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In this section, I discuss the parts of the astrolabe. I use the same terms that are used during the renaissance era. In the section of the article where I describe how to use the astrolabe, I use the same names for the parts that I use here. The reason for this is simple, if you are using an astrolabe at a renaissance faire, you are playing a man of learning. By using terms that are foreign to the patron, you demonstrate your learning. You can call the parts anything you want, but if you use terms like, "this here pointy thing," you won't sound as erudite.
Feel free to bookmark this section and refer back to it often when learning how to use your astrolabe. Later, when the various uses of the astrolabe are discussed, I have included the definitions of some of the more esoteric terms as JavaScript links. If your brouser is enabled for JavaScript, simply hover your mouse pointer over the underlined words to refresh your memory of words if you don't remember. I have also included a Glossary for your reference.
Armilla is Latin for ring. The ring is used to hold the astrolabe when using the sighting vane. It should be loose enough to allow the astrolabe to swing freely, back and forth.
The ring is connected to a projecting shoulder called the throne. The throne is also called the kursi on Islamic astrolabes.
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The entire round immovable portion of the astrolabe is called the mater (Latin for mother). Its parts are the hollowed out portion, known as the womb, and the raised outer portion, known as the limb. A hole is drilled in the center to accept the pin (the small nut and bolt) that holds everything together. Inside the womb are the plate, the rete, the rule, and the pin.
The raised outer portion of the mater is called the limb. The outer portion of the limb contains letters of the alphabet. The letters "J," "U" and "W" are missing so that there are only 23 letters. And, with the cross on top, that's 24 symbols corresponding to the 24 hours of a day. The cross at the top of the limb denotes noon or south. The letter F denotes 6 AM and West, the letter M denotes midnight and North, and the letter S denotes 6 PM and East.
Some astrolabes have diamonds in between the letters for the half-hours.
Inside the letters there is a ring of degrees. The larger marks correspond to 5 degrees, or 20 minutes. This is also called a "mile-way," because a man is expected to walk a mile in 20 minutes. The smaller marks correspond to 1 degree, or 4 minutes.
Note: Whether you are looking at the front of your astrolabe, or at the back, the top is South, the bottom is North, the left is East, and the right is West. While this is counter-intuitive, it is important, and the reasons will become apparent later.
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Most astrolabes today don't come with removable plates. However, for the purpose of clarity, the etched bottom of the womb will be called the plate. Different plates were used depending on the latitude in order to make calculations more accurate. A plate is also called a climate or a tympanum.
Note that the plate has a vertical line etched in it running through the center and also a horizontal line, dividing the plate into even quarters. The portion of the vertical line above the horizontal line is called the south line, or the line meridional. The portion of the vertical line below the horizontal line is called the north line, or the line of midnight. The portion of the horizontal line to the left of the vertical line is called the east line, or the line oriental; and the portion of the horizontal line to the right of the vertical line is called the west line, or the line occidental.
The three concentric circles are the tropics. The smallest of these circles is called the circle of Cancer because the head of Cancer turns on this circle. (I'll discuss this when we discuss the rete.) It is marked at 23 degrees, 30 minutes on the stereographic projection.
The middle circle is called the equinoctal circle and is also called the weigher, or the equator, of the day. When the heads of Aries and Libra are in the equinoctial circle, the days and nights are equal. Again, we'll discuss the heads of the zodiac signs when we get to the rete section. On these two days, one in the spring and one in the fall, we call them the eqinox. The equinoctal circle is marked at the equator of the stereographic projection.
The widest of the three concentric circles is called the circle of Capricorn. This is marked at 23 degrees, 30 minutes on the stereographic projection. The head of Capricorn turns on this circle.
There are non-concentric circles that are etched into the plate. These are known as almucanters, or almucanteras and are used to determine altitude. The lowest line in the almucanters is the horizon - the line where the sky meets the earth. A star or planet that is above the horizon is visible. A star or planet that is below the horizon line is not visible. The point in the middle smallest circle in the almucanters is your zenith - the point in the heavens directly above your head. The individual lines in the almucanters vary in number but represent equal numbers of degrees. On my astrolabe, for example, there is one line for every five degrees of altitude. Other astrolabes have a line for every two degrees of altitude.
The lines that radiate out from the zenith are lines of azimuth. The azimuth angle is the angle from the north. Astrolabes vary in the number of azimuth lines that they have. Count the number of azimuth lines your astrolabe has and divide 360 degrees by that number. The resulting answer will be the number of degrees each azimuth line represents. For example, in Norman Greene's 4-inch astrolabe without interchangeable plates, there are 24 azimuth lines. That means that each line represents 15 degrees of azimuth. His eight-plate astrolabe has 36 azimuth lines. So, each line represents 10 degrees of azimuth.
By knowing both the altitude and the azimuth of an object, you can locate it in the sky.
The lines that radiate downward from the horizon are the lines of unequal hours, also called hours of planets. These lines divide the night by 12 from when the sun goes down until it rises again. They are called unequal hours because sometimes they are shorter than an hour in the summer, and in the winter they are longer than an hour. |
The rete (Latin for net) is sometimes also called a spider. Rete is pronounced ree-tee in the Latin (rhymes with tee-pee). It is set on an ecliptic on top of the plate and spins in any direction. It has three main components. There are some of the visible stars located at the ends of the points, and on the ecliptic that are the twelve houses of the zodiac divided into thirty degrees with each of the larger marks representing ten degrees and each of the smaller marks representing two degrees. The houses of the zodiac are read counter-clockwise. The head of any sign of the zodiac is the beginning letter of that sign. Also, on top of the head of Capricorn, there is a little tab that is always close to the limb called the almuri. This comes in handy for making minor calculations.
Most people who perform at faire use the astrolabe made by Norman Greene. If your astrolabe is made somewhere else, simply consult a star chart to locate the stars on your rete. Here is a list of stars that are on the rete on the 3 and 4-inch astrolabes made by Norman Greene:
| Star Name | Listed As | Location if Not Identified | Constellation |
| Aldebaran | Aldebora | Taurus | |
| Alioth | Alit | Ursa Major | |
| Aliphard | Alfard | Hydra | |
| Alpheca | Alpheca | Corona Borealis | |
| Alpheratz | Alfer | Andromeda | |
| Altair | Altair | Aquila | |
| Arcturua | Arcturua | Bootes | |
| Betelguese | Betelguese | Orion | |
| Deneb | Deneb | Cygnus | |
| Diphda | Diphda | Cetus | |
| Dubhe | D | Near the pin | Ursa Major |
| Markab | Mark | Near Piscea | Andromeda |
| Menkar | Menkar | Cetus | |
| Procyon | Procyon | Canus Minor | |
| Rigel | Rigel | Orion | |
| Rosalhague | Rosalhague | Ophiuchus | |
| Sirius | Sirius | Canus Major | |
| Spica | Near Libra | Virgo | |
| Vega | Vega | Lyra |
The rule is simply the pointer on the face of the astrolabe that is used to find the result of whatever you are calculating and is held in place with the pin.
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Like the rule, the alidade is used to make calculations and conversions. In astronomical and astrological work, it is used for converting days into degrees of the zodiac. Unlike the rule however, the alidade also has a sighting vane.
The sighting vane is used in conjunction with the ring of degrees to determine the altitude of just about anything you can see. Mostly, however, it is used to determine the altitude of heavenly objects.
At night, or when sighting anything other than the sun, the sighting vane is used by holding the astrolabe by the armilla and looking through either the holes or notches and turning the alidade until you can see the desired object through both holes or notches.
During the day, when sighting the sun, the astrolabe is held chest height and looking at the shadow cast on your chest. By turning the alidade until both notches match up, you can determine the altitude of the sun. Caution: Never sight the sun by looking through the sighting vane at the sun!
As with the face of the astrolabe, the back is divided into four quarters. The vertical line at the top descending to the center is called the south line, or the line meridional. The vertical line running from the center to the bottom is called the north line, or the line of midnight. The horizontal line running from the center to the left is called the east line, or the line oriental. And, finally, the horizontal line running from the center to the right is the west line, or the line occidental.
The outermost ring is called the circle of degrees. It is divided into equal quarters, and each quarter is divided into 90 degrees. The longer lines represent 5 degrees, and the smaller lines represent individual degrees.
The next ring inwards is called the circle of the zodiac. It contains the names of the twelve signs of the zodiac: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces. Each sign of the zodiac contains thirty degrees, with the longer lines representing five degrees and the smaller lines representing individual degrees.
The innermost ring on the back of the astrolabe is called the circle of months. It has the names of each month of the year, along with divisions for each five days of the month, as well as smaller marks representing individual days in each month.
The shadow square is a useful tool in solving many trigonometric problems.
The shadow square, also called altitude scale, is the basic element of the so-called geometrical square. The instrument serves to measure heights and distances by simulating the ratio between a gnomon and its shadow. The umbra recta simulates the shadow cast on the horizontal plane by a vertical gnomon when the Sun's ray is inclined between 0° and 45°. The umbra versa simulates the shadow cast on the vertical plane by a horizontal gnomon when the Sun's ray is inclined between 45° and 90°. To each value of the umbra recta corresponds a value of the umbra versa. When the ray is inclined by 45°, the two shadows are equal (umbra media).
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