Thomas Henderson publishes first parallax measurement of Alpha Centauri

Background. Thomas Henderson, the second HM Astronomer at the Royal Observatory, Cape of Good Hope, was aided by Lieutenant William Meadows. Manuel John Johnson informed Henderson that Alpha Centauri had a large proper motion, suggesting it was relatively close. Henderson concluded that measuring its distance via parallax was feasible, as the task was easier for closer stars. This motivated his astrometric observations in the early 1830s.

Observations. Henderson determined the parallax of Alpha Centauri through exacting positional observations of the AB system between April 1832 and May 1833. He analyzed many astrometric mural circle observations to discover the true distance. However, he withheld his results because he suspected they were too large to be true, fearing they might be erroneous given previous discredited attempts to claim stellar parallax measurements.

Publication. Henderson eventually published his results in 1839, after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838. Henderson was relegated to second place due to his lack of confidence and delay. His work was not fully acknowledged at first, leading to Alpha Centauri being considered the second star to have its distance measured.