Analyzing Star Trails, Part Two: Length Of The Lines

In Part One, I talked about the shape of star trails. In Part Three, I talk about color and intensity. Here, I’m going to explore some of the issues regarding how far the stars streak across the frame.

Some people are surprised when I tell them that the length of a star trail varies according the direction in which you point your camera. A person I told this to recently said she was certain that I was just trying to pull her leg. I have no clue why someone would think I would do something like that (insert sound of throat clearing here), but I assured her that I was telling the truth.

To understand this phenomenon, first make sure you read Part One of this series. Now, imagine those lines (star trails) encircling Earth. In effect, they are just like imaginary lines of latitude that are suspended in space above Earth. A line directly above the equator is going to be much longer than one near the poles since it has to travel all the way around the widest part of Earth.

If you were standing on the equator and pointed your camera at a star directly overhead at the zenith and shot a one-hour shutter speed, the light streak would occupy 1/24th of the star’s imaginary line in the example above. If you were standing close to the North Pole and did the same thing, the star streak would still occupy 1/24th of the imaginary line, but the streak would be shorter since Earth rotates a shorter distance at the poles than it does at the equator.

Star trails around Polaris

Stars farther away from Polaris create longer streaks than those closer in. Polaris barely streaks at all since it is nearly at the the celestial pole.

Here’s another and perhaps easier way to visualize what’s going on. When we shoot star trails, we don’t capture the full circle of trails unless we shoot from a location that is in darkness for the full 24-hour period and we point the camera toward the celestial pole. We only capture a slice out of the star’s apparent 24-hour rotation around the pole. Look at a star trail photo that shows star streaks circling Polaris and you can see this plainly. You’ll see that Polaris shows almost no streaking, since it’s nearly at the exact celestial pole, but the star streaks get longer the farther away from Polaris you go. Streaks that are closer to the poles will be shorter because they have a shorter distance to cover in their rotation around the pole.

How different the streaks are depends on the direction you’re shooting and the field of view. A wide-angle lens is going to cover a broader range of the differing star-trail circumferences than a telephoto will, so you’ll get a wider range of streak lengths. The farther away from the north or south celestial pole you go, the longer the streaks will be.

At this point, you might be asking what difference it makes whether or not the length of a star trail varies. Not a lot, actually. I did say in Part One that there is a practical application for this, but it applies more to shooting stars as pinpoints of light than it does star trails. With star trails, it doesn’t really matter how long the streak is as long as it’s a least enough to look good. Sure, you can shoot a shutter speed of only a couple minutes and record a star trail, but it’s probably going to look more like a mistake than a good star-trail image. “Why is that star streaked?” is probably the response you’ll get more than, “Wow, look at those star trails!”

How long the trails need to be to look good is purely subjective. Personally, I like to go with a minimum of 30 minutes for the shutter speed and preferably two or three hours. Interestingly, and in apparent opposition to everything I said above, it doesn’t make a lot of difference where in the sky you shoot for any given shutter speed. That’s because with a wide-angle lens, you’re going to be covering a wide enough range to capture some longer trails even if you point to Polaris and with a telephoto lens, you’ll be covering a narrower field of view, which will make the star trails occupy more of the frame and appear longer. By the way, that’s at least 30 minutes of accumulative time. I probably wouldn’t shoot a 30-minute exposure for star trails, but would shoot six 5-minute exposures instead. When I can, I load a fresh battery and shoot as many exposures as it will take, which is around 3 hours’ worth in my D800.

The issue of shutter speed, focal length, and direction of view when shooting stars as pinpoints of light instead of star trails is such an important consideration that I’m going to write a separate article just for the topic. For here, I’ll give you the short version. Since the length of star trails varies according to the direction in which we shoot, it makes sense that the longest shutter speed we can use before obvious streaking occurs will vary also according to direction.  And the longer the lens, the narrower the field of view, which means any streaking will show up more quickly.

Admittedly, in the grand scheme of things, none of this really matters for most photographers. Just point your camera at the sky and let ‘er rip! But for geeks like me, size really does matter.

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