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Two great circles drawn across a star chart.
Also known as: Ecliptic line, Celestial equator
A star chart carries two reference circles: the celestial equator, which is the Earth's equator projected outwards, and the ecliptic, which is the sun's apparent yearly path. They cross at the equinoxes, which is what defines those two dates.
Mark two reference circles on a star chart: the projected terrestrial equator and the sun's apparent annual path.
Contexts: Technical drawing, Maps
Cross at two points which define the equinoxes, so those dates are a geometric consequence of the two circles rather than a convention.
Contexts: Technical drawing, Maps
Are inclined to one another by about twenty-three and a half degrees, which is the tilt of the Earth's axis and the cause of the seasons.
Contexts: Technical drawing, Maps
Two curved lines crossing a star chart at a shallow angle, meeting at two points on opposite sides.