The Scintillation of Stars

Abstract

How the heart of the astronomer would rejoice if the scintillation of stars were due, as Prof. Hartridge1 considers possible, to physiological causes rather than to non-homogeneity of the earth's atmosphere! It is the practice of many observing astronomers, before ‘opening up’, to look at the sky and note, on some estimated scale, (i) the degree of clearness or transparency of the sky, and (ii) the degree of steadiness or goodness of seeing. (i) may be judged (in the absence of strong moonlight) by the limiting magnitude of stars just visible to the eye at various altitudes, and is important, in the case of photography, for judging exposure times; (ii) may be judged by the scintillation-frequency of the brightest stars; the greater the frequency the worse the ‘seeing’ will be. On the very best nights the stars appear nearly steady, the occasional winking being slow and infrequent. Such nights are suitable for measuring close binaries, the Airy disks and their diffraction ring systems appearing steady, often for many seconds together; on such nights, also, the finest planetary details are to be seen. When scintillation is conspicuous and rapid, the images of stars, viewed through a telescope, appear larger than the Airy disks and show a rapidly changing and uneven distribution of illumination. If the telescope is put out of focus, the star will appear as an illuminated disk showing, on a steady night, a number of fine concentric rings as in a ‘zone plate’. On an unsteady night, rings are no longer visible, the whole disk appearing to ‘boil’. It has been noted by British astronomers that both the scintillation and the telescopic images of stars are, in general, worse when the wind is in the east. Planets are only observed to scintillate when viewed at a low altitude on a night of exceptionally bad seeing.