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 require the optional eclipse data packs. In 2.9.0, Sun, Moon, Mercury and Venus calculations prefer an installed DE440s kernel for supported dates from 1972 onward and use the bundled analytical ephemeris when it is unavailable or out of range. Other planets and navigational stars retain their bundled models. The calculation source is reported by the plugin.

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 planning mode determines the recommendations and their order:

  • Practical Fix favours observable bodies and sound pair or triad geometry.

  • Bright Bodies puts brightness first when visibility is difficult.

  • Lunar Candidates compares the Moon with each potential companion.

  • Show All lists bodies alphabetically without a recommendation ranking.

The All bodies table remains available in every mode. It reports Hc, Zn true, GHA, declination, approximate visual magnitude, the applicable planning score, ecliptic latitude and an explanation. It initially includes every catalogued body above the geometric horizon; Include below-horizon bodies adds the rest. Click a heading to sort that column and again to reverse it. Limit recommendations by Hc optionally applies its editable 10–75 degree default range to recommendations only; bodies outside it stay in the full table. The observability score weights altitude near 40 degrees (65%) and brightness (35%), with a daylight/twilight penalty for stars. A score is a planning aid, not a claim that a body will be visible through glare or cloud. Pairs additionally favour a near-right-angle crossing; triads use the determinant of their bearing-information matrix.

Lunar Candidates lists predicted geocentric Moon–body distance, signed distance change in arcminutes/hour, the approximate number of seconds represented by 0.1 arcminute, the companion’s signed ecliptic latitude, Moon/companion Hc and Zn, illumination, and an assessment. Positive rate means the distance is increasing. Both planners default to fewest planning cautions, then strongest timing sensitivity. The explicit timing-first option changes that priority; below-horizon pairs stay last. Cautions flag Hc outside 10–75 degrees, distance outside 20–100 degrees, absolute rate below 10 arcminutes/hour, magnitude above 2.5, and Sun Hc above -6 degrees for companions fainter than -2. Ecliptic latitude describes geometry without adding a second timing weight. These are planning preferences, not a guarantee of visibility.

Select a companion in All bodies and choose Lunar windows for selected body… for a 24-hour search sampled every 10 minutes. The reported intervals meet these preferences, using the entered course/speed if moving-observer mode is enabled. Boundaries and best times are sampled estimates, not exact events. All eligible above-horizon bodies contribute to pair/triad recommendations; there is no preliminary top-18 shortlist.

The plot has independent magnitude and below-horizon controls. Show ecliptic and Show Moon path can each be enabled or disabled, including both together or neither. The ecliptic uses the date-specific astronomical obliquity; the Moon track follows the ephemeris for the chosen ±3, ±6 or ±12 hour span. The Local sky view shows altitudes and true bearings at the planning position. The SHA / Declination view is better for a longer Moon track because its coordinates do not depend on the changing local horizon. The Sun is yellow, Moon blue, planets red, daylight stars grey and below-horizon bodies hollow. Only a limited number of the brightest bodies are labeled to keep the plots legible; unlabeled dots remain plotted. The ecliptic defaults on and the Moon path defaults off. Moon-track start, middle and end are labelled in UTC, and the selected body is highlighted. Neither plot replaces the navigator’s assessment of cloud, glare, horizon and body-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.

The chart shows the time-based altitude LOP and its entered horizon/time uncertainty band. A bearing adds all nominal position candidates, often one in each hemisphere. Use DR or other observations to distinguish them. The markers are not fixes or confidence areas. Bearing error does not convert to a constant NM-per-degree radius, so the former filled disc is no longer plotted. The Fix solver uses the altitude constraint, not the optional bearing. An easterly bearing belongs to sunrise and a westerly bearing to sunset; the dialog reports a mismatch instead of plotting an unrelated candidate.

Use the full UTC date as well as the time. Near Hawaii, a sunset at 05:55 UTC on 16 July precedes a sunrise at 16:51 UTC that same day: it is the previous local evening. Propagate observations taken while underway to one common epoch using the running-fix controls. A Sun bearing is a celestial azimuth constraint, not a straight terrestrial bearing line through an assumed boat position.

The method applies the ordinary astronomical triangle to upper-limb contact with the visible sea horizon, with dip, refraction, semidiameter and parallax corrections. See the US Naval Observatory rise/set definitions. Refraction, haze, land or an indistinct horizon often dominate the result; a sub-arcminute horizon uncertainty is usually too optimistic. Retain independent navigation evidence.

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.