# Thread: How do I predict the arc of star trails?

1. ## How do I predict the arc of star trails?

Hello everyone!

I'm still working on my 'elemental' collection and I thought I'd represent the moon and stars by using a timed exposure to capture star trails. To keep the moon from appearing like a monster albino bologna, I'm considering stopping down, then opening the lens up all the way for the last minute of so of the exposure. The effect I'm hoping for is for "dim" trails with bright defining moon and stars(this may take a few nights and triple espressos to accomplish!) My question is this: How do I get the star trails to travel in an arc like a "n" rather than a "u" in the sky? Which compass direction do I need to point my camera? I'm in North America(California) and as I recall the last time I shot star trails I had them flying in a "u"---I think I was aiming N or E (or NE?) The reason I ask is that I need to scout out an appropriate foreground I can access from the right direction in order to get the "n" flight path. Can anyone offer some guidence or suggest a resource?

Thanks!

2. ## How do I predict the arc of star trails?

This is really basic...

The earth rotates about its axis once in 24 hours. The axis passes through the north and south pole - that's the definition of "pole".

Directly above the north pole (well - almost) is the polar star. Startrails rotate about that star.

If you're at 45 degrees north, that star will be 45 degrees above the horizon, and always due north. At 60 degrees (where I am), it will be 60 degrees above the horizon.

Now think for a moment about those circles in the sky...

3. ## How do I predict the arc of star trails?

John,

The moon travels essentially along the ecliptic which is near the celestial equator, i.e., 90 degrees from the pole. In that region star trails will appear to be lines rather than circles. Circular star trails are near the pole.

I don't know how you'd capture both moon and star trails in the same exposure. Perhaps expose for the moon and then go back about two weeks later for the stars. I wouldn't worry about stars showing through on the lunar exposure; it won't be long enough for them to register. Don't forget that the moon moves, too, and surprisingly quickly. Depending upon the focal length lens you use, you'll see a blur unless your exposure is short (1/4 sec or so) or you have some means of tracking the moon.

4. ## How do I predict the arc of star trails?

The stars move across the sky at 15 degrees per hour. The moon moves much faster, planets more slowly. My experience suggests the an EI 100 film and an f/8 lens will not give enough exposure- faster film and/or faster lenses will be necessary for good star trails.

5. ## How do I predict the arc of star trails?

As other have said, the stars will circle around the N and S poles. From the northern hemisphere, if you are pointing North, then the trails will be circles around the poles. This makes your "n" shapes or even circles. If you are pointing near the celestial equator (above our equator), you will get fairly straight trails. If you point towards the South, then those stars, which circle the South pole, will make the "n" trail in the other direction. If you aim say NE or NW, and don't get the North pole in the frame, the tracks will be "u" shaped, and on it's side, with the open part of the "u" pointing towards the north pole. If the field of view gets toward E or W, then the stars from those regions will be fiarly straight. If you get the field pointing a little south of E, you will get stars following two arcs, one south, one north, and then some trailing straight in between.

Another way to get "n" shaped trails, is to shoot north but have the pole star below the bottom edge of the frame. Then all the stars will circle up into the top of the frame with the trails pointing downwards. Get a mountain of a bunch of trees high enough to cover the north star and you are set.

Conversely, shoot south, but don't aim the camera too high up to get the equatorial stars in the frame.

It all depends on if the get the trails are coming up, "u", or going over the top, "n", as to which way they are pointing.

You should look for any astrophotography book by Terrence Dickenson. They usually have lots of great photos and should be able to better explain this.

Mark's a bit off with his comment about the speed of the moon, stars, and planets. The difference between the sidereal tracking rate of the stars and the lunar tracking rate is very small. And the planets can be considered to be at the same speed as the stars.

Stopping down the lens will certainly dim the light from the moon and it will still leave a streak the same size as the moon, but it will also dim the light from the stars. The only way you will be able to get star trails without a streaked moon is to shoot the trails without the moon in the frame, and then double expose with the moon. It will not look real, as you will probably have a star trail streaking through the face of the moon, and that will make it obivous that you've done some sort of trickery.

And of course, the angle of view of your lens will have a big effect on how much of these circles you catch. Wider lenses will show more curve, and longer lenses will show less.

6. ## How do I predict the arc of star trails?

Ole, William, Mark, & Kirk,

Thank you so much! Its no wonder I couldn't earn my merit badge at the Starfleet Academy!;-) I'll start scouting locations this week end.

7. ## How do I predict the arc of star trails?

To make a really surreal looking image, include some foreground (saguaro cactuses seem to work well for this), and give a pop from a flash toward the end of your exposure. If you're shooting color, gels on the flash work well to make really strange images.

8. ## How do I predict the arc of star trails?

Check out this site is you are interested in the style of photos that Chad mentioned - there is even a shot similar to what he proposed on the home page.

http://www.thenocturnes.com/

9. ## How do I predict the arc of star trails?

Can't help with the exposure including the moon, but this chart should answer your star trail question. Good luck, and let us know how it turns out.

10. ## How do I predict the arc of star trails?

Brent,

Thanks! That illustration really helps me picture what I'll be needing for the location of my foreground. Thank you everyone for your generous help!

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