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

50 FROM THE EARTH TO THE MOON
Clearly, something else must be going on to cause such a huge
temperature variation. That something else is the tilt of the Earth’s
axis.
Imagine the Earth orbiting the Sun. It orbits in an ellipse, and
that ellipse defines a plane. In other words, the Earth doesn’t bob
up and down as it orbits the Sun; it stays in a nice, flat orbit.
Astronomers call this plane the ecliptic. As the Earth revolves
around the Sun, it also spins on its axis like a top, rotating once
each day. Your first impression might be to think of the Earth’s
axis pointing straight up and down relative to the ecliptic, but it
doesn’t. It’s actually tilted by 23.5 degrees from vertical. Have you
ever wondered why globe-makers always depict the Earth with the
north pole pointing at an angle from straight up? Because it is
tilted. It doesn’t point up.
That tilt may not seem like a big deal, but it has profound implications.
Here’s an easy experiment for you: Take a flashlight and
a piece of white paper. Darken the lights in a room and shine the
flashlight straight down on the paper. You’ll see a circle of bright
light. Now tilt the paper so that the light shines down at about a
45-degree angle. See how the light spreads out? It’s now an oval,
not a circle. But more importantly, look at the brightness of the
oval as you change the illumination angle. It’s dimmer. The total
light hitting the paper hasn’t changed, but you’ve spread the light
out by tilting the paper. More of the paper is lit, but each part of
the paper has to share all the light, so there is less light for each
part. If you tilt the paper more, the light gets even more spread
out, and dimmer.
This is exactly what’s happening to the Earth. Imagine for a
moment that the Earth is not tilted, and that the axis really does
point straight up and down relative to the ecliptic. Now pretend the
Sun is a giant flashlight shining down on the Earth. Let’s also say
you are standing in Ecuador, on the Earth’s equator. To you the Sun
would be straight up at noon, with the sunlight hitting the ground
straight on. The light is highly concentrated, just like it was when
the paper was directly facing your flashlight in the experiment.
But now let’s pretend you are in Minneapolis, Minnesota, which
happens to be at 45 degrees latitude, halfway between the equator