It is still relatively mild and mellow after dark this time of year, a time that Yeats remembered as . . . when white moths were on the wing,
And moth-like stars were flickering out.
But man-made stars can flicker, too. It is an entrancing sight to gaze down from a nearby hill on a town or city on an autumn evening, and see a million lights twinkling like so many diamonds in the gathering dusk. From such a distant viewpoint, the elements of even the most vulgar street-sign acquire a magical quality that quite belies their Philistine intention. But why do the lights of the city dance and flicker so?
The scintillation is closely related to the shimmering effect often seen when looking at the air over a hot surface on a very warm day. The same phenomenon can be seen indoors if you watch the warm air rising over a hot radiator. In both cases the shimmering occurs, not specifically because the air is hot, but because its temperature varies over short distances from one spot to another.
A ray of light passing through air which is at a uniform temperature follows a perfectly straight path. But if it passes from cold air to slightly warmer air, or vice versa, it changes direction ever so slightly at the boundary, a phenomenon called refraction.
Rays of light that have to travel a long distance pass through an atmosphere that is in constant motion, and through layers of air whose temperature and density are changing incessantly from place to place, and from second to second.
These changes cause the rays from a distant street-lamp to follow an erratic zig-zag path to reach our eyes. Indeed, to catch the beam from a given source, we have to "look" each instant in a slightly different direction: to put it another way, the light appears to the eye to dance around in the air, rapidly changing its apparent position, albeit by a very small amount. The perceived optical effect is that of the scintillating street-lamp.
The phenomenon of twinkling is only effective when the light comes from what approximates to a "point source", a little "dot" of light. A distant street-lamp, or indeed the stars in the sky, meet this criterion. If, however, we move nearer to a street-lamp, it will twinkle less and less as we approach; eventually the scintillation ceases altogether when the angular diameter of the lamp - its size as perceived by the eye - has grown to the extent that it no longer approximates to a "dot" of light.