This weekend I had a terrible (though obvious) realization: The Earth moves around the Sun at about 30 km/s. The disk velocity dispersion is also on the order of 30 km/s. Therefore, if you have a survey (like the SDSS Southern Stripe) that has a time span measured in a small number of years, the parallax will be detected at comparable significance to the proper motion. That is, you cannot measure proper motions without also simultaneously fitting parallaxes. Argh—and duh!
You should tell that sun to "take it where you shake it!"
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