Sky Map · Finding Your Way by the Sky
For three centuries this was how sailors knew where they were. One careful sighting of the Sun against the horizon — and the open ocean becomes a map. Watch it work.
Tap any part below — or tap the instrument itself — to light it up on the sextant above.
Latitude is written in the sky. The higher the Sun (or a known star) climbs at its peak, the further south you are; the lower it stays, the further north. Measure that one angle precisely and — with the date — you have your latitude, anywhere on Earth, with nothing but the instrument in your hands.
The sextant’s trick is a double mirror. Looking through the little telescope you see the true horizon straight ahead. A pair of mirrors folds a second image — the Sun — into the same view. Swing the index arm and that reflected Sun glides down until it just kisses the horizon. However the ship pitches and rolls, the two images move together, so the reading holds steady on a heaving deck — the reason it beat every instrument before it.
The arc reads the angle for you. The graduated arc — one sixth of a circle, which is where the name comes from — turns the arm’s swing into degrees. That number, a nautical almanac, and a few minutes of arithmetic fix the ship on the chart. For the age of sail, this small brass triangle was GPS.
An animated Sky Map recreation · the geometry is real, the scene is illustrative.