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abstraction > mathematical concept > geometrical object > conic section > ellipse > circle > great circle

great circle
subject			fact

great circle	has definition A circle on the surface sphere whose diameter equal to the diameter of the sphere.
	is a kind of circle
conic section	has dimensions 2

Kinds of great circle :

• hour circle (2 facts) - A great circle passing through the celestial poles - i.e., perpendicular to the celestial equator.
• meridian (5 facts) - The great circle on the celestial sphere which passes through the celestial poles and the zenith of the observer., Theoretical north-south line on the Earth's surface, or an extension of that line onto the night sky, connecting the observer's zenith with the celestial pole and the horizon. The meridian is used to state directional bearings. Devices and structures - such as meridian arcs - marking the meridian were once common in observatories., The great circle passing through the zenith of the observer and the north and south points on his horizon., A great circle passing through the celestial poles and through the zenith of any location on Earth. For planetary observations a meridian is half the great circle passing through the planet's poles and through any location on the planet.