Rainbow’s Arc — Support

It turns your phone into a camera sextant. You photograph the sun and the sea horizon in one frame; the app measures the sun’s height above the horizon from the photo’s own geometry, works the sight against a built-in nautical almanac using the time the photo was taken, and gives you a line of position — you are somewhere on that line, give or take a stated number of miles. Cross two sights taken hours apart and you get a fix: latitude and longitude, with an honest error radius. It runs entirely offline. It is a fun alternative to try, not a serious alternative to conventional navigation — and if you own a sextant, the drill that scores your sights is the genuinely useful part.

Three things. Height of eye — how far above the water you stand when you take the photo; it feeds the horizon-dip correction. A starting position — start your voyage’s dead reckoning at a known place (the marina you left, a buoy, a chart fix). And a phone clock that’s been online recently — each second of clock error costs up to a quarter mile of longitude, so let the phone sync before a long time offline. That’s it; the almanac is built in.

Sun and sea horizon in the same frame, with the horizon a true sea horizon — open water, not land, haze, or a cloud bank. Hold the phone any way you like: the horizon in the photo is the level reference, so the boat’s roll and your grip don’t matter. When the sun rides high, roll the phone so sun and horizon share the frame corner to corner — the tilt costs nothing. Use the in-app camera when you can; it reads the lens’s own calibration for each shot.

The sight screen offers a practice morning and a practice afternoon: a made-up sun over a made-up sea off Galveston, with the clock and position set to match. Mark both well, then cross them into a fix on the Camera tab — it should land within a few nautical miles of the practice starting point. It’s the whole flow, learnable at the kitchen table.

One image pixel is roughly one nautical mile, so precision pays. The two gold handles go anywhere along the horizon, as far apart as the photo allows — they define the line, not its ends. The blue ring goes on the sun: its center is the usual mark, but when glare hides the middle, put the ring on the crisp lower or upper edge instead and tell the app which you chose — it corrects for the sun’s half-width either way. A loupe magnifies while you drag, and the arrow buttons nudge the selected marker one pixel at a time.

The Earth turns fifteen degrees an hour, so each second of time error moves the answer up to a quarter mile. Photos taken with the in-app camera or your phone’s camera carry their moment with them; photos chosen from your library without a time stamp fall back to the clock, and the app asks you to set it. The displayed time is always UTC.

Horizon dip: the horizon sits below true level because your eye is above the water. Refraction: the atmosphere bends the sun’s light upward, more so near the horizon. Semidiameter: you marked an edge of the sun, but the almanac tracks its center. Parallax: the small shift from observing on the Earth’s surface rather than its center. The same chain a paper sight reduction works through — shown line by line so you can check it.

It’s the honest uncertainty of that line of position. The biggest contributor is how well the app knows your camera’s angle of view: a photo from the in-app camera carries per-shot lens calibration and earns a tight radius; a photo from your library leans on the photo’s own metadata or an assumed lens, and the radius widens to match. Marking precision, refraction, and your height of eye contribute the rest. The app never hides the number.

One sight tells you a line you’re somewhere on — it cannot tell you where along it. Two sights with the sun’s bearing well changed between them cross into a position. That’s why the app keeps saying “one sight is a line, not a position”: it’s the oldest rule in celestial navigation, and the app refuses to forget it.

The fix is found where the lines cross, and lines nearly parallel cross sloppily. The sun’s bearing swings through the day, so a mid-morning line and a mid-afternoon line cut at a healthy angle. The fix screen tells you when the crossing is too shallow to trust.

The sky, almost always — that’s the point of taking sights. The printed gap between the DR and the fix is real information: it’s the current, the leeway, and the optimism in your logged speed, all talking at once. Restart the reckoning from the fix and it starts fresh from truth.

The moment the sun crosses your meridian — highest in the sky, bearing due south or north. The app predicts it from your DR longitude and counts down to it. A sight within a few minutes of noon produces a line running nearly east–west, which reads directly as your latitude; the result card says so when it happens. The card also warns when the noon sun is too high for your lens — shoot morning and afternoon lines instead on those days.

Like the paper one. You start from a trusted position — a known place, or a fix — and log a leg whenever course or speed changes: “0930, came to 250° at 6 knots.” The app advances the starting point along your legs and shows the DR for this moment, stamped with the moment it was computed for. Tap “Stopped” when you heave to, anchor, or drift for lunch — a zero-speed leg holds the plot still.

Everything you enter, you can correct. Tap a leg to edit its course or speed, or delete it. Delete a bad sight or a bad fix from their lists — each asks you to tap twice. Restart the reckoning from any earlier fix. Enter a known position under “I know where I am” to restart it from there. And if the whole story has gone wrong, “Reset the reckoning” wipes the leg log and holds still until you log the next leg.

Yes — each voyage is its own deck log: starting point, legs, sights, and fixes together. Start a new voyage for a new passage; rename or delete old ones. Practice keeps to its own voyage so it never muddies a real log.

When there’s no sun or no sea horizon: at night, under overcast, in fog or haze, or inshore with land across the horizon. In high summer at midday the sun can stand too high for the camera to hold both it and the horizon — the noon card warns you, and the morning and afternoon still work fine, which is where sun navigation always lived. The camera sight is sun-only for now; the moon, planets, and stars are planned for it. (The sextant drill already knows the four bright planets and twenty-one stars.)

A careful sight with the in-app camera is typically good to a few nautical miles, and the app always states its actual estimate. Plenty for the fun of it; not a chartplotter, and not meant as your main way to navigate or even a serious alternative to it. Treat it as what it is: the oldest way there is of finding yourself, made fast and honest, for the pleasure of doing it.

No. No accounts, no servers, no analytics, no network requests. Photos are processed on the device and never uploaded. The optional sextant drill and the deck log’s “Check against GPS” can each read your position once, on the spot — to score a practice sight, or to grade your reckoning — and it is never stored or sent anywhere. See the privacy policy — it’s short because there’s little to disclose.

Yes — that’s the sextant drill. Shoot the sun, a planet, or one of twenty-one named stars, tap MARK at the instant the body kisses the horizon, and enter what the arc read, degrees and minutes. The app applies your index correction, height of eye, refraction, semidiameter and parallax, computes what the sextant should have read from where you truly are (GPS, with your permission, or your DR), and scores you: minutes of arc off, which is nautical miles off. Within 2′ is professional work; within 5′ makes good fixes; past 10′ the app suggests what to check first.

Sextants rarely read exactly zero when the horizon is brought to itself. Check yours: if it reads 2.0′ when it should read zero (“on the arc”), your correction is −2.0; if it reads below zero (“off the arc”), +2.0. Enter it once in the drill and every score accounts for it. An unnoticed index error is the most common reason a beginner’s sights all miss the same way.