Still in their coats on a sunny day(Thanks to [seattle + beyond])

To see what a difference it makes whether sunlight hits the Earth directly or at an angle, just think about a sunny day in summer and a sunny day in winter. In summer, when the sun is out it soon gets very hot. Even cloudy days, when the sunlight has to come through clouds, are often very hot. But in the wintertime, even when the sun is out all day, it’s still cold. That’s because instead of being right overhead, the sun stays lower in the sky all day, and the sunlight hits the ground at an angle.

A man shining a flashlight at the ground

When the sunlight hits the ground at an angle, the same amount of light – the same number of photons – have to cover much more ground than before, so there are fewer photons for each bit of ground. To see this for yourself, take a flashlight outside at night and point it straight at the ground so that it makes a circle of light.  (The flashlight on a cell phone will also work.)

Now slowly tilt the flashlight, so that your circle of light becomes an oval. Is the light dimmer in the oval than it was in the circle? Try tracing your circle and your oval and calculating the area of the circle and the area of the oval. Which one is bigger?