Landing Place, Jamestown, St. Helena

Sea level

Sea level recorders

This dataset contains tabulations of the heights and times of tidal high and low water at St. Helena from 1 October 1826 to 31 October 1827. The tide was recorded by an instrument designed by Manuel Johnson, a future President of the Royal Astronomical Society, while waiting for an observatory to be built. The tabulations in this dataset were obtained by inspection of photographs of Johnson's tabulation sheets that are held in the archive RGO 6/500 in the Royal Greenwich Observatory collection at Cambridge University Library. It is an important record in the history of tidal science, as the only previous measurements at St. Helena had been those made by Nevil Maskelyne in 1761, and there were to be no other systematic measurements until the late 20th century. Johnson’s tide gauge, of a curious but unique design, recorded efficiently the height of every tidal high and low water for at least 13 months, in spite of requiring frequent re-setting. These heights compare very reasonably with a modern tidal synthesis based on present-day tide gauge measurements from the same site. Johnson’s method of timing is unknown, but his calculations of lunar phases suggest that his tidal measurements were recorded in Local Apparent Time. Unfortunately, the recorded times are found to be seriously and variably lagged by many minutes. Johnson’s data have never been fully published, but his manuscripts have been safely archived and are available for inspection at Cambridge University. His data have been converted to computer files as part of this study for the benefit of future researchers. This dataset supports the paper “Cartwright, D.E.; Woodworth, P.L.; Ray, R.D.. 2017 Manuel Johnson's tide record at St. Helena. History of Geo- and Space Sciences”. Richard Ray (National Aeronautics and Space Administration) and Philip Woodworth (National Oceanography Centre) modified and added figures to David E. Cartwright’s original draft paper and sections of text have been updated, but otherwise the paper is as he intended it. This work was undertaken when Philip Woodworth was an Honorary Research Fellow at the National Oceanography Centre in Liverpool in receipt of an Emeritus Fellowship from the Leverhulme Trust. Part of this work was funded by UK Natural Environment Research Council National Capability funding.

British Oceanographic Data Centre
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