semidiameter noun2 •The apparent radius of a star etc, when viewed from Earth. •(no longer used) A radius: half of a diameter.[B]etweene the sphere of Saturne and the Firmament, there is such an incredible and vast space or distance (7000000. semidiameters of the earth, as Tycho [Brahe] calculates) void of starres: [...] — 1638 formssemidiameters
