Dead reckoning, sight planning, capture, and position calculation.
Enter your estimated position at a known UTC, plus course and speed over ground.
Set speed to zero when stationary. Course and speed are assumed constant between the DR time and the sights; include current in the values you enter.
Gold lines: sights adjusted to the common UTC. White point: calculated position. Blue point: DR at that UTC. Axes show local east and north offsets in nautical miles.
| Use | Body / sight | UTC | Ho | Residual · NM |
|---|
Included sights are fitted together using iterative least squares on spherical altitude equations. Every sight keeps its exact UTC and corrected altitude. The calculation moves the trial position along the entered course and speed to each sight time, then compares its predicted altitude with Ho.
At least two sufficiently independent lines of position are required. Nearly parallel lines are rejected. Repeated sights of one body help assess consistency, although they share clock, index, and identification errors. Residual RMS describes agreement between the observations; it is not an accuracy or confidence radius. With only two observations, there is no independent residual check.
The motion model assumes a constant rhumb-line course and speed over ground. Changes in course or speed require a separate session or corrected motion information. The local solver is limited to positions below 85° latitude and a correction within 300 NM of the DR estimate. Large residuals may indicate a timing, identification, correction, or motion error.
Prepare the bodies, capture the instant, record the angle, then reduce each sight.
Enter your expected position and UTC in Plan sights. Aim for bright bodies at 15°–70° altitude with a useful spread of bearings. Use twilight when both stars and the sea horizon can be seen.
Choose Use for sight to open that body’s tab, or add it directly in Capture sights.
Verify the device’s clock before departure. Capture UTC uses that clock, including milliseconds; it does not synchronize with a time service.
Record index correction, height of eye, pressure, and temperature in the selected sight’s details. Each sight keeps its own settings.
Select a free sight slot. Bring the star to the natural sea horizon and rock the sextant to find the lowest point of its arc. For the Sun, select the correct limb.
At the contact instant, tap Capture UTC. An assistant can capture the time while you hold the sextant. Then enter the sextant altitude in degrees and minutes.
Index correction: off the arc = add; on the arc = subtract. Sun lower limb: add semidiameter. Upper limb: subtract it. Stars need neither parallax nor semidiameter.
Moon and planets: use an already corrected Ho. Do not apply these corrections a second time.
Intercept = (Ho − Hc) × 60 NM.
Ho > Hc: toward the body. Ho < Hc: away. One arcminute of altitude difference is one nautical mile of intercept.
Zn is the true bearing of the body, measured clockwise from north. Each result belongs to its own sight time and assumed position.
Take up to five sights in each body tab. Select Sight 1–5 to review its full worksheet and plot. A changed UTC recalculates almanac values automatically; manual overrides are labeled.
A single sight gives a line of position. Use Calculate position to fit included sights at a common UTC using your course and speed over ground.
Before going offline, wait for Offline ready and download the self-contained copy to preserve your sight log and almanac pack.
From AP, lay off 4 NM on 000° true. Draw the line of position through that point on 090°/270°.
From AP, lay off 4 NM on 180° true, opposite the body’s bearing. The line of position still runs 090°/270°.
Diagram distances are illustrative. Use the selected sight’s intercept and true azimuth for your own plot.
Uses your DR advanced to the planned UTC with the entered course and speed.
Select an event to plan at that time. Twilight buttons use a Sun altitude of −9°, midway between civil and nautical twilight. Actual horizon visibility depends on weather.
Preferred altitudes are 15°–70°, with 35°–55° favored. Brightness and separation of lines of position guide the suggested three-body set. Bodies closer than 20° to the Sun are excluded from recommendations. Stars are suggested once the Sun is at least 3° below the horizon; twilight from −6° to −12° is highlighted. In full darkness, a natural sea horizon may be hard to see.
Moon and planets require corrected Ho in sight reduction. Predictions assume a clear sky and an unobstructed horizon; no weather forecast is used. This is a planned set, not a simultaneous fix.
North-up sky plot. Rim = horizon, center = zenith. Gold = recommended; blue = other visible bodies.
| Body | Altitude | Azimuth | Mag. |
|---|
Exact-time geocentric GHA and declination, generated on this device. Sun and Moon semidiameter and horizontal parallax are included. These are computed ephemerides, not pages from an official Nautical Almanac.
No saved passage pack.
One hourly pack is retained on this device; saving replaces it. Exact-time calculations work offline even outside the saved range. Export keeps a portable JSON copy, and “Download offline copy” includes the current pack in the HTML.
| Body | GHA | Declination | SD / HP |
|---|
Solar-system positions: Astronomy Engine by Don Cross (MIT license), bundled locally. Stars: 57 standard navigational stars plus Polaris from HYG v4.1 by David Nash / Astronexus, with J2000 positions propagated using catalog space velocities, then corrected for precession, nutation, and aberration by Astronomy Engine. HYG data and the included star subset are licensed CC BY-SA 4.0. Source data fields are retained in the offline copy.
GHA is westward from Greenwich, wrapped to 0°–360°. Declination is north positive. Planetary and lunar almanac values are geocentric; the planner’s altitude and azimuth are topocentric, without atmospheric refraction. UT1 is approximated by UTC, and Delta T is modeled. Venus uses its center of figure; the official almanac’s center-of-light phase correction is not applied. Lunar semidiameter is geocentric and excludes augmentation; apply official lunar corrections before using Ho. Values are rounded for display; calculations retain full precision. Supported dates: 1900–2100. Independently compare with a current official almanac before relying on the output at sea.
Tap Capture UTC, then enter altitude. Five sights per body; scroll horizontally on a smaller screen.
UTC uses the device clock. Set it accurately before departure. Select a sight for its full date, corrections, and plot.
This position is advanced from your DR to the captured UTC. Individual overrides are retained with this sight.
Synthetic Sun example
Preparing offline cache…
Open this page online once and wait for “Offline ready.” On iPhone or iPad, use Safari’s Share menu to add it to your Home Screen, then open it once while online. Test in airplane mode before departure.
The downloadable HTML is self-contained and works in a browser without a connection or sign-in. Browser caches may be removed by your device. The home-page link and external references need a connection.
This is an intercept-method sight reduction using spherical trigonometry. Hc = asin(sin latitude × sin declination + cos latitude × cos declination × cos LHA). LHA = GHA + east longitude. Intercept = (Ho − Hc) × 60 nautical miles.
Sextant mode subtracts sea-horizon dip (1.76√height in meters), applies Bennett refraction with pressure and temperature scaling, and adds solar parallax and signed semidiameter. Stars use no parallax or semidiameter. Refraction uses apparent altitude after index and dip corrections. This mode accepts apparent altitudes from 5° to 85°; use almanac corrections and the Ho path outside that range.
A single sight gives a line of position, not a position fix. The plot is a local plane approximation centered on the assumed position; it does not advance sights for vessel movement. The Almanac tab generates exact-time values offline. Use Plan sights to select bodies, then capture observations and use Calculate position for a combined fix. Independently check inputs and results before navigational use.