Bad Astronomy: Misconceptions and Misuses Revealed, from ...

92 SKIES AT NIGHT ARE BIG AND BRIGHT
and the naked eye can just resolve a disk that is about 100 arcseconds
across. The best seeing on the planet is usually a half an
arcsecond, but it can be much larger, depending on how turbulent
the air is.
Seeing also changes with time. Sometimes the air will suddenly
grow calm for a few seconds, and the disk of a star will shrink
dramatically. Since the light of the star gets concentrated into a
smaller area, this lets you see fainter stars. I remember once sitting
at the eyepiece of telescope for several minutes, looking for the very
faint central star in a nebula. The star was just at the visibility limit
of the telescope. Suddenly the seeing steadied up for a moment and
the ghostly, pale-blue star snapped into my sight. Just as suddenly,
the seeing went sour and the star disappeared. It was the faintest
star I have ever seen with my own eyes, and it was amazing.
PPP
So why don’t planets twinkle? Planets are big. Well, in reality they’re
a lot smaller than stars, but they are also a lot closer. Even the
biggest star at night appears as a tiny dot to the world’s best telescopes,
but Jupiter is seen as a disk with just a pair of binoculars.
Jupiter is affected by seeing just as much as a star. But, since
the disk of the planet is big, it doesn’t appear to jump around. The
disk does move, but it moves much less relative to its apparent
size, so it doesn’t appear to dance around like a tiny star does.
Small features on the planet are blurred out, but the overall planet
just sits there, more or less impervious to turbulence.
More or less. Under especially bad conditions even planets can
twinkle. After thunderstorms the air can be very shaky, and if the
planet is on the far side of the Sun the planet’s disk will look particularly
small, making it more susceptible to twinkling. But when
a planet does twinkle, the seeing is incredibly bad, and observing is
hopeless for that night.
Another way to increase twinkling is to observe near the horizon.
When a star is just rising or setting, we are looking at it
through more air because our atmosphere is curved. This means
there are more cells between us and the star, and it can twinkle