Planispheric astrolabe (demonstration model).Brass, engraved. After instruments made between the 10th and 16th centuries across the Islamic world and Europe.The circles, scales and the Sun’s place on this model are computed. Its twelve star pointers are schematic: their names are invented and their positions are illustrative only.
The sky in the hand
For some six hundred years the astrolabe was the most useful thing an astronomer could carry. With one its owner could tell the time by the Sun or by the stars, find the direction and the times for prayer, survey the height of a tower, cast a horoscope, and teach the motions of the heavens to a student. It did all this without a single moving part more complicated than a pin: the calculation is in the engraving.
Beneath the pierced star map lies a plate engraved with the sky as it appears from one latitude: the horizon, the circles of equal height above it, and the point directly overhead. Travel north or south and the horizon tilts against the stars, so the plate no longer fits the sky above you. Makers supplied a stack of plates, one for each latitude their client was likely to visit. This model has three.
The map looks strange because it is a round sky pressed flat. Imagine an eye at the south pole of the heavens, looking up through the sphere; every point is drawn where its line of sight crosses the plane of the equator. The north pole falls at the centre pin, the equator and the two tropics become concentric circles, and the Sun’s yearly path becomes the off-centre ring on the rete. The projection stretches the southern sky, but it keeps one promise that matters to an engraver: every circle on the sphere is still a circle on the brass.
Try it
Handle with ordinary care. The originals are behind glass in Gallery III.