semidiameter

The apparent radius of a celestial body when viewed as a disk from Earth.

One half the diameter: especially, in astronomy, one half the angular diameter of the sun, moon, or other object showing a disk; the radius.

Half of a diameter; a right line, or the length of a right line, drawn from the center of a circle, a sphere, or other curved figure, to its circumference or periphery; a radius.

The apparent radius of a star etc, when viewed from Earth

radius, half of a diameter

the apparent radius of a celestial body when viewed as a disc from the earth