ECLIPSE STUDY

New Zealand Herald, Volume LXXIV, Issue 22777, 10 July 1937, Page 21

ECLIPSE STUDY

eecent expedition

HIGH degree of success preliminary indications • METHODS .OF OBSERVATION BY W. r. MERRY i study of tho preliminary report on the observations undertaken by the js'cw Zealand expedition that made Canton Island, in the Pacific, its base, "and observed the total eclipse of tho BUn in June, shows how wonderfully successful the observations proved _to be The conditions wero ideal, the instruments used functioned without a hitch, and the full programme of observations was carried out by the party. The primary object of eclipse expeditions is to obtain records of tho corona either by direct photography or by analvsing the light spectroscopically. The New Zealand party concentrated on tho former method, while the American expedition, that also made Canton its base, employed both methods.

Cumulative Knowledge The corona is a faint outer extension of tho sun's atmosphere, which is too faint to be seen except when the solar disc is hidden during the courso of an eclipse. It shows then a. pearly white light blazing forth in all its clorv from the eclipsed sun, streamers of light extending for over 3,000,000 miles with an intense brilliancy immediately surrounding tho darkened disc. Although no two eclipses appear exactly the same, nevertheless they fall into" definite types according to the state of sunspot activity at the time. Tho eclipse in June was of the sunspot maximum type. The exact nature of the corona has never been explained and the stiidy has been the subject of eclipse expeditions for nearly a century. Each expedition adds to the knowledge and leaves behind a permanent photographic record, so that eventually from this wealth of detail a complete understanding may be reached. Huge Camera used

The principal instrument used by the New Zealand expedition was a coronagraph, specially designed for eclipse observations, and loaned to the party by the ltoyal Astronomical Society. This consists of a horizontal camera with a focal length of 19ft. and a lons of 4in. aperture. Used in conjunction with this is a reflecting mirror, which, driven bv. clockwork, follows the path of the sun, reflecting the imago through the lens of the coronagrapli. This method is adopted owing to the great difficulty of erecting such an instrument so "as to point directly and follow the course of tho sun. This camera was operated by Mr. I. L. Thomson, of the Dominion Observatory. An examination of tho developed plates shows how wonderfully successful wero the observations. Such a wealth of fine detail is shown that the plates can definitely be considered as adding to existing data. Strange Shadow Bands

A direct camera of 78-inch focal length with a lens of s}in. and mounted equatorially also was used. This instrument was' constructed by Mr. C. B. Michie, the leader of the expedition, and embodied an unusual idea in solar eclipse photography. A rotating sector was devised with the view of permitting the faint outer corona to take full advantage of the long exposures while reducing the amount of light from the ' bright inner portion. Strangely enough, erected alongside this instrument was an _ American camera using similar principles. Both of these instruments were purely of an experimental nature and the results obtained showed clearly that the idea was sound and worthy of further development. White sheets spread on the ground were, used to observe the shadow bands which ripple across the ground just before and just after totality. The nature and cause of this phenomenon has never been satisfactorily explained but, with the increasing knowledge now available, science will perhaps in the near future he able to solve this riddle. Another Special Instrument

For the purpose of a closer study of the effects of the eclipse of the sun on the earth's surface and atmosphere continuous meteorological observations were carried out and the data obtained should prove of scientific value. Various visual observations _ were made by members of the party prior to, during totality, and after the sun had appeared again. These observations were

undertaken with a view to comparing the appearance of the corona and the prominences on the darkened sun's surface with the results obtained photographically. A special instrument had been constructed for the visual measurement of the streamers of the corona. This was on the lines of a telescope, but without a lens. At one end was an aperture .for the observer, while at the other end was a graduated metal strip so that close measurements could be made. Satisfactory results were obtained.

Accurate Timing __ . The exact timing of tho phases of the eclipse was recorded visually by means of a telescope and spectroscopically by the rise of prismatic binoculars. By these means the time was placed jis accurately as was humanly possible. f J o ascertain the extent of darkness on the earth's surface through tho cutting off of the sun's light one observer made notes on the stars that could be seen in the sky during totality. The amount ' of darkness on the earth's surface was also recorded photographically by means of an earth illumination camera. It is only -now, from the initial report, that the extent and success of the observations can bo indicated. The full value of the contribution to science by these observations cannot be known until a comprehensive study and comparisons have been made with the photograph:* and reports on provious eclipses.