THE ECLIPSE

Otago Witness, Issue 3581, 31 October 1922, Page 30

THE ECLIPSE

A STRENUOUS DAY. WALL.AL OBSERVATION. IDEAL PHOTOGRAPHIC CONDITIONS. No. 4.

Bt

Dr C. E. Adams,

New Zealand

Government Astronomer.

(See Illustrated Pages.) WALLAL, October 1. The weather experienced during the three weeks preceding the eclipse gave every promise of excellent conditions on the day of the eclipse. The eclipse was total about. 1.30 p.m., and at this time every day observations were taken of the weather; the temperature varied from 79deg to 94deg, the humidity from 25 to 74 per cent., and the wind velocity from 6 to 11.4 miles per hour, and the sky was blue and free from all clouds at this time of the day. On the eclipse day, September 21, the. weather was perfect, and during the morning final preparations were made. Most of the astronomers were up the whole of the night preceding the eclipse, as all the plate-holders had to be filled with plates and carried to and stored at the different cameras before daylight. Soon after noon the first contact was observed, and as the moon moved across the sun the temperature gradually dropped about 9deg F., from 86deg F. to 77deg F. The light diminished rapidly and gave every indication of a dark eclipse. Hurricane lampsvwere lit and placed in position at the telescope's, so that the plate-holders could be seen and handled easily. About six minutes before the beginning of the totality, all observers were at their stations. In the 40ft telescope the image of the partially eclipsed sun was watched on the plate-holder, and the thin crescent of light disappeared at first gradually and finally seemed to vanish suddenly. At this instant the observer recored the time on the chronograph and called out “Go !’’ At this signal the time-counter called outeach second, and all the work at the telescopes began. The pnotograpliing of the corona began at once in the 40ft telescope, with a series of exposures beginning with one-quarter of a second, then exposures of 1,2, 4,8, 16, 32, and 64 seconds. During the longest exposure, which took place at the centre of totality, the observers had time to leave the dark tent and to have a brief look at the eclipse. The sight was veryimpressive, and a good view of the corona was obtained. The sky was very dark, and the blight planets and many stars were easily seen. The recorded drop in temperature was not noticed owing to the strenuous labour of handling the large -plates, some of these being 14in by 17in On returning to the 40ft tent, the series of exposures was continued in reverse order to the shortest ones. In addition to this programme of photographing the corona, an extra programme was carried out of photographing the crescents of the sun for 90 seconds before and after totality. Sufficient plates have, been developed to show that excellent, photographs have been obtained. These will be submitted to detail study, and full reports and illustrations will he published by the Lick Observatoryon the return of the expedition to California. AN ANXIOUS MOMENT. At tiie Einstein cameras, Dr Campbell and Dr Trumpler had an anxious time just before totality, for it was by no means certain that the bright star Beta Virginis, near the sun. would be sufficiently bright during the eclipse to be clearly seen. Exposures of two minutes were made by the cameras, and dui-ing this time it is veryimportant to have some star visible to check the movement of the telescope and to keep it pointing to the same place in the sky. Fortunately just 30 seconds before totality, the observers were able to see this star, and to u«e it for guiding. The sky was so clear and the conditions so good that the star appeared perfectly clear and steady, as clear and steady as on a first-class night. Consequently, these photographs ivere secured under ideal conditions. Another factor which contributed to the success was the very' efficient canvas screen, which prevented any excessive heating of the air round the telescopes. Some of these 17in by 17in jilates have been developed, and are to he partly measured up at Broome. The star images are clear and good, so that accurate measurements can easily be made. The process of measuring the Einstein plates is a differential one —one of the plates taken in Tahiti is clamped to an eclipse plate, and any slight difference in the positions of the star images, due to the action o-f the sun, are at once apparent, and can be readily measured. It is expected that the results of the preliminary measurements made at Broome will lie announced there The final measurements and the

discussion of the results will take place at the Liok Observatory. EIG HTY-THREE STAILIMAGES. A preliminary examination of one of the plates shows no ies3 than 83 star images photographed during the eclipse, and all of these imag.'s are so good that aoourate measurements can lie made. The stars near the sun give good strong images. The preliminary measurement is a very exacting and tiring operation, and it takes about eight hours of steady work to make the measurements in one direction. Before the plates are moved, two other observers remeasure it. The plates are then moved round into a position at right angles to the first one, and the second series of measurements is then made, and these are also checked by two other, observers. It will be seen that every precaution is taken to ensure accuracy, and by changing the observers, any personal bias is reduced. Another check has also been made by taking additional photographs on the same plates of another region of the sky. In this wav any variations, which, if they occur, will be almost microscopically minute, are detected, and their effects allowed for. Dr Moore obtained a successful set of speotograms of the corona, with his prism and grating spectographs. Owing to the lack of suitable facilities at Broome, the results cannot be discussed until after" the return of the expedition to California. EXPECTATIONS EXCEEDED. Work was continued at full pressure after the eclipse in developing plates, and in dismantling and packing up the apparatus Higher w'inds than had been experienced sprang up on the days following the eclipse, and caused a delay on the beach as the rough sea prevented all loading operations. In a few days a light breeze came off the land, and the schooner oame in on the high tide, and was successfully beached. The camp equipment w r as then carted out by the donkey teams and loaded safely when the schooner was high and dry. At tho next tide the schooner was floated off, and was towed to Broome by 11.M.A.5. Geranium. The work of developing and measuring the plates is being continued at Broome, and is carried on night and day in the endeavour to get the preliminary results ready before s.s. Bambra arrives on Octol>er 6. to take the expedition to Perth. Generally, it may be said that the results of the expedition have exceeded expectation, and that the choice of Walla! as the site has been fully justified. The expedition w-as very fortunate in having calm weather to land on the beach. No accidents occurred, and no essential piece of apparatus was lost.