Crepuscular Rays

A fan of sun rays streaming through a cloud or tree or horizon at sunset are called crepuscular rays. It’s a striking and beautiful phenomenon, but may seem to present a quandary.

We know that the sun’s rays that reach the earth do so along almost perfectly parallel lines. The Sun and Earth are so far apart that even if two rays are coming to your location from opposite ends of the vastly large sun, the angle between them is still just over half a degree, and the vast majority of the rays you see are much more parallel than that.

So how is it that crepuscular rays seem to be at very large angles to one another? It seems like the Sun must be much closer than they say...

Actually, it’s just a trick of perspective that can be tricky to get your brain to recognize. Imagine you are in a tremendously long tunnel. Say there are lines painted along the length of the tunnel on the walls. These lines are parallel, but to you they seem to converge at a point.

The suns rays are doing something similar, they are all coming toward you from far away and passing by your head on all sides, so they have this tunnel perspective.

In fact, we can take this further. Consider if you turn around and look the other way in your tunnel, you should get the same effect. Well in fact sometimes if crepuscular rays are visible at sunset, you can turn around and see anti-crepuscular rays on the opposite horizon.