Twinkle, Twinkle: September Skies and Starlight

factors are equal). The continual horizontal and vertical motions of hot and cold air pockets cause light rays moving through the naturally turbulent atmosphere to change direction continuously.

The lower atmosphere, where we stand to view the stars, is mottled with pockets of varying density caused by rising and falling air parcels and strong horizontal winds. When the local air density changes rapidly with time, a condition termed turbulent, the light ray's path also alters rapidly. This slight but perceptible refraction bends the path one way, one moment, slightly different the next. This constant, but random shifting results in the star's image jiving and jiggling, fading in and out, and even changing colours before our eyes. This stellar dancing is what we call twinkling.

In addition to this constant jitter in the apparent position of a star, the turbulent air pockets of also focus and de-focus the starlight, making the stars appear to randomly change brightness. And since the amount of refraction also depends on the wavelength of the light, various colours in the ray may dominate at times, giving the twinkling star hints of colour change. (This effect is not as apparent since our eyes are less light-sensitive in the dark.)

Often on a clear night with calm surface winds, wild twinkling of the stars indicates strong winds, such as the jet stream, high in the atmosphere above the viewer. The greater the atmospheric turbulence, the greater the twinkle effect. Lots of twinkling stars indicate a very unstable, turbulent atmosphere above the viewer.

Twinkling only affects those distant objects whose visual size is smaller than the refractive shifts caused by the atmospheric turbulence. That is why planets do not twinkle. Even though they appear as stars, they present a visually large disk compared to the level of turbulent refraction. So although their light scintillates, the refractions of different light rays coming from across a planet's disk tend to cancel each other out, and the planet's light shines rather steady. In fact, one way, therefore, to determine whether a sky object is a star or planet is to see if it twinkles.

On rare occasions, the turbulence may be strong enough to show some planetary twinkle, but usually only when the planet is near the horizon. A similar effect to twinkling can often be seen in distance surface light sources, such as individual city lights, when there is strong surface layer turbulence. Shimmer over hot deserts or other hot surfaces is another form of scintillation.

One of the reasons that astronomical observatories are located on high mountains is to reduce

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