Arcsecond ($1^{\prime \prime }=\pi /648000\mathrm{rad}$)

An arcsecond is $1/3600$ of a degree, equivalently $\pi /648000$ radians. There are approximately $206265$ arcseconds in a radian.

One thousandth of an arcsecond: $1\mathrm{mas}=\pi /(648000000)\mathrm{rad}$.