The celestial equator is the great circle perpendicular to the rotation axis of Earth; declination measures angular displacement from it.
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The celestial equator is an imaginary great circle on the celestial sphere that lies directly above the Earth's equator. It divides the celestial sphere into the northern and southern celestial hemispheres. In more detail: 1. **Celestial Sphere**: The celestial sphere is an abstract representation where all celestial objects (like stars, planets, and galaxies) are projected onto an imaginary sphere surrounding the Earth.