NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: venus
From: Frank Reed CT
Date: 2004 Oct 13, 20:40 EDT
From: Frank Reed CT
Date: 2004 Oct 13, 20:40 EDT
George H wrote:
"What I suggest is that we take a simple situation that involves little or
no trig and see if we agree then. "
We're thinking along the same lines though for slightly different reasons. I am assembling a post tentatively titled "Really Easy Lunar" using the same observation date and time as Michael Dorl's recent lunar but from a different latitude so that the two objects have exactly the same azimuth --they were nearly there already. I was working from the point of view of the educational value of such sights, but as you suggest, it's also an important check on any calculational tool.
As for oblateness corrections, George H wrote:
" It affects the apparent ALTITUDE of the Moon, by an amount which was
supplied in navigators' table for "Reduction of the Moon's horizontal
parallax", which effectively multiplies the HP by (1- ((sin lat)squared
/300)). "
Do you have a copy of Chauvenet? The logic for dealing with oblateness there involves two steps. First, the HP is increased (yes, opposite procedure to the above) by an amount which is equivalent to multiplying HP by (1+sin^2(Lat)/150). Then at the end, there is a small rotation that adds a further correction. These are not separated as corrections in azimuth and altitude, but they do cover both coordinates. This technique incidentally can be applied regardless of the actual clearing method used.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
"What I suggest is that we take a simple situation that involves little or
no trig and see if we agree then. "
We're thinking along the same lines though for slightly different reasons. I am assembling a post tentatively titled "Really Easy Lunar" using the same observation date and time as Michael Dorl's recent lunar but from a different latitude so that the two objects have exactly the same azimuth --they were nearly there already. I was working from the point of view of the educational value of such sights, but as you suggest, it's also an important check on any calculational tool.
As for oblateness corrections, George H wrote:
" It affects the apparent ALTITUDE of the Moon, by an amount which was
supplied in navigators' table for "Reduction of the Moon's horizontal
parallax", which effectively multiplies the HP by (1- ((sin lat)squared
/300)). "
Do you have a copy of Chauvenet? The logic for dealing with oblateness there involves two steps. First, the HP is increased (yes, opposite procedure to the above) by an amount which is equivalent to multiplying HP by (1+sin^2(Lat)/150). Then at the end, there is a small rotation that adds a further correction. These are not separated as corrections in azimuth and altitude, but they do cover both coordinates. This technique incidentally can be applied regardless of the actual clearing method used.
Frank R
[ ] Mystic, Connecticut
[X] Chicago, Illinois
