semidiameter

English

Etymology

From Middle English semidiametre, semydiametre, from Medieval Latin sēmidiameter, equivalent to semi- +‎ diameter.

Noun

semidiameter (plural semidiameters)

1. (astronomy) The apparent radius of a star etc, when viewed from Earth.
2. (archaic) A radius: half of a diameter.
   • 1638, Democritus Junior [pseudonym; Robert Burton], “Ayre Rectified. With a Digression of the Ayre.”, in The Anatomy of Melancholy. […], 5th edition, Oxford, Oxfordshire: […] [Robert Young, Miles Flesher, and Leonard Lichfield and William Turner] for Henry Cripps, →OCLC, partition 2, section 2, member 3, page 254:

     [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: [...]

Derived terms

• semidiameter correction