"The stars in your eyes, my love, twinkle at so many colours / arcsecond for a given camera displacement.."
Mars and Sirius, 2s exposure. Camera shake shows a constant red squiggle for the planet, while the star goes through all the colours of the rainbow.
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| Mars |
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| Sirius |
This is because the apparent dimension of Mars is considerably larger than that of Sirius, which is a point source of light. Larger objects reflecting (or emitting) light are less likely to be disturbed by the movement of the masses of air in the atmosphere than smaller ones, like Sirius. This is why stars twinkle and may change colour, while planets don't.
The twinkling intensifies as the altitude of the star decreases, because light has to travel through a thicker mass of air, and is more disturbed. Photos of the same star taken throughout the night at various altitudes should (theoretically) show variations in the change of colour - which is the twinkling indicator.
Not very easy to quantify but if camera movement and exposure are constant I guess one could count the number of colours in a given length of the squiggle.