Either of two great circles on the celestial sphere that intersect at the poles; one passes through the equinoxes and the other through the solstices.
From Middle French colure, from Late Latin colurus, from Greek kolouros meaning 'docked' or 'curtailed.' The circles were called 'curtailed' because they appear to cut off portions of the sphere.
Astronomers used colures for thousands of years to map the sky before we even knew why these circles mattered—they mark the critical points where the sun's path creates seasons and determines day length. Ancient navigation depended on understanding these invisible lines.
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