On Sun, 5 Dec 1999 you wrote:

> I don't think my question is news but I'll ask it anyway. My daughter wants
> to use the movement of the stars to predict the rotation of the earth for her
> science project this year in school. She did some reserch on the subject and
> found that taking a time lapsed picture on a clear night might give us some
> clues to a possible answer. My questions is can you give me some suggestion
> on the samera setting and film type to use so that the picture will turn out
> OK. My wife has a 35 mm camera with tripod and all the necessary features (I
> think) but I've never taken this type of picture before.

Star trails are fairly easy to take photographs of. In addition to the
35mm camera and a wide-angle lens (a standard 50mm lens will also work
fine), you will need a cable release button with a locking mechanism (most
camera stores have these). This allows you to open the shutter with the
cable and lock the shutter open for long periods of time without shaking
the camera.

The film you need will depend a little on the sky around the area where
you take the pictures. First, you want to be far from city lights if at
all possible. The closer you are to a city, the slower you want your film
to be (ISO 100 is a slow film, ISO 800 is quite fast). More than likely,
you can get away with ISO 100 or 200 film.

Set the camera focus at infinity. (If you want to become more involved
with astrophotography in the future, there are many guides to making sure
you have the right focus. But infinity is fine for star trails).

Then, just point the camera at the sky, press and lock the cable release,
and wait! Star trails are visible after only a few seconds, but if you
want to measure them to determine the Earth's rate of rotation, then you
will want at least several minutes of exposure. (The Earth turns at a rate
of fifteen degrees per hour, and you will probably want several degrees of
motion). It would probably be best to try several different exposure
lengths, ranging from a few seconds up to several minutes. It may also be
best for the purposes of the experiment to try and point the camera at the
star Polaris, the "North Star", as all stars will appear to move around
that star.

Most of all, be prepared to experiment. Keep a good record of your
exposure times. When you get the film developed, be sure to mention to
that the pictures are astronomical and to develop all images, even ones
that appear blank. (Some labs will play around - I've ended up with
orange skies because some technician thought the sky should be orange.)
Also, take at least one picture in daylight, to establish a reference
frame that shows the lab where frames begin and end.

There is a good article from Sky and Telescope magazing describing how to
get started; I would suggest strongly that you read it, too. If my
information conflicts with theirs, use theirs. I only have limited
experience with star trails. The web site of the article is:
http://www.skypub.com/imaging/astrophotography/camera.html

Good luck!

Sincerely,
Kurtis Williams

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