The sky, flattened
Two discs of brass and a pin. Every curve cut into it is the celestial sphere projected from the south pole onto the plane of the equator, which is the only reason a model of the heavens can be flat.
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01
Circles stay circles
Project a sphere from its own pole and every circle on it lands as a circle on the plane. Every angle survives. A maker with a compass can strike the whole sky onto a disc without ever plotting a point.
r = tan(45° − d/2)
02
It comes apart
A plate cut for one latitude and useless at any other. A pierced star map that turns above it. A rule. A pin. Nothing else.
- Retethe sky, pierced away to almost nothing
- Plateyour horizon, your zenith, your hours
- Materthe body, its rim divided into 360
03
Turn it over
The far side is an almanac and a table of tangents. A ring of every day in the year, spaced by where the sun actually stands that day, so the days crowd in January and thin out in July.
That crowding is the earth's orbit. It is an ellipse, and we are nearest the sun on the fourth of January.
04
Twenty stars
Each spike on the rete is filed to a point that sits at a real star, placed by its right ascension and declination. Anything south of the Tropic of Capricorn falls off the edge of the plate and cannot be carried at all.
It still works
Every line on the instrument in front of you was computed rather than drawn, and checked before it was allowed to exist: the horizon crosses the equator at due east and west, the angles survive the projection to fourteen decimal places, and the altitude read off the engraving agrees with spherical trigonometry to a ten-trillionth of a degree.
Modelled and rendered in Blender. Geometry computed from the stereographic projection. Latitude 39.0997 N. Built by noahlot.
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