The Astrolabe Plate
The astrolabe plate shows the stereographic projection of the celestial sphere on the plane of the equator. It is used to find the positions of celestial objects in the sky as seen by an observer at a specific location.
The curved lines on the plate represent positions in the sky. You can find anything in the sky if you know its angle above the horizon and the direction to look. The angle of something in the sky above the horizon is its altitude and its direction is its azimuth. You look down on an astrolabe exactly as you look down on a compass.
The straight lines that are diameters of the plate represent direction. The vertical line is north and south and represents the meridian for your location. South is at the top. The horizontal diameter is east and west with east at the left side of the plate.
The circles that are concentric with the center of the astrolabe are the tropics. The outer circle, which is the outside of the plate, is the Tropic of Capricorn, the southern limit of the Sun's annual motion. The middle circle is the equator and the inner circle is the Tropic of Cancer; the Sun's northern annual limit.
The web of curved lines inside the instrument show positions in the sky. The lower solid line represents your horizon; the line where the sky meets the Earth. Any celestial object that is above the horizon is visible, or would be if clouds, trees and buildings were not in the way. Any object that is below the horizon is not visible. The circles above the horizon represent lines of equal altitude above the horizon. Objects that fall on the same circle are the same altitude above the horizon. Each circle on the figure represents ten degrees in altitude; therefore, any object that is anywhere on, for example, the 50° circle has an altitude of 50°. Most old astrolabes had altitude arcs for every two degrees, although some instruments were marked for every one degree, three degrees or five degrees.
The point inside the smallest circle where the lines cross is your zenith; the point directly overhead. The altitude circles are also called by their Arabic name, "almucantars," in many astrolabe books.
The arcs that seem to radiate from the zenith are arcs of equal azimuth. The azimuth angle of an object in the sky is the angle from north, measured on the horizon. You can alway locate an object in the sky if you know its altitude and azimuth.
The dashed lines below the horizon show the levels of twilight. The solid arcs connecting the Tropics of Cancer and Capricorn below the horizon were used to determine the unequal hour of the day or night. The arcs radiating from the north point of the horizon were the astrological "Great Houses of Heaven" and were common on many European astrolabes.
Since the plate projection is for a given latitude, a different plate is needed for each location. Most astrolabes included several plates engraved on each side for use at different locations. Depending on the accuracy desired, a plate can be used within a few degrees of the design latitude. The plate for a given latitude is inserted in the mater.
The edge of the mater was always marked in degrees and with the hours of the day on later European instruments. The top of the mater (the throne) was attached to a ring or shackle so the instrument can be suspended for observations.
The astrolabe in the picture is the front of the Classic Edition of the The Personal Astrolabe. The Modern Edition of the Personal Astrolabe uses arabic numerals, does not include the unequal hour arcs or the "Houses of Heaven" and has some additional annotation for ease of use.
Click on the picture to see an astrolabe front with a rete and rule. The astrolabe in the figure is set for an apparent time of 2:30 PM, as shown by the rule on the limb. The rete is set for November 11 (Scorpio 20°) by setting the edge of the rule to that point on the ecliptic. The point of intersecton is the Sun's position. It may be hard to see in the picture, but there are many details available once that astrolabe is set to a date and time. For example, in the picture we can see the Sun's altitude is about 22°. The Sun's declination is about -18°. as shown by the divisions on the rule. The sidereal time is 17:42, as shown by the positin of the vernal equinox (Aries 0°). The positions of the stars are shown by the rete. For example, Orion is at lower culmination.
Once set to a date and time, the front of the astrolabe gives a very complete picture of the sky at that instant. A skilled user can solve many problems that would require significant calculations if you did not have such a wonderful device.