Moving stars or camera shake?

[arkansas72762]

While taking some snaps of the full moon last night I included some distant stars in the frame with some long exposure times of 30 seconds iso 100 with a pentax 1:4/45~125 lens set at 125 on my k20d. I tried several different stops on the the lens and i noticed that the stars all streak from bottom left to top right in every frame I took. At 100% zoom on my computer the streak is about 1mm or less. Any experience with this? ps. of course I was using a tripod. lol

[SpecialK]

Rotation of the earth causes star trails.

[Ex Finn.]

Star trails.
Anything longer than about 2 seconds will result in noticeable streaking with a stationary mount.
Cheers. Mike.

[rparmar]

The Earth! She moves!

[alohadave]

Also, the longer the focal length used, the less time it'll take to see motion in the stars. Since you were at 120mm, you saw it much sooner than you would have at 45mm. Even so, at 30 seconds, you would have seen the trails no matter what.

If you were doing moon shots, you can easily get away with a 1/125-1/500 shutter speed depending on the phase of the moon. It's almost bright enough to handhold, but 120 is a bit short to get a good moon shot without extensive cropping.