The First Measurement of Stellar Parallax

In 1838, Friedrich Wilhelm Bessel made the first observation of stellar parallax (Bessel 1838). While Bessel was the first to succeed, many earlier astronomers had either tried and failed to detect a parallax (Brahe) or published erroneous measurements (Hooke 1669, Cassini 1717 (Halley 1720)). When we consider the difficulties though, it is Bessel’s success, rather than the earlier failures, that is surprising.

To measure the stellar parallax he needed to be able to detect a slow shift in position, over the course of six months, of less than an arcsecond (1/3600 of a degree, roughly 2% the angular size of Jupiter as seen from the earth). He would need to do this while controlling for systematic effects - refraction (which will vary with atmospheric conditions), aberration, instrument imperfections - that are orders of magnitude larger than the shift. He would need to do all this, while observing a star that could well be too distant to even have an observable parallax!

Not only was the observation technically impressive, it was also scientifically significant. It finally determined the distance to nearby stars, and was the first step to finding distances to extragalactic objects.

The comparison with Jupiter shows how impressive Bessel’s observation of 61 Cygni was, and how the observed motion would change if the earth’s orbit or his target were different.

See (Hirshfeld 2001) for an excellent story about the people and instruments that made this measurement.

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