This monument was placed on Calton Hill by the second Astronomer Royal for Scotland in memory of Thomas Henderson.
Henderson was a brilliant mathematician who, for family reasons, entered the Legal profession upon leaving school, and he remained in that profession for eighteen years, six of which were in his native Dundee, and a further twelve here in Edinburgh.
During the latter part of those twelve years he was tutored by Professor Wallace both in his mathematical skills, and in the practical art of observational astronomy.
Thomas Henderson
It was his mathematical skills that first brought Henderson to world attention. In 1824 he produced a leading analysis of Lunar Occultations (events where the Moon passes in front of a star or planet) that was published in the Nautical Almanac for several years. In 1827 he published a correction to a paper by John Herschel (later to become Sir John Herschel) which essentially sealed his future in astronomy.
He worked, initially, as His Majesty’s Astronomer at the Cape, in South Africa, but his main job, to which he was elected in 1834, was as Astronomer Royal for Scotland and Director of the Royal Observatory of King George IV on Calton Hill. The Observatory building still stands there today and the main Transit Instrument is still in place where it was during Henderson’s directorship.
Henderson made over 60,000 observations in the 10 years he was in charge before his untimely death in 1844 due to a heart problem that had plagued him for most of his working life. Due to his parallel mathematical skills Henderson was able to analyse the behaviour of his instruments in order to eliminate any errors in the observations. This has given his observations a premier place in history.
For example, he was eternally interested in the difference of longitude between the Observatory on Calton Hill and that at Greenwich, and his ultimate result, published in 1838, was only 1/20th of a second different from one electronically obtained in 1858.
Thomas Henderson’s Gravestone
But perhaps the largest contribution he made to astronomy was in the measurement of the distance to a nearby star – Alpha Centauri. Henderson was the first person in history to have evidence of stellar parallax in his observations, leading to the distance estimations. Henderson was a conservative person and unfortunately the instrument he was working with had a small flaw causing him to hold back publication pending further more accurate measurements. While he was waiting for this to happen, Friedrich Wilhelm Bessel made a publication of his own, thereby becoming the first to announce the distance to a nearby star.
Thomas Henderson is a most worthy first Astronomer Royal for Scotland and he is interred in a quiet corner of the Greyfriars graveyard.