Articles | Volume 8, issue 1
Review article
27 Mar 2017
Review article |  | 27 Mar 2017

Manuel Johnson's tide record at St. Helena

David E. Cartwright, Philip L. Woodworth, and Richard D. Ray

Abstract. The astronomer Manuel Johnson, a future President of the Royal Astronomical Society, recorded the ocean tides with his own instrument at St. Helena in 1826–1827, while waiting for an observatory to be built. 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.

Short summary
This paper discusses an historical record of the ocean tide made by the astronomer Manuel Johnson (a future President of the Royal Astronomical Society) at St. Helena in 1826–27. It describes how the measurements were made using a tide gauge of an unusual design, which recorded the heights of the high and low tides well, although information on their times were not so accurate. Johnson’s work is not well known. One objective of the present research was to make his measurements more accessible.