Keith's universal astrolabe: description and use of the universal astrolabe displayed with my Java applet
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Keith's Universal Astrolabe

There is a set of Universal Astrolabe displays contained within this program. These displays are revealed by pressing 'u' or 'U' about ten times after which this series of displays repeats. You may also use the menu system to see any of these displays.

The first five displays show the universal plate, over which is superimposed the rete. The remaining displays show separate components of a universal astrolabe.

If you examine the general plate (Menu: Univ.Astr. /Plate general) you will see a set of arcs from the top-centre to the bottom centre. These arcs are the polar arcs, and the points at which they converge are the poles. The arcs which cross these between the left and right sides, and including the straight line across the centre, are the 'parallel' arcs. Although these are not parallel when drawn on a universal astrolabe, they commonly represent the parallel altitude circles or the parallel declination circles. The general plate is commonly used to solve problems of spherical trigonometry or spherical astronomy.

The general purpose rete (Univ.Astr. /Rete general) is very similar to the general purpose plate, and is commonly used with it.

On the normal plate, (Univ.Astr. /Plate normal) the parallel arcs and the polar arcs indicate the declinations and hour angles of the Sun and stars. The positions of several stars are indicated by small circles, and the ecliptic circle is shown as a straight line through the centre, from a position which is about 23.4 degrees below (to the left) and above (to the right) of the parallel arc representing 0.0 degrees declination (across the centre). This line is marked with the zodiac names. In the lower half of the plate, a time scale is arranged in a double arc.

On the normal rete, (Menu: Univ.Astr. /Rete horizon) the straight line down the centre marks the east/ zenith/ west arc with the Zenith at the top and the East/West points superimposed at the centre of the astrolabe. The straight line across the centre which crosses it at right angles represents the horizon. The end of the horizon line which (when it is sloping) is closest to the top of the astrolabe indicates its North position in northern latitudes and its Southern position in southern latitudes. The polar arcs on the rete indicate the azimuth, while the parallel arcs indicate the elevation.

Instead of a rete, universal astrolabes commonly had a regula and a brachiolus. The regula had a scale indicating the hour angle or the ra, and sometimes the zodiac markings. On it was fitted a jointed arm whose end could be positioned over any desired point on the plate. When the regula was rotated through the required angle, as seen on the scale on the outer edge, the point of the brachiolus rotated through the same angle.

Instead of the regula and brachiolus, it was practical to use a thread from the centre on which was a bead, commonly a seed pearl. The thread was moved to an appropriate angle and the bead was moved along the thread until the bead was over the desired point on the plate. The thread was then rotated through the desired angle and the new position of the bead could be read from the arcs on the plate.

Unfortunately, many of the things you might want to do would require this rotation of the regula or thread to be repeated many times, with small changes in the starting position of the brachiolus tip or the bead, until the answer to the problem was found. This iterative procedure is unnecessary with the rete.

The 'brachiolus' provided here uses a disc with a pointer on its edge. This disc is pivotted on an arm from the centre, the arm having markings on its outside edge at 23.4 degrees on either side of a pointer. (After positioning a brachiolus on some point, it is a common requirement to rotate it by 23.4 degrees.)

First display

The first display (Menu: Univ.Astr. /Summer solstice) shows a Universal Astrolabe plate above which is the horizon rete rotated anti-clockwise by the angle of your co-latitude, assuming you have set your latitude, of course. (The angle of your co-latitude is equal to 90 degrees minus your latitude.)

This first display has two uses. First, it allows you to determine the position of the Sun at any time and date, including the times of sunrise, sunset and twilight. Secondly, it allows you to see the positions of the stars at midnight on the day of the Summer Solstice, from which you can determine their positions at other times and dates.

Using the back, you must first determine the position of the Sun on the zodiac scale. On the plate, find this position on the ecliptic line (the red diagonal line) and note the nearest parallel arc. Follow this parallel arc until you reach the (usually diagonal) line on the rete which indicates the horizon. From this point, follow the nearest polar arc on the plate downwards to read the times from the figures arranged in a double arc towards the bottom of the plate. These figures indicate the times of sunrise and sunset (midday is at the left-hand edge).

The Sun moves from the crossing point of the horizon and the parallel arc you have just found above to the edge of the astrolabe on the left, following the parallel arc. At any point along this parallel arc you can read the time from the figures towards the bottom, and the azimuth and elevation of the Sun can be found from the polar arcs and parallel arcs on the rete, measured from the centre of the astrolabe. The rete also shows the -18 degree twilight parallel arc, allowing you to find the time of astronomical twilight.

The positions of the stars shown on the plate are those at midnight on the Summer Solstice. You can find the RA of a star from the polar arcs on the plate, and its declination from the parallel arcs.

They are shown as filled circles if in the eastern half of the sky, and as hollow circles if in the western half of the sky. You can read their azimuths and elevations at that time on the rete using the polar arcs and the parallel arcs, the northern end of the horizon line being that end which is towards the top of the astrolabe in northern latitudes.

Needless-to-say, although the stars are shown in their positions as at midnight at the Summer solstice, they are at the same positions two hours later for each month earlier. Thus they are shown as at roughly 04.00hrs on July 22nd, 02.00hrs on August 22nd, 24.00hrs on September 22nd, 22.00hrs on October 22nd, etc.. (Please don't quote me on those dates!)

To find the position of the stars at any other time or date, imagine them to be moved along the nearest parallel arc by 30 degrees per month and 15 degrees per hour, appropriately, the directions changing when a star reaches the edge of the plate. You will find the calendar scale on the back of the astrolabe useful when working out the angular difference of a different date.

The polar arcs also indicate the hour angle of the star at the times/dates indicated above. The track of a star is along the (nearest) parallel arc on the plate.

However, if a star needs to be considered in a very differnt position from the one shown, it may be more convenient to start from the second display.

Second display

The second display is similar to the first display but shows the rete rotated clockwise by the angle of your co-latitude.

This display also has two uses which are related to the two uses of the first display.

It is used to indicate the Sun's position in the sky at any time and date precisely as with the first display, except that midday is now on the right-hand edge.

It also shows the positions of the stars in the sky at midnight on the Winter Solstice. The explanation of how to find the azimuth and elevation of a star at any time and date is exactly the same as with the first display, but note that although the end of the horizon line which is to the north is still towards the top of the display in northern latitudes, the angle of the slope is now different.

Third display

This shows the rete rotated anticlockwise through an angle of 23.4 degrees. Using this display, you can convert equatorial coordinates (RA and declination) on the plate into ecliptic coordinates (longitude and declination) on the rete. You can find the positions of stars after considering precession by considering them to move along the nearest parallel arc of the rete by 360 degrees in 25800 years, or about one degree in 72 years. Thus, if you are interested in the ecliptic coordinates of a star 720 years ago, subtract 10 degrees from its longitude, moving along the parallel arc on the rete, and for that date you can then read its RA and declination from the plate.

Not shown

If you rotate the rete anti-clockwise by your co-latitude, as in the first display, and then rotate it clockwise by an extra 23.4 degrees, (there is a red line on the rete for this) you will be able to track the ecliptic coordinates of the paths of stars over the centuries, due to precession, at the time of midnight on the Summer Solstice. The stars move along the parallel arcs on the plate by 360 degrees in 25800 years, and their azimuth and elevation for that year can then be read from the rete.

If you rotate the rete clockwise by your co-latitude, as in the second display, and then rotate it clockwise, repeat, clockwise as above, by 23.4 degrees, then using the same red line, you will be able to determine the ecliptic coordinates of the stars as affected by precession as above, as at midnight on the Winter Solstice.

Remaining displays

The remaining displays are intended to be printed. They show two alternative views of the plate, two alternative views of the rete (which should be printed on transparent film), and a regula, followed by two items which if used together allow you to make a brachiolus. The brachiolus can be used to point to any position on the plate, and can then rotated by an appropriate angle to show the new position. The edge of the larger part of the brachiolus has a pointer, as well as marks which are 23.4 degrees on either side of this mark, to simplify making rotations of this angle.


Note that the descriptions of uses above 'do not exclude' other applications. (Thanks are, of course, due to John North for this expression.)

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Copyright Keith Powell 1999-2002