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

106 SKIES AT NIGHT ARE BIG AND BRIGHT
of the bar. Nothing really changes, and it’s just as hard to hear
your friend.
Unfortunately, this also proves wrong the legend of seeing the
Star of Bethlehem reflected from the water in a well. The water
might reduce the brightness of the sky, but it reduces the brightness
of the star by the exact same amount. You’d do better from the
bottom of a chimney. That would change Nativity scenes extensively;
a large smokestack next to the animals in the manger would
take away a lot of the charm of Christmas.
You can see stars fairly easily at night, but not easily or at all
during the day. The reason is just as obvious: at night, the sky is
black and dark, but during the day it’s very bright. The sky is bright
during the day basically because the Sun lights it up. (See chapter 4,
“Blue Skies Smiling at Me,” for a more thorough explanation.)
The Sun isn’t the only source of light illuminating the sky. If
you go out at night during a full Moon, only the brightest stars
will be visible, struggling to overcome the glaring light from the
Moon. City lights also brighten the sky. This is called light pollution,
and it’s bad near cities, but it’s not a good thing even near
small towns. That’s why astronomers try to build observatories far
away from population centers.
During the day the bright sky swamps the rather meager light
from the stars. As a matter of fact, on average the clear, daytime
sky is roughly six million times brighter than that same patch of
sky on a clear, moonless night. No wonder it’s so hard to see stars
during the daytime! They have to fight a fierce amount of light
from the sky itself.
Still, we know it’s possible to see the Moon, for example, during
the day, so it’s possible for some astronomical objects to be
bright enough to be seen against the daytime sky. How bright must
a star be before we can see it against the sky?
The critical item here is contrast. To see an object against a
bright background, the object must be bright enough for your eye
to pick it out over the rest of the light coming from all around the
object. Tests done early in the twentieth century showed that the
eye can pick out a star against the sky background if the object is
roughly 50 percent as bright as the background. It may seem weird,