The City Dome
The Dome was built in 1896 at the time the Observatory proper was being moved to Blackford Hill. It was built for a telescope (the Buckingham telescope) which was removed and scrapped in the 1920s. The lens is now in the Royal Museum on Chambers Street, but this Dome was never used for any serious astronomical purpose. There are also a number of smaller domes, randomly sited, in poor condition, badly built for late telescopes now dismantled and /or removed.
On the outside is the Thomas Henderson plaque – he was jointly the first person to calculate pretty exactly the distance to a star using parallax (parsecs) – he did it while at the Cape of Good Hope – the star was Alpha Centauri and Henderson came back to Scotland to sort out his calculations. The position of Alpha Centauri showed a residual error of about one second of arc (1/3600 of a degree, or 1/1800 of the moon's angular diameter) which was confirmed by observations of the star at the Cape by his successor Thomas Maclear. This was concluded to be the angle of parallax of the star against the background of distant stars, caused by the motion of the earth round the sun, and therefore the first estimate of the distance of a star at about 3.25 light years. (A later measurement gave 0.75 seconds of arc, at about 4.5 light years.)
However, the great German mathematician Friedrich Wilhelm Bessel of Konigsberg, working independently and with a much superior instrument and a different method, announced a parallax for 61 Cygni, a somewhat more distant star in the northern hemisphere, 2 months before Henderson, and thereby got the Gold Medal of the Royal Astronomical Society and all the credit. Henderson had been cautious because his instrumentation was suspect, he needed confirming observations and he feared ridicule because false parallaxes had been announced before. Despite this Henderson and Bessel became friends and even holidayed in Scotland together, and are now jointly credited with being first.