# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Time Sight Computations**

**From:**Chuck Taylor

**Date:**2004 Sep 27, 18:09 -0700

Frank Reed recently described the process of computing a time sight in words, but for some people a worked-out example is helpful. I spent the weekend at anchor and had the opportunity to shoot a few sun sights. The following reductions are of one sight taken of the Sun when it bore approximately WSW. I have shown three methods, one modern (using a calculator) and two traditional methods using log tables. I hope someone finds these examples helpful. For comparison, this same sight reduced by the method of St. Hilaire yielded an intercept of 0.1 nm. Chuck Taylor North of Seattle ============================================= Time Sight of the Sun At anchor, position by GPS: Lat 48d 30.1' N Lon 122d 49.5' W 25 September 2004 Corrected UT (GMT): 23-17-12 Corrected Ho: 24d 54.3' Almanac data: GHA Sun: 171d 27.4' Dec Sun: 1d 17.6' S EqT: + 8m 38s GMT: 23-17-12 GAT: 23-25-50 Method 1: Direct Computation Lat = L = 48.50167 Dec = d = -1.27833 Ho = h = 24.90500 cos t = (sin h - sin L sin d) / (cos L cos d) cos t = 0.66093 t = 48.62894 t = 48d 37.7' For an afternoon sight, Lon = GHA - t = 171d 24.4' - 48d 37.7' = 122d 49.7' === Method 2: Using log tables First compute polar distance: p = 90d + 1d 17.6' = 91d 17.6' h 24d 54.3' L 48d 30.1' log sec 9.12462 p 91d 16.7' log csc 0.00011 2)164d 41.1' s 82d 20.6' log cos 9.12462 s-h 57d 26.3' log sin 9.92573 ------- t log hav 9.22991 t 48d 40.1' For an afternoon sight, Lon = GHA - t Lon = 171d 27.4' - 48d 40.1' = 122d 47.3 If you don't have haversine tables, you can use sin tables as follows: sum 2)19.22991 sin (1/2)t 9.61496 (1/2)t 24d 20.0' t 48d 40.0' This was how it was shown in the 1920 Bowditch. Later editions used haversines. === Method 3: Same as Method 2, except that hour angles are expressed in hours, not degrees. Note that nautical almanacs formerly listed the Equation of Time for every 2 hours rather than the GHA of the Sun for every hour, as is the case now. The user was expected to compute GAT from GMT and use that instead of GHA Sun. Older tables allowed you to extract the hour angle directly in units of time. The computations are the same up to the point t log hav 9.22991 which becomes H.A. log hav 9.22911 H.A. 3h 14m 40s L.A.T. 15h 14m 40s G.A.T. 23h 25m 50s Lon 8h 11m 10s Lon 122d 47.5' Notice that using logs and tables instead of direct computation yields slightly different results for longitude (by about 2'), unless I have made an error. ============================================ __________________________________ Do you Yahoo!? New and Improved Yahoo! Mail - Send 10MB messages! http://promotions.yahoo.com/new_mail