NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Emergency navigation solution?
From: Paul Hirose
Date: 1997 Dec 16, 9:57 PM
From: Paul Hirose
Date: 1997 Dec 16, 9:57 PM
One solution to Mal Misuraca's emergency nav problem is to use Sirius. The dec of Sirius is 16 43' S, and the latitude of Tahanea 16 50' S. I don't have a 1997 almanac, but the 1996 edition should be close enough. Let's first determine the hours of darkness. For position, assume 8 S 136 W, which puts us about the stated 700 mi NE of Tahanea. According to the Almanac, at our latitude evening civil twilight ends at 1828. Morning civil twilight begins at 0505. Looking in the "arc to time" table, we see that Greenwich is 9h 4m ahead of us. So we will be in darkness, more or less, from 0332 to 1409 UT. Meridian passage at Tahanea occurs when GHA Aries + SHA star + east lon Tahanea = 360. That is, when GHAA + 259 - 145 = 360. Therefore GHAA = 246 at meridian passage. Looking at the almanac again, we see that GHAA 246 occurs about 12h UTC. This is within the hours of darkness at our DR position. So, Sirius will be visible in our predawn sky when it's almost directly overhead at Tahanea. I do have "Sirius" doubts about this method for anything more than getting roughly in the vicinity of the destination. You must be able to get directly under the star, and that means judging the zenith quite accurately. Miss by the width of a fingertip at arm's length, and you miss your destination by 100 miles. I don't think I could do that well on land. On the tilted, moving deck of a boat, it'd be even harder. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= =-= TO UNSUBSCRIBE, send this message to majordomo@ronin.com: =-= =-= navigation =-= =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-=
