Calculation of the shadow path of a chosen lunar feature during an occultation.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Calculation of the shadow path of a chosen lunar feature during an occultation. Moser, Nora B. Abstract had two serious limitations: the inaccuracy of visual observation and the influence of limb irregularities. As a development of A. E. Whitford's photoelectric measurements of stellar diameters, mobile equipment has been designed to observe an occultation photoelectrically and record it alongside radio time signals to thousandths of a second. To cancel out the effect of limb irregularities, a prominent lunar feature which will dominate the limb for several hours is selected and two or more observation posts are so placed that at each the star will disappear behind the chosen feature. A suitable occultation having been chosen, Hayn's selenographic coordinates of the edge of the moon as presented to the earth at a given time are computed by Watts' method.i The results are plotted on Hayn's contour charts of the limb region, and a prominent mountain within about 20 of the edge line is picked. The selenographic coordinates of the top of this mountain, together with more or less arbitrary times, will be the starting data for computing the shadow path along which the mountain will occult the star. To obtain maximum precision, the moon s positions, tabulated with an extra decimal place, are requested from the British Nautical Almanac Office, and the earth's selenographic coordinates (librations) are computed from basic formulas rather than interpolated from published tables. The path of the shadow axis on Bessel's fundamental plane is calculated by standard formulas. Some significant figures are lost in subtraction in this process but the relative error between points on the shadow path will be negligible. The geocentric earth's selenographic coordinates and position angle of the moon s axis are referred to the shadow axis by Watts' method for selenographic coordinates, avoiding his approximations. The selenographic coordinates of the shadow axis and of the chosen mountain are transformed to one set of rectangular coordinates fixed in the moon and then projected onto the fundamental plane, resulting in the shadow path of the mountain. Bessel's device of a parametric declination is employed in transferring the path to the earth's surface. The observation positions are tied into the U. S. first-order triangulation and may be assumed correct. The error in the moon's place will be sensibly constant during one occultation. The limb error is under control. An observation equation may be set up to solve for the correction to the lunar parallax. Acknowledgments. Walter D. Lambert of the U. S. Coast and Geodetic Survey and C. B. Watts of the U. S. Naval Observatory have furnished valuable assistance in this project. Dr. John A. O'Keefe and Mrs. Rose L. Rowen of the Army Map Service had a large part in the work described. I.A. J. 48, 170, 1940. U.S. A rmy Map Service, Washington, D. C. Publication: The Astronomical Journal Pub Date: April 1950 DOI: 10.1086/106348 Bibcode: 1950AJ.....55...75M full text sources ADS |