Scintillation

Scintillation, or twinkling of the stars, is a familiar phenomenon to all who have directed their attention to the firmament above us. Under ordinary atmospheric conditions this flickering is possessed only by the so-called fixed stars (see STARS). A planet shines steadily and by this mark can readily be picked out. When near the horizon, however, planets have been observed to scintillate slightly; while stars at low altitudes invariably twinkle more vigorously than stars overhead. This at once points to the atmosphere as an important factor, since the phenomenon is more pronounced when the light has to traverse a greater depth of air. Again, when viewed through sufficiently large telescopes stars cease twinkling altogether. The action of the telescope is to concentrate upon the eye a much larger pencil of rays than could naturally enter it. Instead of one slender ray the eye receives the integral effect of a great number of rays, whose individual features are lost in the general average. In the case of a planet, again, the rays which fall upon the retina converge from all parts of a disc of sensible size; and in the integral effect of this pencil the individual features of the component rays are lost. But a star is so far distant as to be virtually a point of light. In this case we have an excessively slender ray infinitely narrow compared even to the small pencil of light that comes to us from a planet. The vicissitudes of refraction which a star-ray experiences in passing through the infinitely irregular variations of density, temperature, and humidity in our atmosphere characterise its integral effect on our retina, and the result is twinkling. The exquisite chromatic effects that accompany the twinkling of a bright star like Sirius are fully accounted for in terms of this general explanation. It is possible indeed by separating the images of a star produced in the two eyes to observe two different scintillations at one and the same time. Scintillation may thus be said to depend on three factors: (1) The vast distance even of the nearest stars reducing the largest of them to mere points of light; (2) the ever-changing variableness in condition of the atmosphere through which the light must come to us; (3) the smallness of aperture of our eye, which receives an almost ideal single ray of light.