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S E M I - A N N U A L   S T E L L A R   P A R A L L A X
Dr Robert A. Hatch - University of Florida


If we assume that the earth moves, that is, that it rotates daily and revolves annually around the sun, one might reasonably expect the apparent position of a star to shift its relative position with other stars. Put differently, if the earth were fixed and stable at the center of the cosmos, then its distance to the stars would remain fixed and unchanged. But if the earth in fact moved (if the diameter of its annual orbit around the sun was many millions of miles) then the apparent position of stars might be expected to change.  Other conclusions follow from the same assumption.  For example, a close star would have a large apparent shift in relative position (it would have a large angle of parallax) while a more distant star would have a smaller angle of parallax.  But what could this mean?  A number of reasonable inferences seem to follow.  If there was no observable semi-annual stellar parallax, would it be reasonable to conclude that the stars are disproportionately further away than the planets?  Would it be reasonable to infer that the earth did not move?  Historically, as it happens, the absence of observable stellar parallax was a strong argument against the motion of the earth.  Quite simply, putting the earth in motion caused a thousand inconveniences.  The absence of stellar parallax seemed good evidence that the earth did not move.  As it turns out, stellar parallax was not observed until 1838.  It required a superior telescope. The angle of stellar parallax observed about 150 years ago was extremely small, about one-half of one minute. The button below will take you to two illustrations of Stellar Parallax. One shows the large parallax angle for a nearby star, the second shows a small parallax angle for a star further away.

Semi-Annual Stellar Parallax:     Stellar Parallax

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