Sometimes when I watch movies I notice a mark ( circle ) that shows for less than a second on the top right corner of the many movies.
I wonder what is that and why they have it?
This is a cue mark that tells the projectionist that it is time to change the film reel.
A pair of cue marks is used to signal the projectionist that a particular reel of a movie is ending, as most movies come to theaters on several reels of film lasting about 14 to 20 minutes each (the positive print rolls, themselves, are either 1,000 feet or, more commonly 2,000 feet, nominally 11.11 or 22.22 minutes, maximum, with more commonly an editorial maximum of 10 or 20 minutes). The marks appear in the last seconds of each reel; the first mark, known as the motor cue, is placed about 8 seconds before the end of the picture section of the reel. The second mark, known as the changeover cue, is placed about 1 second before the end. Each mark lasts for precisely 4 frames (0.17 seconds).