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Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.
Page 18 of 22
Faster, less precise tables originally designed for air navigation
Volume 1 tabulates altitude and true azimuth directly for seven selected stars per LHA of Aries, requiring no declination entry and giving a fix with minimal work, but it needs replacing every five years or so as precession accumulates. Volumes 2 and 3 cover bodies with declinations up to 29 degrees and are perpetual. Precision is one minute of arc rather than the tenth achievable with NP401, which is ample for ocean passage-making.
Read definition →Volumes of pre-computed altitudes and azimuths for any observation
Sight Reduction Tables for Marine Navigation cover latitudes 0 to 90 degrees in six volumes of 15 degrees each. Entered with whole degrees of latitude, declination and LHA, they give computed altitude, an altitude difference d for interpolating the declination minutes, and azimuth angle Z. They are usable for any body and provide high precision, at the cost of bulk and an interpolation step. The US designation is HO229 and the British is NP401.
Read definition →The average height of the biggest third of the waves
Defined as the mean height, trough to crest, of the highest one third of waves in a record, chosen because it approximates what an experienced observer reports by eye. It is the quantity given in forecasts, GRIB files and buoy data. In a given sea state the highest individual wave in a three hour period is commonly around 1.8 to 2 times Hs, so a 3 metre forecast implies occasional waves near 6 metres.
Sailing reference
Clear explanations of the language used in navigation, pilotage, weather, safety and life under sail.