D7 Astrometry

Topic

Astrometry measures the angular positions of celestial objects with the greatest possible precision.

Parallax—the slight apparent angular shift a nearby star exhibits against the background of distant stars, caused by Earth's own orbital motion—decreases as the actual distance between the star and the observer increases; thus, measuring this tiny angle is, in essence, equivalent to measuring the distance itself.

Proper motion—an object's intrinsic angular displacement over time, distinct from the previously described parallax—combines with the line-of-sight velocity derived from spectroscopy and the distance determined by parallax; this combination yields the object's full space velocity across all three spatial dimensions.

When astrometric precision is sufficiently high and observations are repeated over the necessary period, it is also possible to detect a minute periodic oscillation superimposed on a star's proper motion, caused by the gravitational pull of an unseen planet orbiting that star. Thus, from the angular positions with which this document begins to the oscillation caused by an invisible planet with which it concludes—encompassing distance-related parallax and proper motion combined with spectroscopy—astrometry allows a star's simple position in the sky to eventually reveal, given sufficient patience, its distance, its velocity, and even the presence of worlds we never directly see.

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