THE TIME OF SECOND CONTACT AT THE TRANSIT OF MERCURY ON NOVEMBER 11, 1940

Abstract

Photographic observations of the transit of Mercury on November 11, 1940, have been used to determine the time of second contact. As announced in a preliminary note,1 the ingress of Mercury was photographed by Edison R. Hoge at the 60-foot tower telescope on a fine-grained panatomic film through a red filter with a motion-picture camera running at a speed of 2.5 frames per second. The solar image was 173 mm in diameter, the aperture of the lens 4 inches. Time was recorded on the film by the shadow of a marker fastened to a relay operated each second by a Riefler clock, corrections to which were obtained by comparison with the time signals from the United States Naval Observatory. The whole solar disk was photographed by J. O. Hickox at intervals of one hour during the transit. The time of each exposure was registered on a chronograph with the time signals from a Riefler clock and the signals from the Naval Observatory at 24 hours G.C.T. On each solar image were superimposed the shadows of a set of cross wires, the orientation of which was determined from the direction of drift of the solar image. The scale of the plate was obtained from the distance the sun drifted between two exposures, the times of which were recorded on the chronograph. Final values for the scale and orientation of the plates were obtained from the motion of Mercury. Of the 16 plates exposed, 11 were suitable for measurement. The resulting correction to the relative position of Mercury referred to the sun was -078 d= 074 in right ascension and -f-076 ± 073 in declination. This correction applies to the difference between the positions of Mercury and the sun as tabulated in the American Ephemeris and Nautical Almanac, without the correction of -'-Y!S to the sun's longitude. At the 150-foot tower a series of 16 photographs with an orange filter was started by Richardson just before second con-