| meridian | has definition The great circle on the celestial sphere which passes through the celestial poles and the zenith of the observer. |  |
| has definition 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. | |
| has definition The great circle passing through the zenith of the observer and the north and south points on his horizon. | |
| has definition 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. | |
| is a kind of great circle | |
| conic section | has dimensions 2 | |
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