At this time of year it is mild and mellow after dark - the time of year that Yeats remembered . . . when white moths were on the wing,
And moth-like stars were flickering out.
Let us imagine ourselves on such a night, like Wandering Aengus, gazing at the flickering fairy lights above and finding reasons for their wondrous light fantastic. The Ancients thought of the stars as tiny luminous objects set permanently into the vault of a solid over-hanging sky. Indeed, until about 1700 this remained a perfectly respectable opinion, and it is only since then that stars have come to be universally accepted for what they are - huge concentrations of hot gases, exuding immense light, heat and other forms of radiation. But they are, in fact, a relatively steady source of light; their apparent twinkling is caused by what happens to the light as it passes through our atmosphere.
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. It occurs, not specifically because the air is hot, but because it is unevenly heated. A ray of light passing through air which is at a uniform temperature follows a perfectly straight path, but if it passes from warm air to cooler air, or vice versa, it changes direction ever so slightly at the boundary - a phenomenon known as refraction.
Rays of light reaching our eyes from space pass through an atmosphere which is in constant motion and through layers of air whose temperature and density are changing incessantly from second to second.
These changes cause the rays of light from a star to follow an erratic and inconstant zig-zag path to the eye. In fact, to catch the light from a given star, we have to look each instant in a slightly different direction. To put it another way, the star appears to "dance" in the sky, rapidly changing its apparent position - albeit by a very small amount. The optical effect is that of "twinkling".
There are two other processes which enhance the twinkling effect. A blob of air, warmer or cooler than its surroundings, may sometimes act temporarily as a lens, enlarging or diminishing the image of a distant star.
Similarly, a volume of air may sometimes act as an optical prism, splitting the white light from a star into its constituent colours; at one instant we may see the blue part of the spectrum, and at another the red - which accounts for the flickering multi-coloured appearance of some stars.