Astrolabe: a working model

When

Where

°N °E

The plate (tympan) is cut for this latitude. Polar and equatorial latitudes are held to 5°–85°, where the projection is usable.

Instrument

Ring

Almucantars

Rule follows the Sun

Drag the rete (the brass star map) to turn time. Drag the rule to read the hour scale or a declination. Mouse wheel: ±10 min (Shift: day, Ctrl: year).

Selected

Click a star pointer or a planet on the rete.

How it is made

An astrolabe is a stereographic projection: every point of the sky is joined by a straight line to the south celestial pole and marked where that line crosses the equatorial plane. Circles on the sphere stay circles; the horizon and every almucantar become circles whose diameter is the projection of one chord of the meridian.

Almucantar30°

The highlighted circle on the plate is the projection of the blue chord: altitude 0° is the horizon itself.

Front of a planispheric astrolabe, drawn from the same sky as the Star Chart: stars from the HYG catalogue with proper motion and IAU 2006 precession, so the rete you see for 3000 BCE really is the rete of that epoch; the tropics move with the obliquity of the ecliptic. Planets and Moon from the repo's ephemeris approximations (< 1°). Earth's rotation is only known statistically before 1600 CE, so ancient dates can be off by several degrees of hour angle (about 3° at 1000 BCE, 16° at 5000 BCE); readings are geometric, with no refraction. The Ascendant is the ecliptic point on the eastern horizon. Stars: HYG Database v4.1 (CC BY-SA 4.0).