August 10, 1844

Friedrich Bessel

Bessel infers unseen companions of Sirius and Procyon from their motions

Bessel found a star by watching another star fail to travel in a straight line — and ended the argument with a warning that others should check it.

A letter dated at Königsberg

In a letter dated at Königsberg on 10 August 1844, Friedrich Wilhelm Bessel set out evidence that the proper motions of Sirius and Procyon varied rather than following uniform straight paths.

For Sirius he compared accumulated right-ascension observations against his Königsberg tables; for Procyon he found an increasing declination discrepancy after recalculating the relevant reductions.

The quantities were tiny: Sirius's 1843 right-ascension corrections worked out at +0.318 seconds from Busch's 50 observations and +0.324 seconds from Bessel's own 40.

Invisible does not mean absent

Bessel proposed that treating both visible stars as double systems orbiting centres of gravity would explain the variations, even though their companions had not been seen.

Procyon's declination discrepancy for 1844 was reported at +3.18 arcseconds, and he provisionally associated a period of about 50 years with the Sirius perturbation.

He cautioned explicitly that the result still required strict testing against observations from other observatories.

Not the discovery of binary stars

William Herschel had demonstrated gravitationally bound stellar pairs decades earlier. What was new here was inferring specific unseen companions, too faint or too close to resolve, from astrometric perturbations alone.

Nothing was observed on 10 August, and nothing was published then either — the English translation appeared in the Monthly Notices of the Royal Astronomical Society that December.

Sirius B was first seen in 1862 and Procyon B not until 1896; both are now classified as white dwarfs, a class recognised only decades after those sightings.

Sources

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What the sources leave uncertain

August 10 is secure as the date written on Bessel’s letter. It is not the publication date: the English extract appeared on December 13, 1844.
Bessel spoke of unseen or non-luminous companions as a hypothesis requiring further scrutiny; “deduced” should not be read as direct detection.