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The Question
(Submitted December 17, 1998)
Why do stars twinkle? I have heard that it is because the
light is refracted as it passes through the atmosphere, but
if that is true, why does light from planets not twinkle?
The Answer
You've heard it right.
Twinkling is closely related to what astronomers call 'seeing'
(atmospheric blurring of an image). Both are caused by the
turbulent cells in the upper atmosphere: these are little pockets
of air that have different density, temperature, humidity etc.
than the surrounding air. The density contrast causes refraction,
and as different cells move in and out of your line of sight,
the image of the star (which is point-like) is seen to move around
from one second to the next. This movement is seen as twinkling
by the eyes; if you take a photograph over several minutes, as
astronomers often do, then the image becomes blurred. The seeing
(this blurring) can be as good as ~0.5 second of arc at the best
astronomical sites on Earth, while the worst I've ever seen at a
professional observatory was about 8 seconds of arc (we gave up
observing that night; more typically, 2 seconds of arc would be
considered bad by today's professional astronomers). 1 second of
arc is 1/3600th of a degree.
So why don't planets twinkle? This is because, even though they may
look point-like to naked eyes, they are actually much bigger than
the typical seeing. This means that you observe the combination
of light which has passed through different atmospheric cells. Thus,
the turbulent effects are averaged out, making the planets look steady.
Hope this helps.
Koji Mukai & Maggie Masetti
for "Ask an Astrophysicist"
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