OpenCPN

Offline planning, running fixes and sight analysis

The navigation-planning tools are designed for offshore use. They perform no network access and do not depend on the optional eclipse data packs. Ordinary Sun, Moon, planet and star calculations use the same bundled VSOP87D, ELP2000/82 and navigational-star implementation used by sight reduction.

Sun, Moon and Sight Planner

Open Sun & Moon… on the main Celestial Navigation window. The common header selects both a position and a UTC. A completely manual position and time are always available. Current boat position, chart cursor, selected-sight DR and the most recent calculated fix are conveniences and are used only when the corresponding source is available. While Chart cursor is selected, the position and results follow the OpenCPN chart cursor at a throttled rate; select Manual to freeze that position for planning. Waypoint or place…​ opens a searchable list of OpenCPN marks, waypoints and named route points. The selected item’s stable identifier is saved; the planner resolves its current coordinates each time it is opened and falls back to the retained manual position if the item has since been deleted.

Date and time are entered with separate calendar and clock controls. The input can be interpreted as UTC, the computer’s local civil time, or ship zone time; changing that choice preserves the instant and all calculations are normalized to UTC. Ship zone time defaults to the conventional whole-hour offset suggested by longitude. Disable Auto zone from longitude to select a manual offset in half-hour increments; UTC date rollover is handled automatically. The zone offset controls are enabled only when ship-zone input or fixed-offset display uses them. A temporarily blank or invalid offset retains the last valid value rather than silently becoming the western limit.

The default entry presentation is unambiguous nautical YYYY-MM-DD and 24-hour HH:MM:SS. OpenCPN / platform format instead uses the operating system’s native controls without changing the represented instant. Event times can independently be displayed as UTC, the computer’s local civil time and zone, local mean time derived from longitude, or a fixed UTC offset. The computer-local display observes the operating system’s daylight-saving rules for the event date. The ordinary planner validates dates from 1900 through 2100. The bold Resolved UTC line is the authoritative instant shared by all planner tabs, independent of entry or display format.

Editing an automatically supplied latitude/longitude changes the position source to Manual. Editing an automatically supplied date/time changes the time source to Manual date/time. Valid context, motion, zone-offset and eye-height edits refresh the calculations after a short debounce; Calculate / refresh remains available for an explicit update. Planner choices such as COG/SOG and entry/display formats persist. A newly opened planner starts at Now, and its eye height is initialized from the main plugin defaults. Eye height affects visible-horizon dip for rise/set, not the geometric body-centre Hc in the table.

Enable Time-tagged moving observer to define the position at the displayed reference UTC and propagate it along COG (true) and SOG. Each event row then also reports the propagated observer position.

Events

The daily table contains astronomical, nautical and civil dawn/dusk, sunrise, local apparent noon, sunset, Moon rise/transit/set and the next four principal Moon phases. Rise and set calculations solve apparent upper-limb altitude, including refraction, semidiameter, parallax and eye-height dip. Twilight uses the conventional geometric Sun-centre altitudes of -6, -12 and -18 degrees. Polar day/night and circumpolar Moon states are reported instead of inventing an event.

The table covers the UTC calendar day containing the planning time. At far eastern or western longitudes, the corresponding local rise and set can therefore carry different local calendar dates; both the always-present UTC column and the selected display-time column make this explicit.

The Moon summary includes illumination, named phase, waxing/waning state, approximate lunar age, altitude, true azimuth and angular separation from the Sun.

Bodies and best sights

The body table reports Hc, Zn true, GHA, declination, approximate visual magnitude and a planning score for every catalogued body above the geometric horizon. Click a heading to sort that column and again to reverse it. Limit recommendations by Hc optionally applies its editable minimum and maximum only to suggested pairs and triads; the initial preferred range is 10 to 75 degrees. Bodies outside that preferred range remain visible in the table. The individual score gives 65% weight to altitude near 40 degrees and 35% to brightness, with a daylight/twilight penalty for stars. Planets do not currently receive an equivalent twilight penalty, so score alone is not a guarantee of practical visibility. Recommended pairs additionally favour a near-right-angle crossing; triads use the determinant of their two-dimensional bearing information matrix.

The polar sky plot has independent magnitude limits of 1, 2 or 3 and can show bodies below the horizon. The Sun is yellow, Moon blue, planets red, daylight stars grey and below-horizon bodies hollow. Only the first 14 score-ranked bodies are considered for labels, and overlapping labels are suppressed. An unlabeled dot does not mean that the body is missing. The explanation and plot make the recommendation auditable but do not replace the navigator’s assessment of cloud, glare, horizon and identification risk.

Select a row and use Create selected sight… to open the normal Sight dialog with body, time and DR already filled. No zero-altitude observation is silently created.

Almanac

The almanac page produces an hourly, 24-hour table of body GHA, SHA, declination, GHA Aries, local hour angle of Aries, Hc and Zn for the Sun, Moon, visible planets and Polaris. Longitude is signed east-positive, so local hour angle is GHA + longitude, wrapped to 0–360 degrees. Export CSV… writes a plain-text file suitable for a spreadsheet or an offline printed worksheet. Exported UTC timestamps use ISO 8601 form such as 2026-08-13T04:48:00Z.

Noon and Polaris

The special-workflow page reports local apparent noon and can solve latitude from a corrected Sun meridian altitude or a corrected Polaris altitude. Local apparent noon is solved as the instant when the moving observer’s solar local hour angle is zero, rather than by sampling for the highest displayed altitude. The result is a workflow aid: index error, dip, refraction, limb choice, time and DR uncertainty still apply. The Polaris workflow reports its predicted Hc and refuses a latitude solution when Polaris is below the observer’s geometric horizon. Equal-altitude Sun pairs may be entered as ordinary time-tagged sights and solved with the running-fix mode described below.

Time-tagged numerical running fix

Open Fix… and enable Propagate every sight to a common epoch. When any visible altitude or horizon sight already has a non-zero saved DR Shift, the dialog selects this option and Each sight’s DR Shift automatically. Otherwise it retains the stationary Fix default. The selection can be changed. Enter the common epoch with the calendar and clock controls, selecting UTC or computer local civil time as appropriate. Choose One COG and SOG when the vessel maintained a constant course and speed, then enter COG true and SOG. Choose Each sight’s DR Shift when you have entered the distance and bearing travelled from each sight to the selected common epoch; this allows the vessel to tack or change speed between sights. The last sight at that epoch normally has a zero shift. The dialog initially chooses the latest visible sight’s corrected UTC as the common epoch; confirm that the saved shifts were measured to that time. Do not also apply COG/SOG in this mode. The value is normalized to UTC before calculation. The latitude/longitude labelled DR are the initial estimate at that epoch. For each visible altitude or horizon event, the solver:

  1. propagates the candidate epoch position to the observation UTC on a great circle;

  2. calculates Hc for the body’s position at that UTC;

  3. forms the weighted Ho-Hc residual; and

  4. iterates a two-parameter nonlinear least-squares solution for epoch latitude and longitude.

The table shows each included sight’s saved DR distance and bearing, marking magnetic or true bearings and showing the true bearing used by the solver. These inputs remain visible if the calculation fails. The result also includes each sight’s residual, RMS and an approximate uncertainty ellipse. The selected motion source is applied once: manual DR Shifts are ignored in the COG/SOG mode, and COG/SOG is ignored in the per-sight mode. The legacy stationary plane/sphere/cone solvers remain selectable when running-fix mode is off; they exclude shifted sights and show the exclusion in the dialog.

Horizon-event observations

Use Horizon Event… to record the first visible upper limb at sunrise or the last visible upper limb at sunset. Enter or capture UTC, its uncertainty, eye height, pressure, temperature and horizon quality. An optional compass bearing can be entered as true, or as magnetic with explicit variation and compass deviation. The saved sight retains all of this context and contributes an approximate line of position to the ordinary or running fix.

This is deliberately assigned conservative uncertainty. Refraction, haze, land or an indistinct horizon normally dominate the result, and a bearing-derived estimate without a sextant is not a replacement for a normal multi-sight fix.

Sight Sequence Analyzer

Analyze Sights… compares visible calculated altitude sights with a stationary or moving known/DR track. It plots and lists Ho-Hc, reports mean and standard deviation, robust median/MAD, a time trend and possible outliers. The median is reported as a possible personal bias but is never silently applied to a sight.

Scope and safety

These tools support planning, training and independent navigation. Approximate stellar/planet magnitudes are only ranking inputs. Event accuracy near a refracting or obstructed horizon is normally limited by real atmospheric and horizon conditions. Always retain raw observations, time-source information and an independent DR, and verify important results by another method.