celestial equator

Fall officially kicks off Friday at 11:50 p.m. with the fall equinox, the moment the sun crosses the celestial equator, an imaginary line above earth’s equator.

Standing by the Hunter’s foot, the Great Dog is an easy-to-find constellation visible from most areas of the world, thanks to being positioned just south of the celestial equator.

And whereas Ptolemy based his coordinate system on the ecliptic, Hipparchus used the celestial equator, a system more common in modern star maps.

This makes the sun appear at its farthest below the celestial equator when viewed from Earth.

At the instant of the equinox — and it’s only an instant — the sun will beam directly above the celestial equator, that imaginary line that divides the planet in two.