Instantly compare observed celestial navigation altitudes with high precision, live calculated altitudes. Shoot the Sun, tap pause in the app, and get instant feedback.

This app calculates the exact altitude that should be observed in celestial navigation at sea or on land assuming that the GPS position is correct. No paperwork is required, no tables of corrections. All corrections are included automatically: index error, system time error, dip of the horizon, refraction, temperature and pressure corrections to refraction, altitude above sea level, parallax, and also gravitational deflection of the vertical (a correction of up 2 nautical miles not handled by any other celestial navigation sight-clearing product) are all included automatically. If the GPS position is wrong, due to Spoofing at sea, observations will not match.

The app can function as a celestial navigation trainer: take a sight and immediately compare against the app's display. The app works off-network, even in the middle of the ocean.

If a network connection is available, the app will download temperature and pressure information automatically. The ephemeris data is generated from the same JPL numerical integrations of lunar and planetary motions used by the official Nautical Almanac and Astronomical Almanac. All coordinates are accurate to one second of arc, and displayed altitudes are accurate to better than a tenth of a minute of arc.


Frank Reed
13.5 MB

Requires iOS 8.0 or later. Compatible with iPhone, iPad, and iPod touch.



Age Rating
Rated 4+
© 2018 Clockwork Mapping, Conanicut Island USA


  • Family Sharing

    With Family Sharing set up, up to six family members can use this app.

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