NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: October Lunar
From: Frank Reed
Date: 2008 Oct 04, 20:20 -0400
From: Frank Reed
Date: 2008 Oct 04, 20:20 -0400
Jeremy, you wrote: "Admittedly, this was a poor time to shoot lunars. Both bodies were well below 10 degrees of altitude, and the observed lunar distances were just below 12 degrees, which does not usually help with accuracy, but I wanted to practice, so I proceeded." The low altitudes may be a problem for some lunars but probably not in this case. The short distance is not an issue (two reasons: the altitudes are calculated so their accuracy is not a problem, and also we don't have to interpolate between geocentric distances three hours apart, which was an issue historically but not today). I cleared your sights a couple of different ways and agree that they seem very consistent as a group but show an error of about 1 minute of arc. So what could explain that? You suggested: "Is it strange refraction due to low altitudes?" Because the two bodies were at nearly equal altitudes, we can safely rule this out. First of all, if there were any unusual refraction, it would probably affect both objects by nearly the same amount. More significantly, since the lunar distance was nearly horizontal, the corrections to that arc due to the altitude corrections (which are entirely vertical) is much reduced. You can experiment with this issue by trying different values for barometric pressure on your sights. Though these lead to substantial changes in refraction, they don't change the cleared lunar distance much at all. And you wrote: "Perhaps is an error in my sextant calibration" Always possible. "or perhaps just a personal observation error." A minute of arc at 7x magnification is big. Unless you had very poor observing conditions, you would see this very clearly. By the way, one issue that sometimes comes up with Venus is its large angular diameter and also its phase. Right now, that's not a concern since it's still relatively far away. -FER --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---