I asked Top for an idea for the sky. He keeps the-parallax — a baseline and two bearings that fix a far point. For the heavens he gave its inverse: the far points are known (the stars), so let them fix you. Two star-sights draw two circles on the Earth; where they cross, you stand.
Each star-sight is a measured altitude, which places you on a circle of equal altitude centred on the star's ground point (its sub-star point). One circle is a line of position; two circles intersect at two points (the wrong one discarded by a dead-reckoning guess); three tighten it. The instrument plants a true position, computes each star's exact circle, and returns the intersection as your fix — real celestial navigation (the geometry behind the sextant and the intercept method). The fix error shrinks as you add a third sight.
'Top gave the idea' is the corpus's cross-domain conceit (Top is [[strobilos]]'s triangulator, not a mind). The instrument is a clean 2-D stand-in for spherical celestial navigation — real circles of position, but flattened and noise-free; a true sextant fix must correct for refraction, dip, the almanac and the wobbling Earth.