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

40 FROM THE EARTH TO THE MOON
body of water—as it would from an ocean liner steaming its way
from the United States to England.
Another commonly given incorrect answer is that blue light
from the Sun scatters off dust in the air. As we’ll see, this answer
is close, and certainly better than the one about reflections off
water, but dust is not the cause.
The correct answer, if you want details, is a little more involved.
In the end we can simplify it for our hypothetical five-yearold,
but first let’s look at the whole problem.
When you examine most problems in astronomy, or for that
matter in any other field of science, you’ll commonly find that to
get to the solution you need two separate lines of attack. The color
of the sky is no exception. To understand the blueness we actually
have to understand three things: just what sunlight is, how it travels
through our atmosphere, and how our eyes work.
You may be surprised to learn that when it leaves the Sun’s
surface, sunlight is white. By this scientists mean it is actually a
balanced combination of many colors. The individual colors like
red, green, and blue are all produced by the complex physics near
the sun’s surface. The roiling, writhing gas making up the Sun’s
outermost layers produces light of all different colors. But when
this light gets mixed together, it produces what looks to our eyes
like white light. You can prove this for yourself: Hold a glass
prism up to a beam of sunlight. When the sunbeam passes through
the prism, the light gets “broken up” into its constituent colors.
This pattern of colors is called a spectrum.
This same thing happens after a rainstorm. The raindrops suspended
in the air act like little prisms, breaking up the white sunlight
into a spectrum. That’s how we get rainbows. The order of
the colors in a rainbow is the same every time: red on the outside,
then orange, yellow, green, blue, indigo, and finally violet, which
makes up the innermost curve of the arc. This pattern may be
tough to remember, so it’s usually taught to students using the
acronym ROY G BIV, like that’s a common name or something. Still,
that’s how I remember it, so it must work.
Those colors are coming from the Sun all at the same time, but
a funny thing happens on the way to the ground. Molecules of