- The imaginary great circle around the earth's surface, equidistant from the poles and perpendicular to the earth's axis of rotation. It divides the earth into the Northern Hemisphere and the Southern Hemisphere.
- A similar great circle drawn on the surface of a celestial body at right angles to the axis of rotation.
- The celestial equator.
- A circle that divides a sphere or other surface into congruent parts.
- In <em>astronomy</em>, that imaginary great circle in the heavens the plane of which is perpendicular to the axis of the earth.
- In <em>geography</em>, that great circle of the earth every point of which is 90° from the earth's poles, which are also its poles, its axis being also the axis of the earth.
- Hence A similarly situated circle about any spherical body, or the region adjacent to it.
- The imaginary great circle on the earth's surface, everywhere equally distant from the two poles, and dividing the earth's surface into two hemispheres.
- The great circle of the celestial sphere, coincident with the plane of the earth's equator; -- so called because when the sun is in it, the days and nights are of equal length; hence called also the <altname>equinoctial</altname>, and on maps, globes, etc., the <altname>equinoctial line</altname>.
- the great circle whose plane passes through through the center of the body, and is perpendicular to its axis of revolution.
- See <er>Aclinic</er>.
- An imaginary <xref>great circle</xref> around the Earth, <xref>equidistant</xref> from the two poles, and dividing earth's surface into the northern and southern <xref>hemisphere</xref>.
- A similar great circle on any <xref>sphere</xref>, especially on a <xref>celestial body</xref>, or on other reasonably symmetrical three-dimensional body.
- A short form of the <xref>celestial equator</xref>.
- an imaginary line around the Earth forming the great circle that is equidistant from the north and south poles
- a circle dividing a sphere or other surface into two usually equal and symmetrical parts