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Separating annual parallax from a star’s projected motion
Proxima Centauri’s Hubble-based projected path, released in 2013, illustrates two contributions: the star’s drift across the sky and a scalloped annual component caused by Earth’s orbit.
In the 2013 HST spatial-scanning draft’s one-direction fit, position, parallax and proper motion are estimated together. Known viewing geometry fixes the annual component’s shape and phase; observations at multiple epochs distinguish its parallax amplitude from the star’s drift. Three epochs are necessary for that described fit, but insufficient in practice because image registration adds parameters.
Analyst deduction: a displacement between two images should not automatically be treated as pure parallax. Proxima’s path illustrates the distinction, while the HST epoch requirement belongs to the particular fitting method described.
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Physical Science