]] armillary sphere crowns the top.]]

The globe assembly is supported on a pedestal ''N'', and may be elevated or depressed upon the joint ''O'', to any number of degrees from 0 to 90 by means of the arc ''P'', which is fixed in the strong brass arm ''Q''. The globe assembly slides in the upright piece ''R'', in which is a screw at ''r'', to fix it at any proper elevation.

In the box ''T'' are two wheels (as in Dr Long's sphere) and two pinions, whose axes come out at ''V'' and ''U''; either of which may be turned by the small winch ''W''. When the winch is put upon the axis ''V'', and turned backward, the terrestrial globe, with its horizon and celestial meridian, keep at rest; and the whole sphere of circles turns round from east, by south, to west, carrying the sun ''Y'', and moon ''Z'', round the same way, and causing them to rise above and set below the horizon. But when the winch is put upon the axis ''U'', and turned forward, the sphere with the sun and moon keep at rest; and the earth, with its horizon and meridian, turn round from horizon to the sun and moon, to which these bodies came when the earth kept at rest, and they were carried round it; showing that they rise and set in the same points of the horizon, and at the same times in the hour circle, whether the motion be in the earth or in the heaven. If the earthly globe be turned, the hour-index goes round its hour-circle; but if the sphere be turned, the hour-circle goes round below the index.

And so, by this construction, the machine is equally fitted to show either the real motion of the earth, or the apparent motion of the heavens.

To reset the sphere for use, one must first slacken the screw ''r'' in the upright stem ''R'', and taking hold of the arm ''Q'', move it up or down until the given degree of latitude for any place lies at the side of the stem ''R''; then the axis of the sphere will be properly elevated, so as to stand parallel to the axis of the terrestrial globe, if the globe assembly is to be aligned to north and south by a small compass: once this is done, the user must count the latitude from the north pole, upon the celestial meridian ''L'', down towards the north notch of the horizon, and set the horizon to that latitude. The user then must turn the nut ''b'' until the sun ''Y'' comes to the given day of the year in the ecliptic, and the sun will be at its proper place for that day.

To find the place of the moon's [[ascending node]], and also the place of the moon, an [[ephemeris]] must be consulted to set them right accordingly. Lastly, the user must turn the winch ''W'', until either the sun comes to the meridian ''L'', or until the meridian comes to the sun (moving the sphere or globe at the user's discretion), and then set the hour-index to the XII, marked noon, the whole sphere will be reset. Then the user must turn the winch, and observe when the sun or moon rises and sets in the horizon. The hour-index will show the times thereof for the given day.<ref>Elements of the general description incorporate text from the ''[[Encyclopædia Britannica]]'' First Edition (1771).</ref>