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

TWINKLE, TWINKLE, LITTLE STAR 91
experienced the practical side of this, too. If you don’t compensate
for refraction, you’re more likely to get a netful of nothing than
tonight’s dinner.
Light will bend when it goes from one part of the atmosphere
to a slightly less dense part. For example, hot air is less dense than
cooler air. A layer of air just over the black tar of a highway is
hotter than the air just above it, and light going through these layers
gets bent. That’s what causes the blacktop ahead of you to
shimmer on a summer’s day; the air is refracting the light, making
the highway’s surface look like a liquid. Sometimes you can even
see cars reflected in the layer.
Here on the ground, the air can be fairly steady. But, high over
our heads things are different. A few kilometers up, the air is constantly
whipping around. Little packets of air, called cells, blow to
and fro up there. Each cell is a few dozen centimeters across and is
constantly in motion. Light passing in and out of the cells gets bent
a little bit as they blow through the path of that light.
That’s the cause of twinkling. Starlight shines steady and true
across all those light years to the Earth. If we had no atmosphere,
the starlight would head straight from the star into our eyes.
But we do have air. When the starlight goes through our atmosphere,
it must pass in and out of those cells. Each cell bends the
light slightly, usually in a random direction. Hundreds of cells
blow through the path of the starlight every second, and each one
makes the light from the star jump around. From the ground, the
size of the star is very small, much smaller than the cell of air. The
image of the star, therefore, appears to jump around a lot, so what
we see on the ground is the star appearing to dance as the light
bends randomly. The star twinkles!
Astronomers usually don’t call this twinkling, they call it seeing,
a confusing holdover from centuries past, but like most jargon, it’s
stuck in the language. Astronomers determine how bad the seeing
is on a given night by measuring the apparent size of a star. A
star’s image dances around so quickly that our eyes see this as
a blurring into a disk of light. The worse the seeing is, the bigger
the star looks. On a typical night, the seeing is a couple of arcseconds.
For comparison, the Moon is nearly 2,000 arcseconds across,