NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Raw data for bubble
From: Bill B
Date: 2007 Mar 22, 22:37 -0400
From: Bill B
Date: 2007 Mar 22, 22:37 -0400
Bill B responded to Bill Noyce > With LHA and AP lon we can estimate GHA, from which we can derive estimated > time of transit within a few minutes or better. > > With time of transit, we get a closer approximation of declination and GHA. > We could use the direct computation of azimuth from the formula George > posted a while back as a sanity check. > > "Tan Z = sin LHA / (cos LHA sin lat - cos lat tan dec) > > and the rules for putting Z into the right quadrant, 0 to 360, clockwise > from North, are: > --If tan Z was negative, add 180 deg to Z. > --If hour-angle was less than 180 deg, add another 180 deg to Z" I am sorry, I built a circular argument. It needs further thought on signing conventions for body east or body west, longitude east or west etc. but I think I am getting into the ball park. Lets try this: --Calculate LHA from the declination at best-guess time of transit. cos LHA = tan d / sin L --Add or subtract AP long to calculated LHA = GHAc --Calculate GHA for that estimated time used from the almanac = GHAa --Approx. time off transit in decimal hours = ( GHAc - GHAa ) / 15 --Modify time used for the estimate by the time off. That will get you very close to the transit time for the AP. Now do a sanity check with the formula George posted to determine azimuth. If you are spot on, it will fail as tangent of 90d or 270d won't work on a pocket calculator. Bill --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---