The Bris Sextant
The Bris sextant /ˈbriːs/ is not a sextant proper, but is a small angle-measuring device that can be used for navigation. It is, however, a true reflecting instrument which derives its high accuracy from the same principle of double reflection which is fundamental to the octant, the true sextant, and other reflecting instruments. It differs from other sextants primarily in being a fixed angle sextant, capable of measuring a few specific angles..
When the sun or moon is viewed through the V, it is split into eight images. In use, one waits until an image's edge touches the horizon, and then records the time and reduces the sight using the recorded angle for that edge of the image. In conjunction with a chronometer, it can be used to determine one's longitude.
It is small (fits into a standard film canister), rugged and inexpensively DIY (may be improvised from scavenged materials), perfect for emergency kits, bug-out bags and small vessel sailing. Once well calibrated, it is as (or in practice more) accurate than a sextant, albeit at a limited number of angular points (this is contrary to some accounts which mis-take the mechanism).
This is a truly ingenious solution which may be applied to celestial navigation, and more broadly to surface piloting, range-finding and other applications that require accurate angular measurements but allow discreet intervals. Its calibrated angles imbed trigonometric calculations, in effect, without the math.
This Wikipedia Article is a good place to start, with links to full descriptions:
https://en.wikipedia.org/wiki/Bris_sextant
