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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Venus
From: Bill B
Date: 2005 Dec 12, 19:27 -0500
From: Bill B
Date: 2005 Dec 12, 19:27 -0500
George I see your point now, especially with my parallax vs. phase confusion cleared up. With much greater refraction on the sun's upper limb than Venus, once the suns upper limb has dropped to 50' below the horizon, the elevation of Venus would be approx. 2.5d. Think I will go to plan B, and use a roof line to block the sun when the pair is higher in the sky and I can get perhaps a 4d difference in elevation. If I recall, binocular lens mounting adhesive will not stand up to the heat generated by projecting on a sheet of paper/poster board. As always, thanks for taking the time to clear some of the fog away. Bill Bill wrote:> >> We start with a approx 6.4d difference in declination, back out the sun's >> SD, and account for refraction, would the separation not be greater than >> 3d? > George wrote:> Even if the separation is 6.4 degrees, that is usually at a slant-angle to > the horizon when near setting. > > The difference in ALTITUDES, at setting, will never be greater than that, > and usually less. > > Think, if you were observing setting sky-objects from somewhere on the > equator. Then all objects, stars, planets, Sun, Moon, that have the same > hour-angle will set simultaneously, and their declination will make no > difference at all. That's an extreme case, to bring home the message. > > The difference in altitudes near setting depends on the observer's latitude, > and both declinations and hour-angles. You can calculate the altitudes, near > the moment of setting, just as if you were going to use those two bodies for > a position line. > > I very much doubt whether Venus could be seen at such a low altitude, when > it's so close to the Sun, and therefore in a very bright patch of the > evening sky. Itwould be a good test of Bill's observing powers, and I hope > that my words won't discourage him from trying.