Offline planning, running fixes and sight analysis

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.

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:

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.

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.

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.

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.