The Eclipse and Ern' stein
ONEof the most tragic incidents in the history of science is associated with such an eclipse of the sun as astronomers have congregated in the Far East to watch next Thursday (May 9th). Chinese annalists relate that-the IMperial astronomers Hi and Ho drank so heavily . that they omitted duly to predict the eclipse of 2128 rt.c. Thus the moon's shadow took the land- by surprise : the Emperor was perplexed with fear of change ; Hi and Ho were condemned to a- lingering death. - Our own Astro- nomer Royal is, in no such danger; for he alwaysannouneeS eclipses three years ahead-in the, Nautical Almanac. Nor do we any longer believe that an eclipse is caused by a dragon swallowing the sun, and hold it necessary to pro- pitiate him by sacrifice or frighten him by Chinese music. Yet- even -to-day the people of Tibet think that an eclipse is due to the -shadow -of some celestial- visitant, who can be driven away only by the howling of dogs, pigs, and other : domestic animals; Their tails accordingly get twisted at the -critical -moment, -if they do -not howl of their own accord, as Halley says they did during the last eclip-se which was total m London two centuries ago. " Ralph to Cypthia howls "—but the moon goes on her serene course between the sun and the earth.
It is indeed a pity for astronomers- that -this happens so : seldom, for it -is only during -the fcw minutes of total - eclipse that some of -the -most- important- features of the ' solar Constitution can be studied. An undevout astro- nomer May be mad enough to think that, if he had been present at the Creation, he could have emulated King Affanso in suggesting improvements: If the moon only went round the earth in the plane of the ecliptic, we • should- have a solar eclipse once a month; And if the size - and distance of-the-moon had been a little more carefully adjusted, the total phase might- be visible over the whole daylight -hemisphere ; whereas it is now limited -to so narrow a belt that astronomers have to transport all their apparatus to the isles of Javan and Gadire. Professor Turner reminds us that the need for exact measurements " has converted the weight-of. the packages of instruments into tons instead of -thehundredweights of older days:" But the steamship companies haye been yery.
- Our own parties from Canibridge and Greenwich are - Safely 'established -in the Malay. -Peninsula. All along the track of next-Thursday's shadow; from Sumatra- to the Philippines, Dutch and French, German and American telescopes and cameras have been carefully installed and tested; The only thing left to chance is the weather. As too many of those who went to see our own eclipse in 1927 are aware, months of preparation may be nullified by a few gallons of water in the shape Of cloud. We can only hope that our observers at- Alor Star and Pattani will be as-lucky as the Astronomer Royal two - years ago at Giggleswick.
Many astronomers will be occupied in the study of the corona, that beautiful and still mysterious solar appendage which is probably typified in the winged sphere so common on Egyptian monuments. The red prominences, those awe-inspiring tornadoes of incandescent hydrogen that blaze up hundreds of thousands of miles from the sun, can now be studied any day with the aid of the spectra. scope. But hitherto no method has been found for rendering the corona visible except- during the brief eclipse periods, amounting to only two or three hours in a century. Such fuller knowledge of its constitution as we are slowly gaining may throw valuable light on prob- lems of terrestrial weather. But to the average man Thursday's eclipse is mainly interesting for the assistance it may give- him in deciding whether or not to accept Einstein's new theory of the universe.- It only comes in contact with -the facts - of- observation- at three points comprehensible by those to whom Tensors and Hamil- tonians are worse than- Greek was to Casea. The simplest case is that of the bending of light -in a gravitational field. It was long ago suggested:that rays of light might be deflected from their straight course in, passing close to so massive a body as the sun, In Scott's description of the almost extinct sport of salmon-spearing, he points out that the fish is not where it looks to be : allowance has to be made for the bending of light as it passes from air to water. In somewhat the same- way, a star seen during an eclipse just clear of the darkened sun, so that . the ray of -light from it grazes the solar surface; will seem to be slightly- displaced from its true position.
During next week's eclipse many observers -hope to secure photographs of the stars visible in the neighbour. hood of -the sun. By comparing these with photographs of the same stars taken some months before or after, - when they are shining in the night sky, the exact amount of their displacement can -be measured, small as it is. As long ago as 1801 it was calculated that, on the Newtonian theory of gravitation, this- displacement should amount to about seven.eightlis of -a second of are. - This is approximately equal to the apparent- breadth of a shilling placed on the top of St. Paul's and observed from PrimrOse Hill. But, according to the new theory of gravitation •put forward by Einstein in 1915, the shift - should be just double this or 1.74 seconds. : The first - attempt to apply this crucial test was made by two British expeditions, as soon as the War ceased its annoying interference with- purely scientific- work. In November, 1919, the Astronomer Royal announced that plates: taken - at a station in Brazil showed the actual displacement - to be nearly -two_seeonds. Whilst errors, of -manipulation might account for the small excess over Einstein's cal- culation, it was clear that he was much nearer the truth than Newton. As Lord Haldane said : " Something . like a revolution- in a great- department of -scientific thought was the result." The Lick expedition to Aus- tralia in 1922 obtained a still closer approximation to Einstein's figures. -If the Greenwich thirteen-inch a.stro- graphic telescope tells the same story next week, even , those who would -not presume to understand the .famons Theory of Relativity will• admit-that it has triumphantly passed the most -cogent test which we can at present apply to it. - - • - - -
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