Astronomical or celestial triangle. See also Observed position, OP. Known as the PZX triangle, it is used for working out a sight with spherical trigonometry wherein P is the celestial pole which projects down to the pole on Earth. Z is the zenith of the observer, ie the point immediately above his head, and X is the position on the celestial sphere of the heavenly body.
Beaufort Scale. Compiled in 1808 by Admiral Beaufort to provide a measure of wind strengths with an accompanying description of the associated sea state. Expressed in ‘forces’. Scale runs from force 0; a calm, to force 12; a hurricane.
Cable. Measure of distance, one tenth of a sea mile, equalling 183 metres or 100 fathoms, ie 200 yards.
Course. Direction in which the boat is set to steer. True course is relative to true north. Magnetic course is relative to magnetic north. Compass course is when the magnetic course is corrected to compensate for compass error.
Dead reckoning, DR. Gives the boat’s approximate position on the chart by drawing from the last known position the true course steered and distance sailed through the water.
Departure. Position on the chart, after the vessel has cleared harbour, to mark the start of the passage and from which the measuring log records the distance sailed.
Estimated position, EP. Gives a more accurate position than that by DR. The courses and distances sailed through the water are adjusted to allow for the effects of leeway and tidal streams or currents.
Ex-meridian altitude. A sextant observation to determine latitude by taking the altitude of a heavenly body within a short time before or after its meridian passage.
Fathom. The measurement for the depth of water; used over the centuries until metrication. Equals 6 feet or 1.83 metres. Believed to come from the medieval English word ‘faedm’ meaning ‘to embrace’, and was the span across the outstretched arms of an average man of the day, having a spread of 6 feet.
Leeway. The drift laterally through the water due to the force of the wind. The angle between the course steered and the actual course made good through the water.
Log. A device to record the distance sailed through the water. In Tern, in common with many older boats, this was Walker’s ‘Excelsior’ IV patent log: a rotator towing astern on a plaited line, the log line, with the distance-recording instrument mounted on a bracket on the stern. To give the actual speed, the instrument’s flywheel had a white mark on it; the frequency of rotation as seen through an aperture in the casing enabled the boat’s speed to be derived.
Log or logbook. The ship’s journal. An hourly or daily record of everything happening in or to the boat and her crew.
Meridian altitude. The altitude of a heavenly body, taken by sextant, when the body is on the observer’s meridian, that is due north or south, to give the latitude.
M. Symbol for sea mile and equates to one minute of latitude at the boat’s position. Also indicates a magnetic course or bearing, the angle relating to the meridian through magnetic north, which differs from true or geographical north.
Observed position, OP. The boat’s position at sea as found by observation of heavenly bodies. The method used in Tern II was ‘longitude by chronometer’. The requirement is to find the Local Hour Angle, LHA, which is the angle at the pole between the meridians of the body and the observer. This is done by calculation, incorporating the sextant observation and the estimated latitude. The difference between the LHA and the Greenwich Hour Angle, GHA, which is the angle at the pole between the meridians of Greenwich and the body, gives the longitude. The GHA is extracted from the Nautical Almanac. The position line through the longitude, so calculated, when crossed with the actual observed latitude, gives the observed position, the OP. The astronomical triangle, the centrepiece of celestial navigation, is depicted right:
In the diagram, O is the observer and C the centre of the Earth, PZQS is the observer’s meridian, PXRS is the meridian of the body, and PGS is the Greenwich meridian. The haversine cosine formula is used to calculate the LHA, the equation being solved with logarithms.
The zenith distance, ZX, is the angular distance between the observer’s zenith and the body, being equal to 90° minus the altitude. The declination is the angular distance of the body from the celestial equator, XR, and the latitude is that estimated at the time of the observation.
SAT, Ship’s Apparent Time. The time kept on a daily basis relative to the true sun, ie the one apparent or visible in the heavens, as would be indicated by a sundial.
Sextant. An optical instrument for measuring the angular distance of a body above the visible horizon. Called a sextant because the measuring arc on the frame is one sixth of a circle.