Astrolabe
Astrolabe is like an ancient smartphone for learning the motions of the heavens and finding the time...
Astrolabe is like analogue astronomical computer that models the daily and annual motion of the sky. It consists of a base plate representing the local horizon system and a rotating star map (the rete) centered on the north celestial pole. Together, these components allow the user to determine time, track the Sun’s position, measure star altitudes, and visualize the structure of the heavens from a specific latitude.
The front of the instrument shows a stereographic projection of the sky, including altitude and azimuth lines, the celestial equator, and the Tropic of Cancer. The rete carries the major constellations and an offset ecliptic ring marking the Sun’s zodiac position throughout the year. Rotating the rete simulates the apparent daily motion of the stars.
The back of the astrolabe contains practical tools: an altitude scale, a 360‑degree zodiac ring, a full calendar ring, and an Equation of Time graph. The alidade serves both as a sighting device for measuring celestial altitudes and as a pointer for aligning dates with the Sun’s longitude.
By combining altitude measurements with the Sun’s zodiac position, the astrolabe can determine local solar time, sunrise and sunset, day length, and all twilight phases. At night, the altitude of a bright star is used to orient the rete, after which the Sun’s position on the ecliptic reveals the current time.
