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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Polaris SHA
From: Bill B
Date: 2008 Sep 19, 03:28 -0400
From: Bill B
Date: 2008 Sep 19, 03:28 -0400
I asked: > "Why does the SHA of Polaris shift so dramatically through the month/year > (as opposed to other stars?)" Frank responded > > Fundamentally, this is a "coordinate singularity" effect. Polaris isn't > moving faster. It just happens to be moving fast very close to the North > Celestial Pole. It's a lot like someone walking past one of the geographic > poles on Earth at a distance of fifty feet. You can walk through a dozen > time zones in a few minutes. > > The largest change in the coordinates of the stars (and therefore the > coordinates of Polaris) is due to precession.... Thanks A lot of your answer I had absorbed (in my own vague manner) from your past posts. What you say makes a lot of sense, I just had trouble picturing it. I had briefly seen the magnificent star ball Alex owns (Alex too long absent from the list). How it accommodates some of the changes in star relationships you mention is beyond me. Given the coordinates of the celestial sphere etc. are semi-arbitrary man-made constructs IMHO, sometimes I have trouble syncing up with them lacking the benefit of a formal education in those areas. Regarded precession, I had expected the declination of Polaris to get smaller month-by-month and year-by-year (in my lifetime) with a *extremely* small wobble factor. I would expect the pole star, from an earthbound perspective, to move in an ellipse through the stars on an approx. 26,000-year cycle. Doing a little reading, apparently nutation affects precession so the change is not as constant as I had naively expected. More like a very dull handsaw's teeth applied to a smooth representation of an ellipse. As a side bar, what is the current projection of when Polaris will be closest to the axis of Earth's rotation? Susan P. Howell stated 2101 if I recall. I did not expect SHA to vary so much and move up and down from year beginning to year end. I regarded SHA as an angular distance from the first point of Aires. While the reference point changes with precession, the relationship to the reference point remains relatively constant noting the change of relationships due to proper motion and shifts (relativity) from the Earth moving at nominally 70,000 mph around the Sun and their relationships to the celestial sphere you have mentioned in the past. If I read you correctly, any small changes in the relationships between the stars (for many reasons) that might seem minor on a star ball or a conceptual celestial sphere are magnified by the coordinate construct/system. Bill B. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---