Intercept

Celestial navigation is a perishable skill. The only way to get better is to take sights, reduce them, and compare against a known position — but at sea, reliable ground truth is hard to come by. Intercept uses your device's GPS as that ground truth, giving you immediate feedback on every sun sight you take.

Enter your sextant altitude, the time of observation, and your assumed position. Intercept computes the observed altitude (Ho), calculated altitude (Hc), intercept distance, and azimuth using the same mathematics as manual sight reduction — the altitude-intercept method first described by Marcq St Hilaire in 1875. Your line of position is drawn on the map so you can see exactly where you stand — or not.

Meridian passage

The noon sight is the simplest and most reliable observation in celestial navigation. When the sun crosses your meridian it reaches its highest altitude. Measure that altitude, correct it, and your latitude falls out directly: Lat = Dec ± (90° − Ho). No assumed position, no intercept, no azimuth calculation — just one clean observation and one clean answer.

The Meridian Passage tab applies the standard altitude corrections and gives you your latitude. Compare it against GPS and you will see that a careful noon sight routinely lands within a mile.

Why practice with GPS feedback?

Research by Malkin (2014) found that an experienced observer's median sextant accuracy is around 0.7 nautical miles, with 95% of sights falling within 2 nm. But much of the error is systematic — a consistent personal bias in how you read the horizon, time the sight, or handle the instrument. You can only discover and correct that bias by comparing your sextant position against a known reference, repeatedly, under real conditions.

That is what Intercept is for. Take your sextant on deck, shoot the sun, and enter the data here. The tool computes the intercept — the difference between your observed altitude and the altitude calculated for your assumed position — and plots the resulting line of position on a chart. Compare it against GPS and you will quickly learn where your errors come from.

Resources

Astronavigation Sight Form (PDF) — a printable worksheet for recording sun sights, altitude corrections, and sight reduction. Take it on deck with your sextant.

Celestial Navigation for Yachtsmen — Mary Blewitt's classic reference, compact and practical. The standard text for learning sight reduction by hand.

Leon's Celestial Navigation Course — a thorough video series by a RYA Yachtmaster Ocean instructor, covering everything from first principles to ocean passages.

Mobile app

Intercept is available as a free web app you can install on your phone or tablet — no app store required. Install Intercept