Etymology
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scotoma (n.)

(plural scotomata), 1540s, from Late Latin scotoma, from Greek skotoma "dizziness," from skotoun "to darken," from skotos "darkness" (from PIE root *skoto- "dark, shade.").

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annular (adj.)
"ring-shaped," 1570s, from French annulaire (16c.) or directly from Medieval Latin annularis "pertaining to a ring," from annulus, misspelled diminutive of Latin anus "ring" (see anus). An annular eclipse (1727) is one in which the dark body of the moon is smaller than the disk of the sun, so that at the height of it the sun, due to the moon's remoteness from Earth, appears as a ring of light. Related: Annularity.
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*skoto- 

Proto-Indo-European root meaning "dark, shade." 

It forms all or part of: nightshade; scotoma; shade; shadow; shady.

It is the hypothetical source of/evidence for its existence is provided by: Greek skotos "darkness, gloom;" Albanian kot "darkness;" Old Irish scath, Old Welsh scod, Breton squeut "darkness," Gaelic sgath "shade, shadow, shelter;" Old English scead "partial darkness," sceadu "shade, shadow, darkness," Dutch schaduw, German Schatten, Gothic skadus "shadow." 

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