I ECLIPSE OF THE SUN.

Otago Witness, Issue 3996, 14 October 1930, Page 23

I ECLIPSE OF THE SUN.

r J. C. B.

By

(Special for the Otago Witness.) The eclipse of the sun on October 22 will, for the first time, be the occasion pf eclipse observations abroad by an official New Zealand expeditionary party. Dr Adams, Government astronomer, visited Wallal in Western Australia in 1922, but as a member of an American party from the Lick Observatory. Although eclipses have caused wonder, and even awe, throughout historical times, not until the application of photography at the moments of totality, first attempted in 1851 in Scandinavia and Germany, were the scientific opportunities afforded by the phenomenon adequately realised. Since then astronomers have made long pilgrimages to pbtain records of the rare spectacle. When the sun is hidden momentarily behind the moon, which by contrast appears intensely black, most informative manifestations of the outer envelope and exudations of the great luminary are made accessible to visual and instrumental observation. These altogether escape detection in the ordinary daylight glare. The gaseous character of the fiery atmosphere, or chromosphere as it is called, was first revealed by the spectroscope in 1868, hydrogen, calcium, and helium elements being recorded. It W’as further inferred from the polarised character of the light emitted by the corona, or pearly white halo, which, during the brief period of totality is beautifully silhouetted behind the black body of the moon, that this emanation shines largely by reflected sunlight. A remarkable circumstance about this corona is that its form varies according to the activity of the sun as evidenced by “ spots.” Strangely enough, the minimum period of activity produces the largest streamers. In 1922, a miniinum period, the writer witnessed a total eclipse in Queensland, when streamers were discernible to a distance of about one and a-half solar diameters. The streamers do not shoot or scintillate as with aurorae, but maintain serene, though attenuated, rigidity. In 1878, in the clear air of the Rocky Mountains at an altitude of 14,000 ft, the corona was observed to no less than six diameter widths from the sun, and could also be seen for a time after the total phase had passed. It is not to be supposed that these streamers are confined to the time of eclipse or to the profile projection. No doubt they perpetually extend towards the earth, and, as they are visible to a range of 5 per cent, of the intervening distance, one may speculate as to whether they actually envelop our world in some form or degree. One of the tasks of the New Zealand expedition is to photograph this corona. The Americans have other tasks, among which is a further test of the Einstein effect. This recalls one of the most remarkable of many fruitful eclipse expeditions, that of the British party which observed the 1919 eclipse in Brazil and first recorded the displacement in apparent position of stars lying nearly in line with the sun. Thus was substantiated the famous relativity theory which denies the basic notion of Galileo and Newton that “ forces ” are

necessary to cause' deviation from recti linear courses.

A total eclipse, partial in Dunedin, occurred in the Pacific on September 16, 1876, and this present phenomenon is technically termed the third Saros return of it. The reference is to an eclipse cycle known to the ancients of approximately 18 years 11 days. In three such periods, or about 54 years 34 days, similar eclipses recur near the same region of the earth. A total eclipse was seen in New Zealand in 1885, Wellington being in the zone of totality. Dr Hector and a party of amateurs, including Mr Arthur Beverly, of Dunedin, took up an observing station near Masterton, but were met with stormy weather, although the sky cleared sufficiently to enable them to view the total phase. Their observations were, however, marred. Photographs were also taken in Wellington, but unsettled weather conditions were fairly general. It was noted that at Blenheim, just outside the edge of the totality belt, the corona was nevertheless visible, which is contrary to the statement usually made in astronomical books that only when the sun is entirely obscured can the corona be seen. At Ashburton a crescent sun remained, but some stars were visible. In Dunedin, although only partial, the eclipse was probably more complete than any other witnessed since the settlement.

Total eclipses, although not infrequent on the earth as a whole, are rare in any one place, because the belt under the complete shadow is narrow; in the present case of the order of 15 to 20 miles. The occurrence may be conceived as if a black brush were to descend from the sky and at an enormous velocity of the order of 2000 miles per hour trace a narrow line along about one-third of the circumference of the earth. The velocity is mitigated to some extent by the circumstance that the earth itself is rotating in the same direction, although making a poor race of it in comparison with the sweeping shadow. The darkness occasioned is of about the intensity of deep twilight, but the sudden circumstances create a rather weird effect. The partial shadow is many thousands of miles in width, and in this case includes the whole of New Zealand.

Observers should use a piece of smoked glass or a fogged photographic plate to avoid injury to the eyes. A good method is to project the image of the sun by a small telescope, when such is available, a card being used to focus upon. Those who have access to accurate time might usefully record the exact moments of first and last contact. With the aid of the published elements, the writer has computed the times for Dunedin, and made an approximate estimate for some other provincial places, as follows:—

PARTIAL ECLIPSE OF THE SUN OCTOBER 22, 1930. Begins. Max. phase. Ends.

These are all a.m. or morning hours by New Zealand summer time. At greatest phase the overlap will attain nearly one-third of solar diameter. „ The diagram below shows the relative positions of the sun and moon at the commencement, middle, and end of eclipse. The vertex is the point on the rim of the sun most distant from the horizon:—

h. m. b. h. m. 11. m. s. Dunedin 8 44 -40 9 37 10 29 15 Oamaru 8 43 9 36 10 29 Pembroke 8 41 9 35 10 27 Invercargill 8 46 9 36 10 24