BC Gnomonics
Planispheric Astrolabe
Latitude specific or Universal, Altitude, Equal and Seasonal hours
Astrolabes are two-dimensional models of the celestial sphere, with their key feature being the ability to model the sky and positions of the sun and stars. With this, the astrolabe is able to replicate and predict the paths of the sun and stars across the sky at any point in time.
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The astrolabe is rightly termed the "mathematical jewel of astronomy." Its utility is vast, encompassing the functions of many other similar instruments.
The computational might of the astrolabe results from its ability to manipulate three key variables: the altitude of a celestial body, the date, and the time. With any two of these points known, the third can be determined. This allows the astrolabe to solve a great many problems relating to the position of the sun and stars.
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The planispheric astrolabe's most distinct feature is the rete. This lattice holds the markers for each included star, as well as the ecliptic ring for the sun. The rete represents a close intersection of art and science, offering the designer a wide range a creative freedom; as long as the star indicators are positioned correctly, the design of the rete itself can take any form from the utilitarian to the intricately ornate.
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Planispheric astrolabes are often equipped with multiple latitude plates, supplying them with a projection of the local sky for each location of use.
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The reverse sides of these astrolabes often contain additional instruments. Some examples include unequal or equal hour quadrants and shadow squares.
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In addition to the uses highlighted below, the astrolabe can be used to simply model the sky for any moment in time, displaying the coordinates of the sun and stars both above and below the horizon. Most uses are derived from this one, and it is helpful to understand it before approaching other problems.
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How to use
(Finding time during the day)
1. Insert a tympan appropriate for the latitude of use.
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2. Consult the back of the astrolabe and take note of the current date. With the alidate set to the date of use, determine the sun's ecliptic degree using the zodiac scale. On the front of the astrolabe, set the rule to this ecliptic degree.
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3. Suspending the astrolabe by the throne, position the alidade so that the shadow of the upper sight falls exactly in footprint of the lower. Take note of the solar altitude measured by the alidade's edge.
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4. Turning to the front of the astrolabe, rotate the rete until the ecliptic point corresponding to the current date sits at the altitude previously measured by the alidade (if this measurement is taken before noon, this altitude should be set on the left side of the tympan. If taken after noon, it should be set on the right side).
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5. Read the position of the rule along the astrolabe's outer rim. This measures local apparent time. To determine civil time, add the appropriate longitude correction and equation of time correction (more information here). If applicable, add one hour during daylight saving time.
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