NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Star ID method
From: Paul Hirose
Date: 1998 Jun 23, 00:45 -0400
From: Paul Hirose
Date: 1998 Jun 23, 00:45 -0400
I used Pub. 229 to identify the mystery star from Silicon Sea Leg 42. Latitude is 37, so we know the volume and column to search. The problem says the star is a little to starboard of dead ahead, our course is 98 true, so I guessed 100 for the azimuth. Azimuth angle, Z, is therefore 80. Altitude was observed at 30 deg. So we're looking for an entry where Hc = 30 and Z = 80. Open the book to a random page, look at the left page (dec same name as lat), go down the lat 37 column, locate the Hc 30. Unless we're lucky, Z won't be 80. So we try another page. Instead of scrambling around hapazardly, get systematic. Turn one page, find Hc 30 again. If Z is closer to 80, you're going the right way. Eventually you'll find lat 37, alt 30, Z 80 all come together for LHA 70/290, dec 25 same name. LHA 70 would put the star to the west. It's east, so the correct LHA is 290. Convert LHA to SHA. Nunki fits pretty well. That's how I found the star, but in retrospect it could have been done better. The problem with searching for a known Hc, then checking if Z is correct, is that the same Hc can occur twice on the same page. E.g., on the LHA 70/290 "same name" page, lat 37 column, see how Hc 38 occurs twice, once at dec 45, again at dec 87. So you'd have to check Z at both places. It's more efficient to find the closest Z, then see if Hc is correct. In any column, Z decreases continuously as you go down the page. You won't have to worry about your Z appearing twice in the same column. If you can't find your Hc and Z on the left-hand pages, then of course it must be on the right. Star identification by Hc and Z loses accuracy as altitude increases because it's hard to measure or estimate a decent Z when the body is way up there. Remember, just because only one of the 57 navigational stars is close to the expected SHA & dec, you haven't got a positive ID. Many of these stars have neighbors of near-identical magnitude. Take Nunki. It's part of the handle of the Sagittarius "teapot". The other 3 stars of the handle are almost as bright. If you're looking through a hole in the clouds without being able to see the star field, you could easily shoot one of the neighbors by mistake. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@roninhouse.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=