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plural semidiameters
(astronomy) The apparent radius of a star etc, when viewed from Earth. examples
(archaic) A radius: half of a diameter. quotations
[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, Democritus Junior for Henry Cripps, partition 2, section 2, member 3, page 254