Do you know what an astrolabe is? | Behind the Lore
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When Hubert finds out Rafal owns an astrolabe, he immediately concludes the boy watches the stars: “If you have an astrolabe, you must be looking at the stars.” The object betrays the young man’s forbidden passion and becomes the very tool of the heliocentric quest the two will pursue together.
The subject in depth
A planispheric astrolabe is a map of the sky folded onto a disc [Wikipedia] . Its geometric principle is stereographic projection: the celestial sphere is projected onto a plane from the pole, which turns the circles of the sky into circles on the metal, easy to engrave.
The instrument stacks several parts. The mater is the graduated body, marked in degrees and hours. The tympan carries the local coordinates (horizon, altitudes, hour lines), engraved for a given latitude. The rete is an openwork rotating grid showing the bright stars and the Sun’s yearly path. On the back, a pivoting rule, the alidade, is used to sight a body and read its altitude on the graduated rim.
Using it takes two steps: measure the altitude of a known star, then rotate the rete to the matching position. The whole configuration of the sky then snaps into place, giving the time, the orientation, and what you need to compute latitude. That versatility made it the reference instrument of the medieval scholar.
One question often comes up here: if the stars are already engraved on the instrument, why must you still sight one? The answer is the subject of its own page, why measure a star with an astrolabe.
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Why measure a star with an astrolabe
The stars are already engraved on the astrolabe, so why sight one? The instrument is a calculator that derives the time, your orientation and your latitude.
Star magnitude
Rafal marvels at seeing a magnitude-six star in Orb. We explain what magnitude is, why the scale runs backwards, and the limit of the naked eye.
Using an astrolabe
Rafal observes the sky for Hubert in Orb. We walk you through, step by step, how to use an astrolabe to measure a star's altitude and read the time.
Heliocentrism
Hubert defends a forbidden idea in Orb: the Sun at the center, the Earth turning. We explain heliocentrism, its history, and why it caused such an uproar.
Ptolemy's geocentric model
Before heliocentrism, the Earth sat at the center of the world. We explain Ptolemy's geocentric model, its epicycles, and why it lasted over a thousand years.