The astronomical or universal ring is a solar ring which is used to find the time of day in various places on earth, whereas the usage of the one we just discussed, is limited to finding only certain latitudes. [1] Its shape is represented in the Plates on Gnomonics , [Plate II] Figure 22. See also Dial .

This instrument is made in different sizes. It can be from two to six inches in diameter. It consists of two slender rings or circles whose width and thickness are in proportion to the size of the instrument. The exterior ring A represents the meridian of the place where the instrument is being used. It contains two divisions of 90 degrees each, diametrically opposite each other, and which serve as the northern hemisphere and southern hemisphere, respectively. The interior ring represents the equator and exactly turns within the first ring using the two pivots which are at 12 o’clock in each ring. Across the two circles is a small ruler or strip with a cursor marked C , which can slide down the middle of the ruler. In this cursor is a small opening that allows the rays of the Sun to pass through.

Consider the axis of the ruler to be the axis of the world, and its extremities as the two poles. On one side are the signs of the zodiac, on the other the days of the month: on the meridian is a piece which can slide and to which a small pendant is attached which carries a ring to hold the instrument.

Use of this instrument . Place line A , marked in the middle of the pendant, on the degree of latitude where the instrument is placed, for example, 48 degrees 50 minutes for Paris. Place the line which crosses the opening of the cursor on the degree of the sign or on the day of the month. Then open the instrument so that the two rings make a right angle together and suspend it by the pendant H , so that the axis of the ruler which represents that of the instrument should be parallel to the axis of the world. Then turn the flat side of the ruler toward the sun, until the ray that passes through the small hole falls exactly on the circular line which is traced through the middle of the concave circumference of the interior ring: the sun’s ray marks what hour it is on the concave circumference.

It must be noted that the hours of midnight and noon are not given by the dial, because the exterior circle is within the frame of the meridian, preventing the rays of the sun from falling on the interior circle. The dial also does not give the hour when the sun is in the equator, because then its rays are parallel to the frame of the interior circle.

There is still another sort of astronomical ring built on the same principles as this last one, except that instead of having two, it has three circles. It has certain advantages over that one, in that it gives the hour of noon and marks when the Sun is in the equator; it is even a little more accurate. Anyway, we hardly ever use these instruments anymore, the use of watches having made all these dials that do not give the time very accurately superfluous.

Astronomical Ring is also the name of the instrument that is used at sea to take the height of the Sun. It is a type of zone or circle made of metal. See the Plate on Navigation , figure 1. In this zone there is a hole C , which cuts across it parallel to its plane; this hole is separated by 45 degrees from its support B ; and it is the center of the quarter circle DE , one of whose radii, (ending CE ), is parallel to the vertical diameter, and the other ( CD ) is horizontal and perpendicular to the same diameter BH . To divide the arc FG of this ring into 90 degrees, you draw on the plane a circle FGC equal to the interior zone of this ring . From point C , taken at 45 degrees from point B , as the center, and from one radius taken at will, draw a quarter circle PQR , of which the radius ending PC is perpendicular to the diameter BD, and the other CR is parallel to it. Then divide the quarter circle by degrees, and draw through the center of C and through all the points of division of the quarter circle radii which cut the circumference FDG into as many points as correspond to the degrees of this quarter circle. These divisions or degrees taken and applied respectively to the astronomical ring from F to G , will divide it perfectly.

To observe the height of the Sun with this instrument, you need to suspend it by the buckle B and turn it toward the Sun A, so that its ray passes through opening C ; it will mark at the bottom of the ring from F to I , the degrees of the height of the Sun between the horizontal radius CF and the radius of the star CI . The section IHG mark its distance to the zenith, determined by the radius CI of the star, and the vertical radius CG .

Observations done with the astronomical ring are more exact than with an astrolabe, because proportionately to its size, the degrees of the ring are larger. See Astrolabe .

1. The article immediately preceding this one is “Anneau solaire” [Solar ring].