Compass Observation App

The compass check and celestial navigation toolkit for deck officers, yachtsmen and navigation students. Everything is calculated on your phone, offline, with no account, no ads and no data collection.

COMPASS & GYRO ERROR • Gyro and standard compass error from any body's true bearing, with variation (World Magnetic Model) and deviation worked out for you • SOLAS gyro-error limit check for your latitude • Amplitude worksheet: compass error at sunrise or sunset, with the exact bearing for the Sun's centre on the visible horizon (dip, refraction and parallax applied) as well as the classic formula • Deviation cards with a fitted deviation curve and a PDF report

SIGHT REDUCTION & FIX • Enter your sextant altitude in degrees and minutes as the sextant reads it • Index error, dip, refraction (with air temperature and pressure), semi-diameter and parallax, including the Moon's augmentation • Intercept and azimuth (Marcq St-Hilaire), drawn on a plotting sheet • Fix from two or more saved sights by least squares, including running fixes with course and speed, plotted with every line of position

SKY & EPHEMERIS • Sun, Moon, planets and the 57 navigational stars plus Polaris, computed to almanac accuracy (VSOP87 and Meeus algorithms, with nutation, aberration, refraction and ΔT) • Rise, set, transit and twilight times • Star Dome: the whole sky at a glance, with GoTo to any body • AR Sky: point your camera at the sky to see stars, planets, the Sun and Moon, all 88 constellations, classic Hevelius constellation art and the Milky Way. Search any star, planet or constellation and the app points you to it.

AT SEA • Logbook of compass checks, sights and fixes, with CSV export • Dark, light and red night-vision themes that protect your night vision on the bridge

Star data: HYG Database (CC BY-SA 4.0). Moon and Milky Way imagery: NASA Scientific Visualization Studio. Constellation art: Johannes Hevelius (1690), public domain.

Compass Observation App is a training and cross-checking aid. It is not a substitute for official nautical publications or the ship's primary means of navigation; always verify with approved sources.

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