Himmelsäquator

The Himmelsäquator, or celestial equator in English, is an imaginary great circle on the celestial sphere. It is the projection of the Earth's equator onto the sky. Just as the Earth's equator divides the planet into the Northern and Southern Hemispheres, the celestial equator divides the celestial sphere into the northern and southern celestial hemispheres.

The celestial equator is defined by the plane of Earth's equator. It is always equidistant from the celestial poles. The celestial equator is tilted relative to the ecliptic, the apparent path of the Sun across the sky, by an angle equal to the Earth's axial tilt, which is approximately 23.5 degrees. This tilt causes the celestial equator to intersect the ecliptic at two points, known as the equinoxes.

For an observer on Earth, the celestial equator appears as a line that rises in the east, passes overhead, and sets in the west. Its exact position in the sky depends on the observer's latitude. At the Earth's equator, the celestial equator passes directly overhead. At the North Pole, it lies along the horizon.

The celestial equator is a fundamental reference line in astronomy, used for defining celestial coordinates, specifically declination. Declination is analogous to latitude on Earth, measuring the angular distance of an object north or south of the celestial equator. Stars and other celestial bodies are located by their right ascension and declination, forming a celestial grid system.