Castaway Sky User Guide

Splash screen

Shows CASTAWAY SKY and COLORS CHOSEN FOR FUNCTION: RED ON BLACK PRESERVES YOUR NIGHT VISION. With the green display color it reads GREEN ON BLACK. FOR NIGHT VISION, CHOOSE RED IN SETTINGS. It fades after 3 seconds; tap anywhere to go straight to the main screen (Aim).

What you need

• An iPhone or iPad with iOS / iPadOS 18 or later.
• For star photos: a clear, dark night sky, and a device whose back camera can take RAW photos.
• For sun and moon sights: a view of the sun or the moon.
• A correct clock. The device's clock stays accurate even without a network.

CASTAWAY SKY (top left)

The app name.

MENU (top right)

Opens MENU (section 5): photos, SUN & MOON FIX, MANUAL FIX, SKY CHART and more.

Large frame

The area the photo will cover (phone held upright).

Small cross in the middle

The camera direction (optical axis).

Circle with a cross

The zenith (straight up). When it is inside the frame, the zenith is in the photo. The closer to the zenith you aim, the better.

Triangle outside the frame

Points toward the zenith when it is outside the frame. Tilt the device that way.

ALT 85°

The elevation of the camera direction above the horizon. ALT --° means no sensor reading.

STABLE / WAITING FOR STABILITY

Whether the device is still.

POINT AT THE SKY / LOW ELEVATION

Shown when the camera points below the horizon, or less than 30° above it.

CAPTURE / ABORT

CAPTURE starts a capture; it can be pressed only when the device is still and the star data is loaded. During a capture it becomes ABORT, which stops and returns to Aim.

While the app computes a position from the photo, it logs each step. The number at the start of a result line is the time the step took (ms).

IMAGE … (first line)

Only for a loaded photo: file name, size, and where gravity came from (GRAVITY MAKER NOTE or ZENITH ASSUMED). For SIMULATE: SIMULATED SKY OF ….

CHECKING MOTION...

Whether the device moved during the exposure (exposure time and rotation).

BINNING SENSOR DATA...

Prepares the sensor data for analysis.

EXTRACTING STARS...

Finds the stars in the photo (STARS is the number found).

SOLVING ATTITUDE...

Matches the star pattern with the catalog to find the camera direction and field of view (MATCHED is the number of stars matched).

READING GRAVITY...

Reads the direction of gravity during the exposure.

COMPUTING POSITION...

Computes latitude and longitude.

ESTIMATING ERROR...

Estimates the error (north-south and east-west, km).

POSITION FIX ACQUIRED and VIEW POSITION

A position was found. Tap VIEW POSITION to see the result (Plot).

Heading (source)

How the position was found. None for a star photo; IMAGE <file name> for a photo; SIMULATED SKY OF … for SIMULATE; SUN FIX — 3 SIGHTS, MOON FIX — n SIGHTS or SUN & MOON FIX — n SIGHTS (a SUN, b MOON) for sights; MANUAL FIX — NOON SUN and so on with ROUGH ESTIMATE for manual methods.

35°41'N, 139°46'E

The estimated latitude and longitude in degrees and minutes. When there are several candidates (sun and moon), each is listed as A, B.

ZENITH ASSUMED — ROUGH ESTIMATE

The photo had no gravity data, so its center was taken as straight up. The error is larger.

±111 km N-S ±113 km E-W

The estimated error (1σ), north-south and east-west.

2026-01-15 12:00:00 UTC 143 STARS

The time of the observation (UTC), and for star photos, photos and SIMULATE the number of stars matched.

Map

A world map of coastlines, borders and a grid (no place names). The cross is the estimated position and the ellipse the error; the ellipse may be tilted. Each candidate gets its own cross, ellipse and A/B label. Drag to move; pinch to zoom.

NEW FIX

Discards the result and returns to the main screen.

SKY

Opens SKY CHART for this position and time.

Tap MENU on the main screen. Tap an item to open it; tap CLOSE or the dark area to close the menu.

LOAD PHOTO

Choose a starry-sky photo from your library and analyze it (section 6).

LOAD FILE

Choose an image file from the Files app and analyze it (section 6).

SUN & MOON FIX

Measure the sun and the moon to find your position (section 7).

MANUAL FIX

Classic methods without the camera, and clues (section 8).

SKY CHART

See the sky for any place and time; try SIMULATE (section 9).

TIP JAR

Optional tips (section 10).

SETTINGS

Choose the display color, red or green (section 11).

ABOUT

Version, safety note, privacy policy, support (section 12).

LICENSES

Licenses of the components and data the app uses (section 12).

Preview and file name

A small red preview of the photo. Library photos are named PHOTO.

TIME (UTC)

When the photo was taken (UTC), read from the photo and editable. The line below tells the source: EXIF (with a time zone offset), TIME ZONE ASSUMED UTC+09:00 (no offset in the photo; the device's time zone was assumed), NO TIME IN IMAGE — ENTER UTC, or MANUAL (edited).

FOV (DEGREES)

The horizontal field of view, computed from the 35 mm equivalent focal length and editable. Source line: EXIF 26MM (plus DIGITAL ZOOM x2.0 if zoomed), UNKNOWN — ENTER DEGREES, or MANUAL.

GRAVITY

Where the direction of gravity comes from. MAKER NOTE uses the device's accelerometer data stored in the photo. For photos without it, rotated (portrait) photos, or front-camera photos, the center of the photo is taken as straight up (ZENITH ASSUMED — …), which gives a rough estimate.

ANALYZE

Checks the inputs and starts the analysis (section 3).

DO NOT LOOK AT THE SUN. AIM USING THE SCREEN.

Never look at the sun; aim with the screen.

TAKE SIGHTS OF THE SUN, THE MOON, OR BOTH: …

Take three or more sights, hours apart, pointing the back of the device at the body.

SUN / MOON

Choose the body to measure (SUN at first).

Aim box

Covers ±30° around the camera direction. The cross is the camera direction, the faint circle the usable range (10° from the center), and the dot the sun or moon. A body outside the box is shown on its edge. No camera image is shown.

Text under the box

SUN CENTERED (inside the circle; ready), CENTER THE SUN ↗ (the dot lies that way; turn the device toward it), NO SUN IN VIEW, STARTING CAMERA..., AIM AID UNAVAILABLE (camera cannot be used), or a camera-paused message (section 2). For the moon, SUN reads MOON.

TAKE SIGHT

Takes the sight. While shooting it reads TAKING.... Then the result appears on the right, for example SUN ALT 40.5° (or the reason for a failure). The sun is one photo; the moon is a burst with different exposures.

SIGHTS rows

One row per sight: number, time (UTC), body and apparent altitude (for example SUN ALT 35.7°). Tap a row to use or skip it in the solution (skipped rows are dimmed). Sights older than 24 hours start out skipped.

CLEAR

Deletes all sights. The app asks CLEAR ALL SIGHTS?; tap YES to delete (the hints are reset too) or NO to keep them.

AT NOON THE SUN WAS:

Whether the noon sun was north or south of you (NORTH OF ME / SOUTH OF ME / UNKNOWN). It picks between mirror-image candidates. At midday, shadows point away from the sun.

AT ITS HIGHEST THE MOON WAS:

Whether the moon at its highest was north or south of you. The moon's hint.

RESULT

The estimated position and error (±km). With too few sights: TAKE AT LEAST 2 SIGHTS. or ONE MORE SIGHT NEEDED — WAIT 1–2 HOURS.

VIEW ON MAP

Shows the result (Plot) on the map.

TILT METER

Measures the altitude of a star or the moon using the top edge of the device as a sight. Look along the edge at the star; the large number (for example 41.3°) is its altitude. Never sight the sun.

HOLD

Freezes the average of the last second as HELD 41.3°. Then USE HELD buttons appear beside the height fields; tap one to copy the value.

POLE STAR — LATITUDE

NORTH: POLARIS / SOUTH: SOUTH POLE

Polaris in the northern hemisphere; the south celestial pole in the southern. Find Polaris by extending the two end stars of the Big Dipper's bowl about 5 times, or from the W of Cassiopeia. The south celestial pole lies about 4.5 times the long axis of the Southern Cross beyond its foot.

HEIGHT (°)

Its altitude, measured with the TILT METER, or at arm's length (a fist ≈ 10°, a finger ≈ 2°).

CALCULATE

Gives the latitude (±1° with Polaris, ±2° with the south pole). Outside 0–90°: ENTER A HEIGHT BETWEEN 0 AND 90°.

NOON SUN — LATITUDE AND LONGITUDE

Stand a straight stick upright on level ground and mark the tip of its shadow every few minutes. The shortest shadow is local noon. You can also note two times with equal shadows, one in the morning and one in the afternoon, and use the time halfway between.

STICK HEIGHT / SHORTEST SHADOW

The stick's height and the shortest shadow, in any one unit. With both, the sun's altitude appears, for example SUN HEIGHT 42.7°.

OR SUN HEIGHT (°)

Enter the sun's altitude directly instead (takes priority over the stick).

TIME OF SHORTEST SHADOW (UTC)

When the shadow was shortest (UTC). It starts at the time you opened the screen; NOW sets the current time.

AT NOON THE SUN WAS: SOUTH OF ME / NORTH OF ME

Whether the noon sun was south or north of you (south at first).

CALCULATE

Gives latitude (±1.0°) and longitude (±1.5°).

MOON TRANSIT — LATITUDE AND LONGITUDE

The moon is highest when it crosses due south (northern hemisphere) or due north (southern hemisphere). Measure its altitude with the TILT METER every few minutes and take the highest, or use the time when its shadow is shortest.

MOON HEIGHT (°) / TIME OF HIGHEST (UTC)

The moon's highest altitude and its time (UTC; with NOW).

AT ITS HIGHEST THE MOON WAS: SOUTH OF ME / NORTH OF ME

Whether the moon was south or north of you then.

CALCULATE

Gives latitude (±1.0°) and longitude (±2.5°), corrected for the moon's parallax (about 1°).

RESULT

LATITUDE 35.8°N ±1.0° (NOON SUN)

The latitude, its rough error and the method; LATITUDE — until calculated.

LONGITUDE 28.4°E ±1.5° (NOON SUN)

The longitude. Polaris alone gives no longitude (LONGITUDE NEEDS THE NOON SUN OR THE MOON TRANSIT.).

VIEW ON MAP

Appears when both are known, and shows the result (Plot) headed MANUAL FIX — <method> and ROUGH ESTIMATE.

Input problems show messages such as ENTER THE STICK AND SHADOW, OR THE SUN HEIGHT, ENTER THE MOON HEIGHT and ENTER THE TIME AS YYYY-MM-DD HH:MM:SS (UTC).

CLUES — WHICH PART OF THE WORLD?

No position, but clues that narrow down where on Earth you are.

• The latitudes where the sun and the moon pass overhead today. If one is highest to your south, you are north of that latitude.
• The moon's phase now (for example WAXING GIBBOUS, 91% LIT). If the moon you see looks different, the device's date may be wrong.
• Noon sun to the south: northern hemisphere or the tropics. To the north: southern hemisphere or the tropics. Overhead: the tropics.
• Polaris visible: northern hemisphere. Southern Cross visible: south of about 27°N.
• In the southern hemisphere, the lit side of the moon is mirrored.
• A full moon is highest around midnight, a first quarter around sunset, a last quarter around sunrise.
• A line through the horns of a crescent points roughly south in the north and north in the south (±20–30°; unreliable near the equator).
• Days of about 12 hours all year mean near the equator; very long or short days mean far from it.
• Coconut palms, coral and a warm sea: the tropics.

PRESET

TOKYO, NEW YORK, LONDON, SYDNEY, RIO DE JANEIRO, CAPE TOWN, ST PETERSBURG, COLOMBO, NORTH POLE, SOUTH POLE. Tap one to fill latitude and longitude; the matching button is highlighted.

LAT / LON

Latitude and longitude as degrees (35.68, -33.87), degrees and minutes (35°41'N) or degrees, minutes and seconds (35°41'22"N). Use S and W, or a minus sign, for south and west.

TIME (UTC)

The time (UTC). NOW sets the current time.

MAG LIMIT

The faintest stars to draw (magnitude 0 to 5.5 in steps of 0.5). The value shows as ALL (≤ 5.5) or, for example, ≤ 3.0. Tap or drag sideways to change it; it is remembered while the app is open.

Chart

Centered on the zenith, as seen looking up (north up, east left). The outer circle is the horizon, the dashed circles are 30° and 60° altitude, the small cross is the zenith, and the faint outline is what SIMULATE photographs. Brighter stars are larger. The moon is drawn with its phase and labeled MOON; the sun is a ring with a dot labeled SUN (both only above the horizon). At the poles, right ascensions (0h and so on) replace the compass labels.

If an input is missing or invalid, the chart is replaced by ENTER LATITUDE, LONGITUDE AND UTC TIME and an example.

35°41'N, 139°46'E 1394 STARS ABOVE HORIZON

The place and the number of stars above the horizon.

SUN: ALT 45° / MOON: ALT 54°, WAXING GIBBOUS, 91% LIT

Altitudes of the sun and the moon, and the moon's phase and lit fraction (only when above the horizon).

N UP, E LEFT (LOOKING UP). FRAME: CAMERA AT ZENITH

How the chart is oriented, and that the outline is a camera aimed straight up.

SIMULATE

Renders a photo of this sky taken straight up and runs the same analysis as a real star photo, so you can try the app in daylight or indoors. The result is marked SIMULATED so that it is never mistaken for a real fix.

If the app helped you, you can leave an optional tip. Tips unlock nothing; every feature is free. This is the only screen that uses the network.

Price buttons

Three tips in your local currency, lowest first. Tapping one opens the App Store purchase sheet (the only screen not in the app's color).

LOADING…

Loading the prices.

THANK YOU.

The tip was received.

WAITING FOR APPROVAL.

Waiting for Family Sharing approval.

PURCHASE FAILED.

The purchase failed.

THE APP STORE IS NOT AVAILABLE. TRY AGAIN WHEN ONLINE.

The App Store cannot be reached (for example in airplane mode). Tap RETRY when you are online.

Whether you sent a tip is not saved.

Open it from MENU > SETTINGS to choose the display color (the one color of text and lines). The background stays black.

DISPLAY COLOR: RED / GREEN

Takes effect at once on every screen: text, lines, the map, the star chart and the aim box. RED is the default.

RED PRESERVES YOUR NIGHT VISION BEST. GREEN IS EASIER TO READ IN DAYLIGHT.

Use red when you look at the night sky; it disturbs dark-adapted eyes the least. Green is brighter and easier to read in daylight or at dusk.