NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
About French (in)accuracy. was :Lunars for dummies like me
From: Zorbec Legras
Date: 2004 Sep 26, 16:08 +0000
From: Zorbec Legras
Date: 2004 Sep 26, 16:08 +0000
George Said: "So I ask Zorbec, what moment, around sunrise, do French navigators choose to time when they adopt this procedure? Do they make any adjustment to allow for refraction; or is any such adjustment included in the tables?" The tables give the result of the formula P = TanLat Tandec. Since sin/cos = tan, P = sinLat sindec/ cosLat cosdec With a correction, it is the instant of rise for the center of the sun on the celestial horizon When a French navigator wants an exact instant, he takes it from the almanach (Ephemerides nautiques) or he computes it. For exemple the rising time "Trs" of the sun is computed with a given altitude of : minus 53'. Why 53', let say it's French inaccuracy... P = asin( sin - 53' - ( sin lat sin dec) / (cos lat cos dec)) Trs = 12 - P - Eq. So, on 25/09 Lat : 50 N, dec : 1 05.7 S, Hv : - 0 53', Eq = 8mn27.9sec Asked : time of sunrise ? P = asin( sin -0 53' - (sin 50 sin 1 05.7)/(cos50 cos 1 05.7) P = 90 4.2' = 6h 0.3mn 12-P-Eq = 5h 51mn 20.1sec as a first approximation, a second computation gives 5h 50mn 50.1sec At this time the French navigator can continue the iteration or he can take the mean value between the limits. The mean value is 0h 0mn 15sec and the asked result is 5h 51mn 05.1sec. Very close to the nautical almanach which gives 5h51mn, but certainly closer to the real value. -- ___________________________________________________________ Sign-up for Ads Free at Mail.com http://promo.mail.com/adsfreejump.htm
