Where twinkling comes from
The atmosphere isn't uniform: areas of warm and cold air with slightly different densities are constantly moving through it. As a beam passes through them, it bends slightly - and does so differently every fraction of a second.
For a point source, this means its position can seem to wobble, its brightness can fluctuate, and its color can even seem to change - especially for bright stars low above the horizon, where the light passes through more air.
A planet's disk looks like a point to the eye, but its angular size is tens of times larger than a star's. That's enough for deviations across its different parts to combine and cancel each other out.
How this is useful
Twinkling is the main enemy of ground-based telescopes, and there are two ways to fight it: build observatories high in the mountains, where there is less air, and use mirrors whose shape changes hundreds of times per second to compensate for distortions.
The second method is why ground-based telescopes can now match space telescopes in some respects. A flexible mirror adjusts to current atmospheric conditions, using a bright star or an artificial laser beacon as a reference.
This clue has a practical use, too. If a bright point in the sky doesn't twinkle and shines steadily, it's most likely a planet; a strongly wobbling, shimmering point is almost certainly a star.
From space, by the way, stars don't twinkle at all. Outside the atmosphere, light travels without distortion - and that's one of the main reasons telescopes are sent into orbit.
Why do stars twinkle?
Why don't planets twinkle?
Sources
- Twinkling is caused by irregularities in the atmosphere that refract light
- for point sources, fluctuations do not average out, while for extended objects with a larger angular size, they partially cancel each other out. In ground-based astronomy, distortions are corrected by placing observatories at high elevations and using adaptive optics systems
Next in the “Stars” series
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