Rotating Earth
With a fixed camera we need to take short exposures to avoid trails
Then use software (app) to combine many separate exposures into one,
making a long total time
How short must each exposure be?
See the calculator on my web site: www.grelf.net/star_trails.html
Length of exposure depends on the required size of the final image:
for printing or only for screen display?
www.grelf.net/star_trails.html
The calculator looks like this: