Celestial equator

Common card · No. 290604

The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of a planet, by convention generally Earth. By extension, it is also a plane of reference in the equatorial coordinate system. Because of the Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit), but has varied from about 22.0° to 24.5° over the past 5 million years as a result of Milankovitch cycles and perturbation from other planets. An observer standing on the Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be of infinite reach (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on...

Read on Wikipedia · Image Dna-webmaster, CC BY 3.0