Part of your planning for a star-trail photo is if you want the stars to appear as lines or circles. When you point your camera into different directions then you will get different effects, for example lines or circles. This article will try and show you the different effects when you point the camera into different directions. The primary thing to remember is that the earth rotates in relation to the stars. This rotation then causes the stars to form lines in long exposure photos due to the stars being in “fixed” positions. Thus the star lines will have different shapes depending to where you point your camera. (To clarify: what I mean by “fixed” is: the stars move a lot slower in relation to the speed that the earth rotates, plus the milky way moves at 828 000 kilometres per hour, but from our perspective the stars “seems” stationary. Another interesting fact is that the earth rotates at approximately 1600 kilometres per hour)
Circles are generated at the South and North celestial Poles. Imagine a spinning car wheel. When you look at the side the centre will be going round but when you look at the spinning wheel from the front the lines will go down (or up). This is the same effect when the earth spins on its axis.
If you want your star trails to be straight lines then point your camera to 90 degree east or west from the South or the North Pole. This will then cause the star trails to move in straight lines. Top: Remember the tip around a car wheel: when you look at the spinning wheel from the front the lines will go down (or up). In a wheel the outside of the wheel will spin faster than at the poles, and it is the same, the Earth “spin” faster at the equator, and thus the star trails will be linger at the east and west of the celestial north and south poles.
It is your choice what kind of effect you want your photograph to have, and I would suggest that you see what your foreground scene looks like, and what would be the best fit with the lines or circle star trails.