RO and RGO Solar Plates

Regular observations of the sun were begun at Greenwich because of the association of the fluctuation of the Earth's magnetic field with sunspots and other activity on the sun. There was a greatly enhanced interest given to the observations when it was found that the intensity and frequency of magnetic disturbance was due to changes taking place many millions of miles away. Two observers one working on spots on the sun and the other studying the movement of the magnetic needle independently found that the phenomena they were studying had a regular cycle and that when the cycles were compared they proved to be the same. It was shown that as the spots on the sun became more numerous the daily swing of the magnetic needle becomes stronger and as they diminish the needle moves more feebly. It was therefore felt necessary to supplement the Magnetic Observatory by one devoted to the direct study of the solar surface by photography.

Observations of the sun had been made during the 1860s on a photoheliograph designed by Warren De La Rue operated from the Kew Observatory. Early in the 1870s the Royal Society took over control of the Kew Observatory from the British Association and in 1873 the 'Kew' photoheliograph was moved to Greenwich. At about the same time five photoheliographs were ordered from Dallmeyer for use on the transit of Venus expedition organised by Greenwich. In order to photograph the sun it is not necessary to have a very large telescope and the one in use at Greenwich from 1875-97 was only four inches in aperture and its focal length was five feet.

In 1873 G.B. Airy appointed E.W. Maunder as Assistant for Photographic and Spectropic Observations. From that time until 1977 these instruments were used to make one or two direct photographs of the sun's surface on every day when it was visible, gaps in the record being remedied as necessary by observatories overseas. This has produced a record of the size, position and movement of every sunspot that has appeared on the visible dish of the sun during more than a century. In 1904 E.W. Maunder published his famous 'butterfly' diagram as a representation of sunspot activity that vividly displays its cycle of eleven years. Besides direct photography, initially by wet-plate process, Maunder carried out spectroscopic sweeps round the sun's limit to detect prominences. Major changes were made by W.H.M. Christie; a conversion was made to dry plates and, in order to keep as full a record as possible, arrangements were made with the Solar Physics Observatory at South Kensington to supplement the Greenwich photographs with those taken in India and Mauritius. An increased amount of time was also spent on the reduction of photographic material.

A number of special investigations based on the accumulated material have been made. Maunder examined the sunspot cycle and solar rotation. He is best remembered for introducing the 'butterfly' diagram as a means of demonstrating the change in the latitudinal distribution of spots within the solar cycle. He also demonstrated that the magnetic storms on Earth tended to recur at twenty-seven day periods corresponding to the rotation of the sun as seen from Earth. Mander also indicated that there were specific regions of the solar surface, not necessarily sunspots, the affected the Earth's magnetic field.

In 1910 the Cape Observatory began to take regular photographs of the sun and the Royal Observatory received copies; even wartime photographs eventually arriving. Solar observations were the only series carried on at Greenwich during the Second World War and continuity was even maintained on the move to Herstmonceux. The photographs were taken every day until 1977 when it became a weekly exercise, unless there was particularly active sunspot, until finally the last plate was taken on 8 May 1979.

It will now be useful to look at the actual process of taking the plates and their use afterwards. A magnifying lens is used so that the photograph is about eighth inches in diameter. The exposure is for a fraction of a second and this is achieved by arranging a very narrow slit in a strip of brass which is made to move in a grove across the principal focus. Before the exposure this is fastened up so as to a prevent all light from entering the camera part of the telescope. When all is ready the strip is released and drawn down very rapidly by a powerful spring; the slit flying across the image of the sun gives exposure to the plate for a fraction of a second.

Two of these photographs were taken every fine day at Greenwich but in winter weeks could pass with no opportunity. It was arranged therefore that photographs taken with precisely similar equipment should be taken in India and Mauritius and sent over to fill the gaps. Towards the end of the nineteenth century the 'Dallymeyer' photoheliograph was superseded by the 'Thompson' a photographic refractor of nine inch aperture and nearly nine foot focal length; the image of the sun obtained after enlargement is seven and a half inches diameter.

The photograph has next to be measured; the four following measurements particularly being determined for each spot. These are: its distance from the centre of the image of the sun; the angle between it and the north point; the size of the spot; and the size of its dark central position or umbra. The size is measured by placing a thin piece of glass on which a number of cross lines have been ruled one-hundredth of an inch apart in contact with the photograph, the lines over counted and the measurements made. Dark spots are not the only ones of interest there being also bright marks usually near the edge of the sun. These are called 'foculae' and, like the spots, have times of abundance and scarcity changing, on the whole, at the same time as the spots.

After the photographs have been measured these results must be reduced and the position of the spots, as expressed in latitude and longitude on the sun, computed. The study of individual spots is interesting but of more interest is that they increase and diminish in a regular cycle and affect different regions of the sun at different points of the cycle.

Spectacular demonstrations of the connections between sun-spots and magnetic activity happen occasionally. In November 1882 a great spot was seen on the sun covering an area of more than three thousand million square miles and was visible to the naked eye. This great disturbance, in the nature of a storm in the solar atmosphere, stretched over one hundred thousand miles on the surface of the sun. Simultaneously with the spot's appearance the magnetic needles at Greenwich began to react excitedly and over the British Isles telegraphic communication was greatly interfered with. Such events are rare but are a dramatic demonstration of this link and provide even greater incentive for this phenomenon to be studied.

RGO 50 is the product of a solar observation programme begun in 1873 at the Royal Observatory, Greenwich and continued, from 1953 to 1979, at Royal Greenwich Observatory, Herstmonceux. It primarily consists of a sequence of glass plate negatives. The sequence represents an intent observe daily for where no plate was produced at Greenwich or Herstmonceux the gap has been filled by sourcing plates from observatories elsewhere around the world particularly the Indian observatories at Kodaikanal and Dehra Dun. Observation times for these plates may be given in Universal (Coordinated) Time (UT) or a location specific time such as Indian Standard Time (IST) or Pacific Standard Time (PST). A sequence of 6798 plates from Royal Observatory, Cape of Good Hope, South Africa (1918-1977) was amalgamated with the sequence at an unknown date. Plates produced at Greenwich from circa 1873-1917 are thought to have been destroyed in the First World War. After 17 January 1977 the plates were produced at roughly weekly intervals.

The class is a continuation of the main part of RGO 51: RO and RGO Solar Plates Contact Prints (1873-1917), which is itself essentially a continuation of RGO 67: Kew Observatory solar photographs (1858-72).

Some additional material in other formats such as 35mm film was obtained from a variety of sources and used to complete or augment the series.

This catalogue has been written to include entries for dates where no plate is know to be extant to allow these gaps to be marked explicitly. Exceptions to this rule are:

- 18-20 September 1920 and 10 April 1947: there are no plates extant and no numbers assigned

- RGO 50/1231-1250, 1464, 3200-3999, 5505, 8921-8930, 9266, 9643, 12929, 14377-14576, 16841-16940, 19391-19399 and 20255: there are gaps in the reference code sequence but there are no plates missing (i.e. date sequence is continuous)

-RGO 11002-11062: the numbers were assigned to dates duplicating plates RGO 10941-11001 and have thus been deleted

Additionally there are entries coded RGO 50/888, 1888, 6434, 6435, 16710 but these do not represent an extant plate.

Produced or accumulated by the Royal Observatory, Greenwich and the Royal Greenwich Observatory.

N.B. THIS SERIES IS NOT STORED AT CAMBRIDGE UNIVERSITY LIBRARY. Requests for access may be directed to the Royal Greenwich Observatory Archivist by sending an email to mss@lib.cam.ac.uk by sending an email to mss@lib.cam.ac.uk. Items RGO 50/747a, 1155a, 50/14971 and 22230 are marked as missing in the original manuscript listing.

Please cite as Royal Greenwich Observatory Archives, RO and RGO Solar Plates, RGO 50

Data from the plates was published in tabulated format on a regular basis as ''Greenwich Photoheliographic Results''. An article drawing on research on the solar images by D.M. Willis, V.N. Davda, and F. Richard Stephenson and entitled ''Comparison between oriental and occidental sunspot observations'' appeared in the Quarterly Journal of the Royal Astronomical Society, 37: p189–229 (Blackwell: Oxford, 1996).

Contact prints of many of the plates in RGO 50 and also of destroyed plates (circa 1873-1917) are to be found at classmark RGO 51. Further records of solar observations can be found in RGO 57, Kew Observatory sunspot observations; RGO 58, Heinrich Samuel Schwabe: sunspot drawings; RGO 64, RGO Solar Department: papers; RGO 65, RGO Herstmonceux Solar Records: Lyot filter negative films; RGO 81, Observations of the minor planet Eros to determine the solar parallax: papers and records; RGO 138, RGO Solar Eclipse Plates and Photographs; RGO 143, Royal Observatory, Cape of Good Hope Solar Records: Lyot filter negative films and records; RGO 144, Stonyhurst College and Zurich Observatories: sunspot drawings; RGO 157, RGO Solar Observation Records (including photoheliograph registers); RGO 169, Royal Observatory and RGO Solar Eclipse expeditions: records; and RGO 202, Ely Observatory Solar Plates Contact Prints: from Canon William Selwyn's observatory.

A handlist is kept in the Manuscripts Reading Room but contains a considerable number of keying error rectified in this electronic version. A tabulated catalogue, combined with other RGO solar image lists, and which may be sorted by various data e.g. former reference.

The online catalogue for Janus was compiled from an existing list in May 2009 by Zoe A. Rees, Department of Manuscripts and University Archives.