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 a line, a latitude, a fix or the time, 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, Venus, Mars, Jupiter, Saturn and the navigational stars
• Checked against JPL Horizons at 40 instants from 2000 to 2035: the Sun, the Moon, Jupiter and Saturn within half a minute of arc, Venus and Mars within a minute
• Star positions carried from the catalogue epoch to today with precession and proper motion

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
• Hc, Zn and the intercept in nautical miles, toward or away
• A series of shots brought down to one sight along the slope the almanac gives, with a misread shot found and left out
• Sights below 15 degrees are marked; below 5 degrees a sight stays out of the fix until you confirm it

NOON, POLARIS AND THE MOON
• Local noon at your DR, carried along your course and speed, and the latitude from the meridian altitude
• Longitude by equal altitudes, solved for the sun's change of declination rather than halving the times
• Latitude by Polaris, the triangle solved for the moment instead of three tables
• The lunar distance: cleared, and turned into the time at Greenwich and your chronometer's error

THE CLOCK
• Chronometer error and daily rate, applied to every sight
• The rate found from your daily comparisons with a time signal, with each comparison's distance from the line
• 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, with a running fix along your course and speed
• The angle of cut, named poor below 30 degrees
• Three sights give a cocked hat, not a point: its size in miles, its centre, and the mean intercept that shows a systematic error

TOOLS
• Compass error by azimuth or by amplitude at sunrise and sunset, and the deviation once the chart's variation is in
• Which star was that: name a body after the shot from its altitude and a rough bearing
• A note before morning and evening star time and before noon, and a widget with the next one

PRACTICE
• Simulated sights with realistic error and a known answer: the intercept, the noon latitude, Polaris, a compass check and plotting a fix
• You work them; the app gives the difference with its sign
• When the difference matches a known mistake, it names it: a missed dip, a reversed sign, seconds of clock
• No points, badges or streaks

LORE
Short articles on the craft: the clock, the four corrections, low sights, the cocked hat, why stars move, the noon sight, Polaris, the compass, the lunar distance and where the line of position came from.

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, 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, opens Venus, Mars and the navigational stars, the multi-sight fix, practice and the unlimited sight log, and removes the ads.