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