The radian (rad) is the SI unit of angle measurement, defined as the angle subtended by an arc equal in length to the radius of the circle. It is the natural unit for trigonometric functions in mathematics and physics — sin, cos, and tan all assume radian input. One radian equals approximately 57.296 degrees.

The arcsecond (″) equals 1/3,600 of a degree, providing extremely fine angular resolution for astronomy, optics, and satellite imaging. The Hubble Space Telescope has a resolution of about 0.05 arcseconds, and stellar parallax measurements use milliarcsecond precision. One arcsecond equals 1/3600 degree or about 0.00000485 radians.