Analyzing Star Trails, Part One: Shape Of The Lines

Warning: Geek Alert. Analytical discussion ahead. You do not need to bring the right side of your brain.

Okay, you’ve been warned. (Patti, I know YOU’RE still with me. J)

A question I hear often when I show star trail photos is, “Why are the lines going in different directions?” Good question. Here is the short answer for any right-brain folks who may still be with me:

Star trail lines are not always in the same direction because, in effect, those lines orbit a globe (Earth) and therefore will look differently from different viewing angles.

Star trails North, South East, West

Four star trail photos shot in the four cardinal directions of the compass. Photos were shot from 35 degrees north latitude.

Now, for all you left-brainers, let’s get into some really fun stuff.

Back in the 1990s when I was shooting star trails with film, I remember well the first time I pointed the camera to the South and captured both convex and concave star trails in the same photo. What the heck was going on? I asked an astronomy friend about it and he couldn’t answer. In fact, I couldn’t find an answer anywhere. (This was before I had even heard of Alta Vista, much less Google.) So I did what I always do in a case like this. I figured it out for myself.

I started thinking about what the star trails would like from space. Of course, I knew that the stars didn’t actually move. It was the rotation of Earth that caused the effect. A stationary camera on Earth that was exposing the film for more than a minute or so would have to record the star as a streak since the star was still but Earth was moving. (Stars do move, but imperceptively so for us.)

So I imagined all of those star streaks circling the globe as viewed from space. It would look like a dense grid of latitude lines. Those at the equator would be longer than those close to the poles, since they have a greater distance to cover. And all of them would be circular, since they are circling a globe.

Next, I imagined what those same lines would look like if I moved from space to the surface of Earth. I kept those lines pictured in my mind as I travelled back to Earth and looked back up to the sky. Now I could see why the lines had to be in different directions depending on the angle of view and latitude. From the North Pole, I’d see circles directly overhead, circling Polaris at the north celestial pole. As I moved away from the pole, I’d see Polaris sinking toward the horizon until at one point it was no longer visible. From the equator, if I looked directly overhead and used a wide angle of view, I’d see convex and concave lines meeting in the center of the frame. If I kept going, I’d then start to see star trails circling the southern celestial pole.

Lighthouse and star trails

I was very careful to compose this image so that Polaris was exactly behind the lens of Bodie Island Lighthouse so that the star trails would appear to radiate from the light.

Once you wrap your head around this mental image, it’s easy to determine what the star trails will look like from any point on Earth. And why would you want to do that? Well, I can’t think of a good reason other than to satisfy the curiosity corner of the left side of your brain. The truth is, there is no practical application for this knowledge for the night photographer. You’re not going to start shooting a star trail image and adjust your composition based on what direction the stars are going to streak. You’re going to compose based on foreground elements and open views of the sky.

The one exception to this is when you want to capture those circular star trails around Polaris (or Sigma Octantis in the Southern Hemisphere). You’ll need to identity Polaris and compose it in the scene for the effect you’re after. A good compositional approach is to center a strong foreground object, such as a tree, lighthouse, cactus, or tent, and place Polaris directly above or behind the object.

Remember I said that star trails at the equator are longer than those at the poles are. Think about it. If the exposure time is the same, the streaks at the equator have to be longer since they have a greater distance to travel to circle the globe. It’s the same reason why the surface of Earth rotates faster at the equator than it does at the poles. Unlike the shape of star trails, understanding this phenomenon can have a practical application for the night photographer. We’ll talk about that in Part Two. In Part Three, we talk the color and intensity of star trails.

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