semi·di·am·e·ter /-daɪˈæmətɚ/
半徑
Sem·i·di·am·e·ter n. Math. 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.
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semidiameter
n : the apparent radius of a celestial body when viewed as a
disc from the earth