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Java applet page: Keith's Astrolabes |
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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.)
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.
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.
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.
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