The observation ledgers collected in this class represent observations made at the Royal Observatory, Greenwich, from 1911 to 1936.

The Cookson Floating Zenith Telescope was manufactured to the design of Bryan Cookson (1874-1909) at the celebrated makers the Cambridge Scientific Instrument Company and T. Cooke & Sons of York where the object glass and some of the iron-work was produced (the records of both these companies are retained in the collections of the Department of Manuscripts and University Archives of the University Library, the C.S.I. Co. as a distinct collection, and Cooke & Sons as part of the Vickers Archive, as the company was eventually taken over by Vickers Ltd. Sometime after Cooke amalgamated with Troughton & Simms in 1922). A novel feature of the design was a circular basin of cast-iron, which was filled with nearly 65kg of mercury. To maintain the precise position of the vertical axis of the instrument true to the zenith in the local gravitational field the structure of the FZT, also of cast-iron, was floated in this mercury bath.

Cookson was MA (Oxon) and MA (Cantab) 1899 and 1905 respectively. A grandson of Robert Stirling Newall, Cookson was appointed an assistant to his uncle Professor Hugh Frank Newall at the University Observatory in 1906 and served until his untimely death in 1909. It had been in 1900 that the Cookson’s FZT was installed in the dome over the centre of the 1823 building of the Cambridge University Observatory, where it remained for eleven years.

The purpose of the instrument, reflecting the history of zenith telescopes back to the time of James Bradley’s 12½ft Zenith Sector of 1727, was to measure stellar aberration as well as the nutation (movement back and forth or ‘nodding’) of the Earth’s axis in addition to the variation of latitude due to short-term influences affecting the position of the poles - the Chandler Wobble (identified by an American astronomer Seth Carlo Chandler in 1891 but predicted from theory by Leonhard Euler in 1755). To determine these values observations of meridian transits of pairs of stars close to the zenith were recorded on photographic plates with the resulting data recorded in these ledgers.

Having been donated to the University of Cambridge by Cookson’s widow, the CFZT was in 1911 transferred on loan to the Royal Observatory Greenwich at the request of the Astronomer Royal, Frank Watson Dyson. Initially the RO borrowed the CFZT for seven years but the period of the loan agreement was subsequently extended. Up to 1911 RO astronomers had been using the Reflex Zenith Tube made to George Airy’s design in 1851. Sixty years on it was felt that a modern instrument was required for measurements which so critically affected the reductions of observations from the other telescopes at Greenwich. The CFZT was first installed in a dedicated hut close to the meridian at the perimeter of the main courtyard at Greenwich but in 1936 it was transferred to the Christie Enclosure that had been constructed some 300m to the east of the principal RO buildings, where it was close to the Cooke Reversible Transit Circle of 1933. From the time of this move the CFZT was used for latitude variation observations only.

Remaining the property of the University of Cambridge the CFZT was unused during World War II and by 1951 it was dismantled, its work being taken up by the Photographic Zenith Tube of 1955 installed at the new home of the newly named Royal Greenwich Observatory at Herstmonceux Castle north of Eastbourne in East Sussex. The CFZT was transferred to the Science Museum, South Kensington, by the University of Cambridge in 1956 and it is today part of the Museum’s collections, though at the time this collection history was written (August 2015) it is not on display.

Perhaps by chance a senior member of the Observatory’s staff of the mid-20th Century was responsible for this collection surviving today as a separate class in RGO Archives with a late date-of-accession. Robert d’Escourt Atkinson (1898-1982) was a distinguished British physicist and astrophysicist who in 1937 returned from eight years teaching at Rutgers University in New Brunswick to take up the post of Chief Assistant to the Astronomer Royal at Greenwich, then Harold Spencer Jones. Though Atkinson was away on war work 1940-1946 he returned to Greenwich and in addition to his scientific work was Officer-in-Charge at Greenwich during the protracted move down to the RGO Herstmonceux Castle after Spencer Jones moved to Sussex.

In his obituary of Atkinson in the Quarterly Journal of the Royal Astronomical Society, Vol. 25, No.1, pp.100-104, 1984, Andrew Murray noted his subject’s 'continued interest in fundamental astronomy' shown in a series of papers in which he argued strongly against the use of the Earth’s rotation axis as the definition of the instantaneous pole of celestial observations, since latitudes and longitudes of observatories theoretically have rapid though small quasi-diurnal changes when referred to this pole.

When Atkinson retired from the RGO Herstmonceux in 1964 he returned to the USA, to the University of Indiana where he took up another academic teaching position. Eager to use the raw observational data in the CFZT observations to take further his argument, he was it seems permitted by the Astronomer Royal, Spencer Jones’s successor Richard Woolley, to take with him the collection of ledgers now in this class of RGO Archives. Staying in Bloomington after his retirement from the University in 1979 Atkinson died there in 1982. The ledgers, packed into two wooden crates 600mm x 900mm x 450mm, remained in the store of the Astronomy Department at Bloomington for a further 24 years. By 2006 Professor Catherine Pilachowski at Bloomington and Adam Perkins in Cambridge were in correspondence about possible action to have the ledgers returned but it was not until the end of July 2008 that the contents of the crates, now in six parcels, arrived in the University Library after the receipt of assistance on the part of the Science & Technology Facilities Council, Swindon.