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Glossary

Alilade: The pointer on the back of the astrolabe. Used for finding the sun in the zodiac, calculating the seasonal hour and solving basic trigonometric problems using the shadow box.

Almucanters: Lines drawn on the front of the astrolabe that circle the zenith. They are used to determine the height of an object above the horizon. Think of them as imaginary lines in the sky that are parallel to the horizon. The horizon is an example of an almucantar.

Altitude Scale: The scale on the back of the astrolabe labeled with degrees from 0 to 90. This scale is designed for use with the alilade when it is used as an instrument for observing the height of celestial objects.

Azimuths: Lines drawn on the front of the astrolabe that radiate like spokes from the zenith. They are used to determine direction relative to the cardinal points (i.e. north, south, east and west.) Think of them as imaginary lines drawn from the zenith straight down to the horizon. The meridian is an example of an azimuth.

Calendar Scale: The scale on the back of the astrolabe used in conjunction with the zodiac scale to find the sun along the ecliptic. It is the inner, slightly off-center circle labeled with the months of the year.

Celestial North Pole: For those living in northern latitudes, the celestial north pole is the point about which the entire sky appears to rotate. In our century, the North Star (Polaris) is very close to the celestial north pole. On the astrolabe, the north celestial pole is represented by the hole in the center of the astrolabe.

Celestial Equator: The line representing the point halfway between the north celestial pole and the south celestial pole. On the astrolabe, it is represented by the dark circle passing through east and west on the horizon.

Declination: The angular distance of an object from the celestial equator. Used for locating objects in the sky.

Ecliptic: The path that the sun follows through the sky during the course of a year. The sun passes through the twelve constellations of the zodiac as it travels along the ecliptic. Each constellation is divided into 30 degrees. On the rete, the ecliptic is represented by the off-center circle labeled with the constellations of the zodiac.

Height: The angular distance of a celestial object from the horizon. Height can be measured using the vanes on the alilade and the altitude scale.

Horizon: On the astrolabe, the horizon is represented by the dark line labeled with the directions north, east and west.

Latitude Plate: The central part of the front of the astrolabe representing the horizon and the visible portion of the sky. Each latitude plate is specifically drawn for a particular latitude.

Mater: The outer portion of the front of the astrolabe labeled with Roman numerals. The Roman numerals represent the twenty-four hours of the day. In Medieval times, the mater acted as a "holder" for the latitude plates, which could be removed and replaced.

Meridian: An imaginary line drawn through the sky from south to north and passing through the zenith. On the astrolabe the meridian is represented by the central vertical line on the front plate. When a star crosses the meridian, it has reached its maximum height for the day. When the sun crosses the meridian, it is noon.

Rete: The rete is a map of the northern half of the celestial sphere. In addition to stars and constellations, the rete contains a circle representing the path of the sun through the heavens (the ecliptic). On a modern astrolabe, the rete is printed on a transparent disk. On Medieval astrolabes, the rete was cut out of a piece of brass with pointers indicating the position of stars.

Rule: The pointer on the front of the astrolabe. Used for finding the time of day by positioning it next to the sun and reading off the hour on the mater. It also has a declination scale that allows one to measure the declination of any object on the rete.

Seasonal Hours: An old way of dividing up the day into hours. Every day was divided into twelve seasonal hours no matter how many "real" hours it actually lasted.

Seasonal Hour Arcs: The arcs drawn on the top half of the back of the astrolabe. Used for calculating the seasonal hour once the declination of the sun was known.

Shadow Boxes: The shadow boxes are used for solving simple trigonometric problems, especially those involving the length of shadows.

Throne: The tab at the top of the astrolabe containing a single hole. The purpose of the hole is to provide a place where the astrolabe can be vertically suspended when taking observations with the alilade. The thrones of most Medieval astrolabes were extremely ornate and hung by a simple metal ring.

Tropic of Cancer: The line in the sky representing the maximum declination of the sun (which occurs during the summer solstice.) On Earth, the tropic of cancer is the latitude where the sun will be directly overhead during the summer solstice. On the astrolabe, the tropic of cancer is the smaller circle just inside the celestial equator.

Tropic of Capricorn: The line in the sky representing the minimum declination of the sun (which occurs during the winter solstice.) On Earth, the tropic of cancer is the latitude where the sun will be directly overhead during the winter solstice. On the astrolabe, the tropic of cancer is the very edge of the latitude plate.

Twilight Almucantars: The almucantars drawn below the horizon for calculating how dark the sky would be before sunrise or after sunset.

Zodiac: The string of twelve constellations that lie along the ecliptic. In ancient times, the constellations were defined so that the sun spent exactly 1/12th of a year in each of these constellations.

Zodiac Scale: The scale on the back of the astrolabe used in conjunction with the calendar scale to find the position of the sun along the ecliptic. The zodiac scale is the outer circle labeled with the constellations of the zodiac.

Zenith: The point directly overhead. On the latitude plate, the zenith is represented by the circle just above the celestial north pole.