NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Determination of longitude and the prime vertical
From: Joel Silverberg
Date: 2007 Apr 08, 04:18 -0700
From: Joel Silverberg
Date: 2007 Apr 08, 04:18 -0700
Thomas Sumner in his "New and Accurate Method of Finding a Ship's Position at Sea" (1843) states repeatedly that there is but one proper moment in the day when a single observation can determine the Latitude of the ship, unless the apparent time at the ship is accurately known. And when the Latitude is uncertain, ther are only two proper instants per day (when the sun bears exactly East or exactly West) when the solar altitude can be used to find the Longitude by Chronometer with accuracy. If one makes an observation other than at those times, he stresses, the unavoidable errors in measurement may cause frequent errors which are very great. He also mentions more than once that in his latitudes, the sun in not observable in the proper E and W points for seven months of the year (taking into account that the altitude must exceed 6 or 7 degrees to be reliable. My question is : what are the crucial issues around the sun being on the prime vertical (that is, either due E or due W) when his (her?) altitude is taken? Am I correct in thinking that the hour angle between the observer's meridian and the sun's meridian would be exactly 90 degrees when the sun bears true E or W ? Are we dealing with a Pole-Sun-Zenith triangle with a right angle at the Zenith? If so, could we not calculate not only the hour angle (at P) but also the Latitude? { sin(D) = sin(L) sin(h) would give us L and cos(D) sin(HA) = cos(h) would give us HA , where D is the declination, L the latitude, h the altitude, and HA the hour angle .} Or is it more a question of sensitivity to errors in Latitude when the zenith angle is close to 90 degrees? Did people calculate longitude when the sun was on the prime vertical without using any asumptions as to latitude, or did they always (never say "always" or "never") use an oblique triangle together with their best estimate or guess for latitude? --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---