Why measure a star with an astrolabe? | Behind the Lore
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Hubert is not after the time of day. He needs Rafal to record the positions of the stars night after night, because it is those accumulated readings that feed his calculations on the motion of the sky. The very same measurement that would simply give a traveller the hour becomes, here, the raw material of an inquiry into the shape of the Cosmos.
The subject in depth
The astrolabe already carries, engraved on the rete, the positions of the bright stars. If everything is known in advance, sighting a star seems pointless. The key lies in a distinction: a map of the sky gives its shape, that is, the positions of the bodies relative to one another, but not its orientation. And the sky turns, so at every instant that same map is presented at a different angle.
The instrument can be compared to a clock face with all its numbers printed but its hands removed. The numbers are the known stars; on their own, they do not tell what time it is. Sighting a single star and reading its altitude is like looking at where the hand points: that one measurement fixes the orientation of the whole vault. Once the rete is set to that altitude, the position of any other body, even one hidden below the horizon, can be read off directly.
The procedure amounts to solving an equation with one unknown. The known terms are the positions of the stars (the rete) and the latitude of the place (the tympan, engraved for a given location). The unknown is the orientation of the sky at the moment of observation. A single measurement is enough to determine it, and the instrument then delivers the rest: the time first of all, then the orientation (where north lies) and the length of the day.
Two uses mirror each other. If you already know where you are, measuring a body gives the time; this was the most common everyday use, in an age without reliable clocks, to know the hour at night or the time left before dawn. If, conversely, you measure a body whose true position is known, such as the Sun at noon or the pole star, you deduce not the time but your own latitude: this is the principle of navigation. In both cases the astrolabe measures a direction, an angle, never a distance. The distance to the stars stayed beyond the reach of any instrument until the 19th century [Wikipedia] .
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