Principles of Celestial Navigation

Supplemental Material » Parallel Light Rays

You will notice on certain illustrations, we have the starlight hitting the Earth in parallel rays; that is, the light appears to come from the same direction in the sky. You don’t really need to understand why this is true to know how celestial navigation works, but the illustrations will make a more sense if you do.

So, why does starlight hit the Earth in parallel light rays? Simply, it is because stars are so very far away. To illustrate, let’s look at light sources at three locations.

First, let’s have the light source be located directly over Location 1, but pretty close to it. We see that the light rays arriving all the locations are at very different angles. In this illustration, the rays hitting Location 2 and Location 3 are about 90 degrees different. Let’s now move the light source farther, but still keeping it above Location 1. We see that the light rays arriving at all the locations are still at varying angles, but the differences are smaller. Simply by moving the light source farther, the light rays arriving at different positions on Earth are become more parallel.

Now imagine that we move the light source very, very far off the diagram, say many miles away, but still keeping it above Location 1. The light rays arriving at Locations 1, 2, or 3—actually any location on Earth—will appear parallel. (In reality there is a very, very slight difference in the angle, but it is way too small to measure).