Sextant: The Modern Astrolabe

An elegant, accurate instrument using mirrors became the modern version of the mariner’s astrolabe around 1730. Its first version was called the octant, thus named because its measuring surface for angles was one eighth of the circle. The sextant, the modern instrument, has a measuring arc of a sixth of a circle. The two simultaneous but independent inventors of this device, intended to measure the angle between a celestial object and the horizon, were an English mathematician, John Hadley (1682-1744) and an American glazier, Thomas Godrey (1704-1749). Isaac Newton (his name looms large) tends to be credited with inventing the principle of using mirrors in such an instruments, but while he wrote down the idea, he did not publish it.

The best way to convey the use of the sextant is by demonstration. Pictures are better than words, animations even better. Herewith the first image of an award-winning series produced by Joaquim Alves Gaspar called “Animation of the use of a marine sextant to measure the altitude of the sun”. You can see the animation by clicking on this image. It comes from Wikipedia (link).

As we can see, the user can locate the sun (in this case) and then, while watching the horizon through half of image provided by the visor, cause it to overlay the horizon line precisely by moving the index arm. Modern sextants come with filters so that looking at the sun does not endanger the eyes. When the sun is precisely on the horizon, the reading of the index arm against the arc below provides the exact altitude—exactly the same service that the astrolabe provided; in the animation that altitude is 40°. That number, can then, by means of an equation I discussed in the first post on this subject, provide the exact latitude at which the sighting was taken.

Latitude provides one of two (the horizontal, east-west) coordinate needed to locate ourselves on this planet. Longitude (the vertical coordinate, north-south) required high-precision clocks—at least before satellites rose into the sky. I’ll summarize that history in a future post.

This is the second of two posts in a series. The first is here.