The Western Equatorial was cobbled together from exisitng instruments for the Astronomer Royal, John Pond, in about 1824. It was made from a 30-inch telescope by Tulley that had been kept in the Great Room (Octagon Room) and the Sisson 5-foot Equatorial Sector from the West Dome, that had been described by Pond as useless and incapable of repair (RS MS371/70). Like the instrument it replaced, the Western Equatorial was housed in the West Dome in the extreme north-west corner of the Observatory.
The Western Equatorial was described by Airy in the 1843 volume of Greenwich Observations as well as those for 1844 and 1845:
‘This is a small telescope mounted on a long polar axis; the two pivots of the polar axis being at the two extremities of the polar axis, and the pivots of the short declination-axis being both on the same side of the telescope. The length of the polar axis is about 10 feet 3 inches; the diameter of the declination-circle and that of the hour-circle, 2 feet. The subdivision is effected by verniers. The length of the telescope is 2 feet 10 inches; the aperture of its object-glass, 3.5 inches. This is an instrument of an inferior class.’
In a letter to John Quincy Adams of the Harvard Observatory in 1839, Airy was rather blunter in his assessment, describing the Western Equatorial as ‘a very worthless instrument’.
The second equatorial sector by Sisson that had been in the East Dome was replaced by the Schukburgh Equatorial in 1816. It was with this instrument that Pond observed the Great Comet of 1823 in the early months of 1824. Between them, the East and West Domes (which had been created from Flamsteed’s summerhouses), were able to command a view of the whole sky. But each had a large section of sky obscured by the towering presence of the Great Room which stood between them. Taken together with matched instruments, the domes were arguably useful. On their own, their potential was distinctly limited. One might speculate that it was the sighting of the Great Comet of 1823 that prompted Pond to convert the instrument in the West Dome. Despite the inferior nature of the instrument that resulted, he at least now had a means of observing the whole sky with an equatorial! The inventory taken at the visitation of 1824 (RS MS371/70), together with a list of instruments compiled in 1818 [RS MS372/170], and an undated list from about 1823 (RS MS372/171), suggest this conversion took place in 1824.
There appear to be no references to observations being made in the West Dome from 1811 (when the great comet of 1811 was observed with the equatorial sector), until Enke’s Comet was observed from there in 1828 with the Western Equatorial.
Click here to view the observations of Enke’s comet made with the modified instuments in 1828.
Despite Airy’s implied criticism of the Western Equatorial, it was used in conjunction with the Schukburgh Equatorial to observe Halley’s comet during his initial weeks in office in the last months of 1835. By 1838, both instruments had been superseded by the newly acquired and rather superior Sheepshanks 6.7-inch refractor.
Click here to view the observations of Halley’s comet made with the instument in 1835.
The Western Equatorial remained in-situ until 1845 when the polar axis, the declination-circle, and the hour-circle were sent to the Cape of Good Hope for the use of Mr. Maclear, the Astronomer Royal at the Cape. [Greenwich Observations 1845]. In the autumn of 1847, the Cape’s 46-inch achromatic with an aperture of 3½ inches by Dollond was mounted on the polar axis and placed under a revolving dome. (Contributions to astronomy and geodesy, London, 1851, p.3). According to Brian Warner, the telescope was ‘thrown out’ by Gill soon after his arrival as H.M. Astronomer at the Cape in 1879.
Meanwhile, the telescope was mounted as a ‘Detached Telescope’, where in the Introductions to the Greenwich Observations from 1851 to 1908 it was described as having a focal length of 36 inches and an object-glass of aperture 3.6 inches. After 1908, it appears to dissapear from the records.
The West Dome at Greenwich was incorporated into the dwelling house by Airy in 1848. Today, the upper floors are used as offices by the staff of the Royal Museums Greenwich.
© 2014 – 2024 Graham Dolan
Except where indicated, all text and images are the copyright of Graham Dolan