Telescope: Photographic Zenith Tube (1955)


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The Photographic Zenith Tube. From an RGO photo published in 1959. Image courtesy of Phillip Gething

The Photographic Zenith Tube (PZT) was designed and constructed in order to improve the accuracy of time-determination. Although technically a zenith instrument, its mode of operation was more akin to that of a highly specialised transit instrument. Like its two predecessors at the Observatory, it was also used for variation of latitude measurements. It was sited with the other meridian instruments to the north of the castle at Herstmonceux. 

Operations with the PZT commenced in 1955. It took over time-determination from Small Transit C on 16 October 1957. It was superseded by the satellite laser ranger, which had been brought into use at Herstmonceux in 1983. Operation of the PZT ceased on 30 June 1984. It was then mothballed by the National Maritime Museum, with a view to it possibly being put on public display at Greenwich in due course. In the end, it was turned down by the National Maritime Museum and transferred to the Science Museum instead (Object No. 1990-758).


The need for improvements in time-determination








Published descriptions of the Instrument

The photographic Zenith Tube of the Royal Greenwich Observatory. D.S. Perfect, Occassional Notes of the Royal Astronomcial Society, No.21, (Vol 3, November 1959)

Photographic Zenith Tube, Instrument and Method of Reduction. D.V. Thomas, Royal Observatory Bulletins, No.81 (1964)



Contemporary accounts

Reports of the Astronomer Royal to the Board of Visitors:

1944 p.5 1945 pp.5–6 1946 pp.4–5 1947 pp.6–8 1948 p.8
1949 pp.7–8 1950 pp.8–9 1951 pp.7–8 1952 pp.7–9 1953 pp.7–8
1955 p7 1956 p.5

Each of the above Reports has a specific section about the development, construction and installation of the PZT. Once in service, the instrument was only mentioned in the section of the Reports relating to the Time Service. This can be accessed via the link below:

Extracts from the Reports of the Astronomer Royal relating to the Time Service (1912–1964)

See also:

Recollections of Phillip Gething


The Spencer Jones Group from the south. Left: the pavilion for the Reversible Transit Circle. Centre: the pavilion for the Photographic Zenith Tube (PZT). Right: the building, housing the control room for the PZT, the observers' room and accessory services. A further pavilion built to house a small transit instrument is out of sight on the left