What geometric difference defines the celestial equator versus the ecliptic?
Answer
Celestial equator is defined by Earth's spin; ecliptic by Earth's revolution.
The fundamental difference distinguishing these two critical great circles lies in the terrestrial motions they represent. The celestial equator is derived from and defined by the Earth's rotation, or spin, around its axis. In stark contrast, the ecliptic is defined by the Earth's revolution, or orbit, around the Sun. This distinct geometric separation is why the Sun appears to cross the stationary celestial equator only twice annually, marking the start of spring and fall at the equinoxes, whereas the ecliptic itself moves relative to the celestial equator over the course of the year.
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