Wiktionary
second of arc
n. An angle, one sixtieth (1/60th) of a minute of arc or one 3600th of a degree.
Usage examples of "second of arc".
Somewhere in the distance, all but lost in the maze of three-dimensional neon that partitioned one tiny second of arc in the incredible datasphere of one small world, I sensed rather than saw those soft, hazel eyes waiting for me.
The readout was accurate to the hundredth of a second of arc: a little over ten inches of movement by the seasled would be discernible.