SightReduce Celestial

SightReduce Celestial is a paid, local-first worksheet for offshore sailors, celestial-navigation students, instructors, and traditional navigators. Enter an observed sextant altitude, index correction, height of eye, assumed latitude and longitude, and GHA and declination copied from your own nautical almanac. The app applies dip and standard refraction corrections, converts Hs to Ho, calculates Hc and Zn with spherical trigonometry, and derives a nautical-mile intercept with a clear Toward or Away instruction. The reduction screen keeps each correction visible instead of hiding the result behind a single answer. Carry the calculated azimuth and intercept into the local plotting aid, which draws the intercept point and perpendicular line of position on a clean navigation grid. Save multiple sights in a passage sight book, duplicate a full worksheet as a revision, retain observation time and notes, and export the reduction log as CSV with the original inputs and computed outputs. SightReduce does not provide almanac ephemerides and does not replace official navigation publications or prudent cross-checking. No login is required. The app has no subscription, in-app purchases, ads, analytics, tracking, cloud sync, AI processing, or developer-server upload.