Intercept started as a personal tool. I wanted a way to check my sextant sights against GPS.

The result is a progressive web app that runs entirely in the browser. You enter your observed altitude, the time of the sight, and your GPS position. Intercept computes the calculated altitude and azimuth using spherical trigonometry, applies all the standard corrections (dip, refraction, semi-diameter, index error), and draws your line of position on a Leaflet map.

The entire astronomical calculation runs client-side; the only network request is loading the map tiles (which cache aggressively).

The stack

• Astro for the static shell and routing
• TypeScript for all sight reduction logic
• Leaflet for map rendering
• astronomy-engine for solar position calculations

The app lives at sailorslog.io/intercept.