Erected by Flamsteed in the Great Room (Octagon Room) of Flamsteed House, the so called ‘Sirius Telescope’ was set up to enable a series of observations to be made in order to determine whether or not the Earth was isochronous (spinning on its axis at a steady rate). Such an investigation would have been impossible before the construction of the first pendulum clock in 1656 as all earlier clocks were simply too inaccurate for the purpose. Flamsteed was the first astronomer to investigate the matter.
Since the Observatory had been established with the specific and practical purpose of ‘rectifying the Tables of the Motions of the Heavens, and the places of the fixed Stars, so as to find out the so much desired Longitude of Places for perfecting the art of Navigation’, it was important to establish if the Earth was isochronous at the outset. Measuring longitude differences by measuring time differences depended on it being so.
Flamsteed described the telescope and his use of it in a letter written to Richard Townley on 24 March 1677/6:
‘…I have now fixed a payre of convex glasses of six foot focus in brasse cells upon an Iron Ruler to one of our Walls and three or 4 times observed the transits of Sirius over it. my last note was the 21th instant when hee passed it 3 min before 5 a clock at which time the sun was more than 10 degrees high and yet I saw the star as plainely as you usually see the planet♀ [Venus] upon the sun rise in winter with your bare eyes….’
In essence, what Flamsteed had constructed was a fixed telescope (where the lenses were supported on a ruler rather than by a tube), in front of which, the ‘Dog Star’, Sirius would pass each time the Earth completed one turn on its axis. By measuring the time interval with the Tompion (Great) Clocks (with their 4 metre long pendulums) set into the wall in the north-east corner of the room, Flamsteed was able to determine that the Earth was indeed, isochronous as well as how the length of the solar day varied with the season. Had the Earth proved other than isochronous, the Observatory would probably have been rapidly shut down.
This timing quoted by Flamsteed, implies an azimuth for the telescope of about 154º – In other words the telescope was aligned by about 26º to the east of the meridian, suggesting the telescope was located on the west side of the room opposite the clocks with the OG looking out through the south west window and the eyepiece adjacent to the east window.
Following the invention of the highly accurate Shortt-Synchronome free pendulum clock in the 1920s, astronomers were able to show that the Earth is not completely isochronous, leading eventually to a change in the way in which intervals of time were defined in the 1950s.