Royal Observatory & history of astronomy
Eclipses and Greenwich
The first recorded astronomical observation by John Flamsteed, the first Astronomer Royal, was a solar eclipse from his home in Derby 12 September 1662, when he was just 16 years old. Solar astrophysics started at the Royal Observatory with Airy's observation of the solar eclipse of 1842.
The two most famous sets of solar eclipse photographs in the archive are de la Rue's pictures taken at the solar eclipse of 1860 and the Sobral eclipse in 1919.
From 1842 until 1954 there were many eclipse expeditions from the Royal Observatory. The observatory archives contain photographic plates from the eclipses of:
1868
1900 Portugal
1905 Tunisia
1919 Brazil
1926 Sumatra
1929 Malaya
Solar eclipse, Turin 1842
Airy writes to Whewell at Cambridge from Turin in 1842:
The sun arose badly and the sky was very cloudy, but we saw the beginning and progress of the eclipse clearly, and saw the totality well. But it is difficult to get an idea of it. The gloominess increased, the country seemed annihilated, the clouds looked horribly black and threatening, a little sunlight on the ground looked (as I fancied) dusky red, till at the actual totalization the whole of these horrors were suddenly and greatly increased (the light excepted, which vanished) and the aspect of things was the most fearful that I ever saw. I could with difficulty read the face of my watch, though I held it almost close to my own face.
The moon was seen like a black patch in the sky surrounded by a ring of light (very slightly red I think) whose breadth was about 1/8 of her own diameter. I think that if the sun and moon had actually vanished they would not have appeared so alarming(the suddenness of darkness excepted), but this appearance increased the horror in the same degree in which the discovery of a man's mangled carcase increases the first horror of merely missing the living man. As touching the ring of light, about which so much has been said, I have no hesitation in believing it to depend neither on the sun's atmosphere nor on the moon's but simply to arise from our own atmosphere. It was exactly like the aurora which you may see at night when a street lamp is just hidden by the corner of a house. On examining with a telescope, there were some singular little red flames (stationary) from the moon's edge, which as I have no doubt depend on the inequalities of a great cloud that touched the moon there. When the sun's limb appeared, all seemed light again.
Airy's view about the significance and origin of the light he saw around the eclipsed sun quite changed after he witnessed the 1851 eclipse in Sweden. Afterwards he wrote to his wife in these words:
Some important points are made out from this. First, the red flames certainly belong to the sun. Second, they certainly are in some instances detached. Third, they are sometimes quite crooked. Fourth, they seem to be connected with spots.
Solar eclipse, Spain 1860
De la Rue built a special telescope for observing the sun and recording sunspots. He took this instrument to Spain and for the first time was able to photograph solar prominences, which can only be seen during a solar eclipse. He was delighted with his observations because he had expected the prominences to be too faint to be detected on his low sensitivity plates. In fact he was even able to see features on his plates invisible to the eye which it was rightly deduced must come from short wavelength light to which the eye is not sensitive.
These observations, as well as confirming that the prominences were part of the sun and not an atmospheric effect, stimulated research into their nature and origin.
Solar eclipse, Sobral 1919
Albert Einstein's theory of General Relativity published in 1915, pointed out that not only do we live in space-time not space and time but that matter bends space-time. Sir Arthur Eddington at Cambridge quickly realised the importance of the theory and that it could be tested during a solar eclipse.
In 1917 the Astronomer Royal Sir Frank Dyson pointed out that the 1919 solar eclipse provided the ideal opportunity to test the theory as the eclipsed sun would be close to the Hyades star cluster amongst a group of relatively bright stars. During the eclipse rays of light from stars, passing close to the sun would be bent so that the star images seen from the earth would seem to move away from the sun. The effect for a ray of light grazing the sun is 1.75 seconds of arc which is twice as large as the shift predicted by Newton's theory.
This is the same phenomenon as gravitational lensing, where a cluster of galaxies can produce multiple magnified images of a galaxy twice as far away, which has been seen using the Hubble Space Telescope.
Two expeditions were organised, one to Sobral in Brazil and the other including Sir Arthur Eddington, to the island of Principe off the west coast of Africa. At Principe there was a thunderstorm in the morning which cleared in time to take photographs, only two of which showed star images. At Sobral the clouds also cleared in time to make observations and 7 usable plates were obtained.
The shifts were typically about one fifth the diameter of the star images and difficult to measure accurately. However, in November 1919 Sir Frank Dyson announced to a joint meeting of the Royal Society and the Royal Astronomical Society, that the results confirmed Einstein's theory.
The following is a recreation of a letter from Albert Einstein to his mother, upon hearing that observations of the 1919 eclipse had confirmed his theory. Click on the image to enlarge or read the letter below.
Dear Mother,
I have good news for you today. H. A. Lorentz has cabled to say that the English expedition has really proved that light is deviated by the Sun. I am sorry to hear from Maja that not only are you in a lot of pain, but also thinking gloomy thoughts.
How much I would love to keep your company again so that you will not abandon yourself to that horrible melancholy. However, I have to stay on here for a while and work.
I will also go to Holland for a couple of days to prove my gratitude to Ehrenfest, although this means a loss of time to me that is quite painful.
I send you all my love,
Warmest regards
Your Albert
Not everyone was convinced and in 1922 a party was sent from the Royal Observatory to Christmas Island but was clouded out. However observations made of the same eclipse by the Lick Observatory confirmed Einstein's result.
Solar eclipse, Giggleswick 1927
The following extract is from the Report of the Astronomer Royal, Sir Frank Dyson, to the Board of Visitors, 2 June 1928.
An expedition to observe the total eclipse of the Sun on June 29 was kindly accomodated on the school grounds at Giggleswick. The principle part of the programme was to photograph the Corona on a large scale with a 6-inch lens of 45-feet focus; to photograph the spectrum in the infrared with a Littrow spectrograph; and to compare the intensity of the Ca doublet in the infrared with the H and K lines by means of a small dispersion Littrow spectrograph. The weather previous to the eclipse was extremely bad, but the eclipse itself was observed through a break in the clouds. Small-scale photographs were also secured from an aeroplane kindly placed at the disposal of the Observatory by the Daily Mail. By permission of the Hydrographer of the Navy two observers from Greenwich were accommodated on board the Survey Ship H.M.S. Fitzroy which was engaged near the belt of totality in the North Sea. Owing to clouds no results were obtained. A full report is given in the Monthly Notices of the Royal Astronomical Society, Vol. 87, p. 657.
