Jack - I'm replying to you through Paul's reply, since Paul was so helpful as to in-line the figure. Please take a careful look at the figures again. 'N' is the actual position in space of the observed star. 'E' is obviously the Earth and 'S' the Sun. The Sun and the star are fixed in relative distance and direction to each other. The observer is obviously on the Earth in both cases. 'Au', I believe, refers to the one astronomical unit (AU) average distance between the Earth and the Sun. Top plot: - The Earth 'E' revolves around the Sun 'S'. - The two lines from 'E' through 'N' show the directions to the star, half a year apart. There is obviously a parallax - the angle shown as \theta. Bottom plot: - Take the Jan. part of the top-plot: the 'E', the 'S', and the 'E'-'N' and 'S'-'N' lines, and move them down by 1AU so the 'E' is in the centre. - Take the July part of the top-plot: <same as above> and move them up by 1AU so the 'E' is in the centre. - Result: The exact same situation as above but seen from the Earth's perspective. - And the exact same parallax, \theta. The bottom plot, however, is NOT equivalent to a geocentric Solar system, where the stars would be fixed (actually have constant velocities) with respect to the Earth, and there would be no parallax. There is nothing secret or Earth-rattling about that bottom plot. Regner Paul Deema wrote:
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