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

TWINKLE, TWINKLE, LITTLE STAR 93
madly. Ironically, if you happen to be looking over a city, the air
can be more stable. There are commonly smog layers over cities
which stabilize the seeing, perhaps their only beneficial effect.
As it happens, different colors of light are refracted more easily
than others. Blue and green, for example, bend much more than
red. Sometimes, in really bad seeing, you can see stars change colors
as first one color and then another is refracted toward you. Sirius
is the brightest nighttime star, and it usually appears to be a
steadily white color to the eye. But sometimes, when Sirius is low,
it can flicker very dramatically and change colors rapidly. I have
seen this myself many times; it’s mesmerizing.
It can also lead to trouble. Imagine: You are driving along a
lonely road at night and notice a bright object that appears to follow
you. As you watch it flickers violently, going from bright to
dim, and then you notice it’s changing colors, from orange to
green to red to blue! Could it be a spaceship? Are you about to be
abducted by aliens?
No, you are a victim of bad astronomy. But the story sounds
familiar, doesn’t it? A lot of UFO stories sound like this. Stars
appear to follow you as you drive because they are so far away.
The twinkling of the star changes the brightness and the color, and
imagination does the rest. I always smile when I hear a UFO tale
like this one, and think that although it may not have been a UFO,
it was definitely extraterrestrial.
PPP
Twinkling stars may inspire songs and poetry, but astronomers
consider them an inconvenience. One of the reasons we build big
telescopes is that they help increase our resolution of objects.
Imagine two objects, one of them half the size of the other, but
both smaller than the seeing on a given evening. Because of seeing,
they will both get blurred out to the same size, and we cannot tell
which object is larger. This puts a lower limit on how small an
object we can observe and still accurately measure its size. Anything
smaller than this lower limit will be blurred out, making it
look bigger.