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    Re: star-to-star distances
    From: Jan Kalivoda
    Date: 2004 Sep 30, 23:33 +0200

    Bill Noyce wrote:
    > This is called "annual aberration" or "aberration of starlight"
    > Historical note -- this effect was first discovered by astronomers
    > who were trying to measure parallax of stars (using the earth's orbit
    > as a baseline) to determine how far away they are.  It turns out that
    > parallax is (almost always?) a much smaller effect.
    In all cases. The annual ellipse of the apparent (= observed) star position 
    around the true position (becoming the circle in the poles of the ecliptic 
    and the straight line on the ecliptic) due the annual aberration has the 
    major axis of some 20" of SHA, while even the greatest annual star parallax 
    (still Proxima Centauri, if I hadn't missed anything) lies below 1".
    Another important factor for obtaining the apparent (= observed, almanac) star 
    positions is the nutation, caused by perturbations by the Moon, which rocks 
    the position of Earth's axis around its mean position that is in turn 
    steadily shifted by the precession (cca 50" of SHA in a year). The nutation 
    has several components, the largest one amounts to cca 17" of SHA and 9" of 
    the declination with the period of 19 years (other nutation components have 
    much smaller amplitudes and periods).
    All can be summed up this way:
    Mean celestial body's position for some epoch, obtained by celestial mechanics 
    for planets/Moon/Sun=Earth or tabulated in astronomical catalogues for stars 
    + its own intermediate motion + intermediate precession = the mean position 
    for the actual date
    Mean position for the actual date + nutation = true position for the actual date
    True position + annual aberration + the correction for the finite light speed 
    for planets/Moon/Sun + annual parallax for stars  = apparent geocentric 
    position (tabulated in the Nautical Almanac)
    Apparent geocentric position + daily parallax (+ daily aberration, max 0.3") = 
    apparent topocentric position
    Apparent topocentric position + refraction + instrument errors = measured position
    Jan Kalivoda

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