Sextant: Celestial Navigation

Sextant is celestial navigation on your phone, without GPS and without a network. It is three things at once: a nautical almanac for any moment, a sight reducer that turns a measured altitude into an intercept and a fix, and a practice book of sights with a known answer.

The app never asks where you are. There is no location permission at all. You give it an assumed position, from dead reckoning, a port or typed coordinates, and it makes you do what a navigator does. Everything is computed on the device from its own ephemeris, so it works in airplane mode.

ALMANAC
• GHA Aries for any instant
• GHA and declination of the Sun, the Moon, the navigational planets and the navigational stars
• Star positions carried from the catalogue epoch to today with precession and proper motion
• Laid out like an almanac page, in degrees and minutes

SIGHT REDUCTION
• Hs to Ho in four steps, every one shown with its number: index error, dip, refraction, and semi-diameter and parallax
• Dip from your height of eye, which the app asks for and never assumes
• The Moon's semi-diameter and horizontal parallax from its real distance at the time of the sight, not a fixed value
• Hc, Zn and the intercept in nautical miles, toward or away
• Sights below 15 degrees are marked, because refraction there is uncertain; below 5 degrees a sight stays out of the fix until you confirm it

THE CLOCK
• Chronometer error and daily rate, applied to every sight
• One second of time is a quarter of a mile, and the app shows it that way
• The phone's own clock is offered as a suggestion, never taken as the time

FIX
• Two or three lines of position drawn to scale
• The angle of cut, named poor below 30 degrees
• Three sights give a cocked hat, not a point: its size in miles and its centre, and the mean intercept, because three lines that all err the same way make a small triangle in the wrong place

PRACTICE
• Simulated sights with realistic error and a known answer
• You reduce them; the app gives the difference in miles, with its sign
• When the difference matches a known mistake, it names it: a missed dip, seconds of clock, a body too low
• No points, badges or streaks

LORE
Short articles on the craft: why the clock matters more than the sextant, the four corrections, why low sights are not trusted, two lines and two points, the cocked hat, and why stars move.

Sextant is a calculator, not a sextant. A sight is as good as its horizon and its clock, and the app says so in its own article rather than in small print.

Terms used as a navigator uses them: GHA, SHA, LHA, Hs, Ha, Ho, Hc, Zn, intercept, LOP. Angles are entered in degrees and minutes with a tenth, not in decimals.

Free to install, with ads. A single one-time purchase, no subscription, removes them and opens the planets and stars, the multi-sight fix, practice and the unlimited sight log.