The ancients thought of the stars as tiny, luminous objects set permanently into the vault of a solid, overhanging sky. Indeed until about 1700 this remained a perfectly respectable opinion, and it is only since then that stars have come to be accepted for what they are, huge concentrations of hot gases, exuding immense quantities of light, heat and other forms of radiation.
Then around 1850 some bright spark drew attention to the fact that if you look intently at a star it sometimes seems to change position every now and then by a quite significant amount. The phenomenon was said to be most readily observable at twilight and also when the star in question appeared low in the sky.
It was seen to move with little jerks parallel to the horizon, then to come to a standstill for five or six seconds, before apparently returning to its original position.
Many observers tried to explain it, like the well-known twinkling of the stars, as a consequence of sudden variations in the temperature, and therefore of the optical properties, of successive layers of the atmosphere.
But as any astronomer could have told them, it was no such thing. These apparent shifts in position are of the order of half a degree of angular movement, viewed with the naked eye. Seen through a moderately powerful telescope, the shift would be magnified a hundred-fold, and the star would seem to shoot across the field of vision like a meteor.
But even when atmospheric unrest is at its worst, displacement due to scintillation seen through a telescope remains below the limit of perceptibility: stars just sit and twinkle in their designated spots.
The shifting-star phenomenon is an illusion. It has its origins in the fact that when viewing a single star there is frequently no fixed object nearby relative to which the star's position in the sky can be mentally related.
We are unaware that, as we look, our eye continually performs little involuntary movements, and we naturally ascribe the consequent displacement of the star's image on the retina to some corresponding displacement of the source of light.
The same phenomenon can be observed when we watch a distant aircraft at night as it makes its lonely way across the sky: when watched intently, it seems to move in little jerks, and yet we know it is moving steadily. As the Roman writer Phaedrus once remarked: "Non semper ea quae videtur sunt" - "Things are not always what they seem".