Rainbow's Arc Positioning

A camera sextant, for the fun of it. Every boat carries GPS these days, usually several. This app is not a contingency plan for the day they all fail at once — though it would serve. It is a fun alternative to try, and it holds your position three ways — a camera sextant, a drill for your real sextant, and an honest deck log — that reinforce each other: the log carries you between sights, the sights correct the log, and the gap between them is the current talking. No signal needed, no accounts. The nautical almanac (sun, four planets, twenty-one named stars, computed on the phone through 2050), the math, and your deck log all live on the device and work anywhere, signal or none. THE CAMERA SIGHT Photograph the sun and the sea horizon in one frame; the picture and the clock do the rest. The horizon in the frame is the level reference, so the boat’s roll and your grip don’t matter. Mark the sun and horizon with draggable handles, a magnifier loupe, and one-pixel nudge arrows — a pixel is roughly a nautical mile, and the app lets you mean it. The full correction chain (dip, refraction, semidiameter, parallax) is shown line by line, the in-app camera reads its lens’s own per-shot calibration, and the result is a line of position with a stated radius. Cross a morning sight with an afternoon sight for a latitude-and-longitude fix. THE SEXTANT DRILL Own a real sextant? This is the genuinely useful part. The drill is your practice partner: shoot the sun, a bright planet, or a named star, tap MARK at the instant of the sight, enter what the arc read, and the app — clock, almanac, and answer key in one — scores you against your true position. Minutes of arc off is nautical miles off — index correction handled, and the app says what to check. THE DECK LOG A real dead reckoning, and the clearest way to see what set and drift do to a course: start from a known position, log course-and-speed legs, tap “Stopped” whenever the boat isn’t moving. A clear status strip says what the boat is doing; every sight reduces against the DR for its own instant; every fix can restart the reckoning with one tap. Check it against the GPS aboard and the gap comes back as set and drift, the current and leeway your legs couldn’t see; the course-to-steer card then turns that current into the heading that holds your track. Everything you enter, you can correct. HONEST BY DESIGN, FUN ON PURPOSE A camera sight is typically good to a few nautical miles — plenty for the fun of it, and not a chartplotter. Every result carries its own error radius, and when a sight isn’t trustworthy — sun too low, camera angle too uncertain, a line falling absurdly far from your reckoning — the app says so in plain language and refuses to pretend. One sight is a line, not a position, and the app never forgets the difference. This app is not meant as your main way to navigate, or even a serious alternative to conventional navigation. It’s a fun app — and if you own a sextant, a genuinely useful one. LEARN IT AT THE KITCHEN TABLE A built-in manual walks through all three ways and how to learn each, and an information button beside each setting explains it where you meet it. Practice mode draws a made-up morning and afternoon over a pretend sea, so you can mark sights, cross them into a fix, and restart the reckoning from it — the whole day’s work in ten minutes at the kitchen table. Never look directly at the sun. The screen does the looking; that rule is built into the app and worth repeating here. From the maker of Rainbow’s End Anchoring. Fully offline, no ads, no analytics, no data collected — location is read only if you ask the drill or the GPS check to referee, used on the spot, and never stored. May you find a rainbow’s end and treasure found wherever you sail or set your anchor. Apps for sailors, by sailors.