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    Re: Star finder, 2102-D, further modification
    From: Gary LaPook
    Date: 2010 Nov 09, 14:23 -0800
    Giving it even more thought, the scale around the star finder base is really a right ascension scale, or 360° minus S.H.A., so zero is always the Vernal Equinox, 90° is the Summer Solstice, 180° is the Autumnal Equinox and 270° is the Winter Solstice and this will always be the case on the 2102-D so the S.H.A. of the sun will not change as the years go by. But, due to the inclination of the earth's axis to the plane of the ecliptic, the sun's declination will change, making a slight change to the drawn-in ecliptic necessary for maximum accuracy. For instance, the sun's declination this year at the Winter Solstice will be 23° 26.3' south while at the same point in 1910, one hundred years ago, it's declination was 23° 27.1' south a difference of 0.8' so this could present a problem if you live long enough. ( I wouldn't lose too much sleep over this since the width of the pencil line marking the ecliptic is much broader than this.)

    But the positions of the stars will change much more, making it necessary to buy a new 2102-D or to re-plot their positions. Taking three stars at random, Deneb, Enif and Polaris, for example, and comparing their positions at the Winter Solstice this year with their positions at the Winter Solstice in 1910, the S.H.A. of Deneb changed from 50° 24.6' to 49° 33.1' and it's declination changed from 44° 57.7' to 45° 19.4' north, a change of 42.3' in it's position. Enif moved from an S.H.A. of 35° 03.2 to 33° 49.0 and it's declination went from 9° 27.9' north to 9° 55.7' north a change of 1° 18.2'. For Polaris, the S.H.A. changed from 338° 08.9' to 318° 20.1 and it's declination went from 88° 50.1' north to 89° 19.0' north, a change of 34.3'. (Part of these changes are due to proper motion but most are due to the Precession of the Equinox.) So, if you can tolerate about one degree of accuracy then you will never have to replot the positions of the stars on your 2102-D but your grandchildren will have to do it on your star finder that you pass down to them.


    On 11/9/2010 11:16 AM, Gary LaPook wrote:
    On 11/9/2010 8:22 AM, Luc Desmedt wrote:
    The book published by David Burch (star-finder book) explains why the ecliptic have not drawn on the 2102-D but I can not find the page today (I'll read it again) but I guess it changes with the precession of equinox?
    Yep, that's the problem and in only 13,000 years an ecliptic drawn on the 2102-D will be off by 180°. If you are worried about this then use a pencil and, to maintain one degree accuracy, simply erase the ecliptic and draw in a new one after 72 years. But wait, the whole star base will be off by one degree after 72 years since the plotted positions of the stars are based on the position of the equinox at the time that the 2102-D was manufactured so you will have to buy a new star finder in 2082 A.D. (or earlier, depending on how old your 2101-D is already.) In fact, giving it more thought, the ecliptic will remain in the correct place (so you won't have to redraw it) but you will have to replot all the stars.


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