Planispheric Astrolabe

This is a paper reconstruction of one of the four brass planispheric astrolabe by well known 15th century German astronomer, Georg Hartmann. It was originally made in 1532 in Nuremberg, Germany (49°27’0” N 11°5’0” E) to which this astrolabe had been calibrated to. The original astrolabe is currently in the permanent collection of British Museum.

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Operational Guide

  • Telling the position and visibility of stars for a given night at a given time

First you set up ant the back of the mater by using the alidade to point at the date on the calendar scale (at the inner most ring). Once aligned read the matching zodiac on the zodiac on the zodiac scale, which gives the position of the sun on that given day. Turn to the front of the astrolabe and locate the position of the star on the ecliptic ring on the rete then use the ruler to align with that point. Now as if the rete and the ruler are both fixed together, turn the ruler so it points to a time of interest for that given night. Once in position, the entire astrolabe is done setting and ready for observation. Every star pointer that lies within horizon line, indicated on the climate (with the stereographic projection of the entire night sky relative to the viewer’s zenith), is visible at that particular day and time.

  • Using the position of the Sun or star to tell time

First one finds the start of interest in the sky. Hang the astrolabe by its throne so aligns itself relatively perpendicular to the ground. Use the calendar scale and zodiac scale at the back of the ring to convert the given date to zodiac. Then use the alidade to point towards the star or sun using the two sights so that their sight hole aligns (note use the shadow of the sights to align in the case of sun). Once they align record the altitude using the elevation scale at the outer most rings. Now flip to the front. Locate the star of interest on the star pointer as a reference point. Rotate the rete so that this reference point intersects with the altitude (as recorded previously) on the stereographic projection on the climate. Once set, rotate the ruler so it points to the zodiac on the ecliptic ring on the rete. At the other end of this ruler indicates the time.

  • Measuring height or depth of objects using the shadow square.

Measure the distance between where the astrolabe is set and the object(such as a building) to measure. Hang the astrolabe at its throne so it falls relatively perpendicular to the ground. Then uses the alidade at the back of the astrolabe to sight the top of the building. Take the value pointed by the alidade on the Umbra Recta on the shadow square and divide by 12. Take one over that result and multiply by the distance between the astrolabe and the object of interest. The result gives the height of the object.

Mark L.

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