Keith's astrolabes: description and use of the plate displayed with my Java applet
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The following describes the plate of the conventional astrolabe displayed by the accompanying program. The plate of an equinoctial astrolabe has similar features but is different in many respects. It doesn't describe the plate of a universal astrolabe. A spherical astrolabe has no plate.
On the display of the basic astrolabe, the pink area, which represents the hemisphere of sky above the user, contains a 'spider's web' of arcs and circles. These allow you to find the altitude above the horizon and the compass bearings of any celestial bodies shown on the rete. The centre of this 'web' is the zenith, the point in the sky immediately above your head.

The spider's web of coordinates initially indicates azimuths and elevations at 5 degree intervals, but the intervals can be changed to 3 or 15 degrees by clicking on the coord+ or coord- buttons in the button panel on the left or by using the menu:

Menu: Astrolabe /Coordinates
which offers a selection of 3, 5 or 15 degree intervals.

If you have a latitude setting in the northern hemisphere, the point in the centre of the astrolabe (where the black horizontal and vertical straight lines cross) shows the position in the sky which is immediately above the northern celestial pole - and hence shows the approximate position of the pole star, Polaris in 2000 AD. With a latitude setting in the southern hemisphere, the centre of the astrolabe shows the position in the sky above the southern celestial pole. (There is no bright star above this pole at present but beta-Hydrus - mag. 2.8 - will be roughly in that position in 4000 AD.)

Also shown on the plate is an arc drawn a little below the horizon arc. This is the 'twilight' arc and is shown at 18 degrees below the horizon. When the Sun is more than 18 degrees below the horizon, there is insufficient light from the Sun to affect astronomical observations. On some astrolabes, arcs may be shown at 6 degrees and 12 degrees below the horizon. The 6 degree arc represents civil twilight. The 12 degree arc indicates Nautical twilight, and when the Sun is beneath this arc it is usually impossible to distinguish between the sky and the sea on the horizon.

Also shown on the plate are circles representing the equator and the tropic of Cancer. The tropic of Capricorn is represented by the inside edge of the scales around the perimeter of the astrolabe.

Within the lower half of the plate of the astrolabe you can see curves between the tropic of Cancer and the tropic of Capricorn circles. These usually showed the 'unequal hours'. Before the days of mechanical clocks, the day and the night were each divided into twelve hourly intervals. Although the methods differed in different countries, the day commonly began at sunset after which there were twelve equal-length hours until dawn, followed by twelve equal-length daytime hours.

Except at the times of the spring and autumn equinoxes, the length of each hour during the night was different from the length of each hour during the day, and these lengths changed daily throughout the year. If the astrolabe was set according to a star's angle in the sky, the time could be determined from the position of the sun on the ecliptic circle in relation to these unequal hour curves.

To allow you to display (or omit) the unequal hour curves, as well as curves for other methods of indicating the time, you can repeatedly press the '1' keyboard key or use the menu:

Menu: Astrolabe /Front - Hour arcs

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Copyright Keith Powell 1999-2002