Our Astronomical Column

Abstract

MEASUREMENT OF PLANETARY DIAMETERS.—In a paper giving particulars of measurements of the polar diameter of Mars (Astronomical Journal, No. 354), Prof. Campbell gives an interesting summary of the conditions of planetary measures in general. He points out that measurements of diameter are affected by a variety of errors, among them being spherical and chromatic aberration, imperfect atmospheric conditions, irradiation, diffraction, and imperfect focus, all of which tend to increase the apparent diameter of the object; while, in addition, personal equation and accidental errors may also affect the results. The effects of spherical and chromatic aberration, as well as of diffraction, may be regarded as constant throughout a series of measures of any given object. Differential refraction can be satisfactorily corrected for, but the irregular refraction caused by the unsteadiness of the atmosphere, and resulting in “poor seeing,” may produce very large errors indeed. The apparent increase of diameter due to irradiation may be regarded as sensibly constant with any given telescope, eyepiece, planet, and background. Imperfect focus may produce considerable and variable errors; in the 36-inch Lick telescope, an error of a thousandth of an inch in focussing increases the diameter of a planet by 0″˙02. Experiments as to the best method of procedure were made by Prof. Campbell in June and July, 1894, with the result that the following programme was adopted in the case of Mars: (a) All the observations were made with the sun above the horizon, and the advantages of a bright sky background were very marked; it was believed to reduce all the errors, except possibly that of personal equation, (b)Observations were only made in a tranquil atmosphere, (c) The same eyepiece was used throughout. (d) An eyepiece cap with a very small aperture was employed, (e) The observer's eyes were always similarly situated with respect to the threads of the micrometer. (f) The micrometer threads were always placed parallel to the great circle passing through Mars and the sun. (g) The micrometer threads were placed directly upon the opposite limbs of the planet.