Overlapping sugar maple leaves create a kaleidoscopic display of fall color. Bob King
Happy first day of spring! That’s what they’ll be saying tomorrow in the southern hemisphere. Here in the north the first moment of fall ticks off Monday morning Sept. 23 at 2:50 a.m. (Central Time). The sun has been slowly sinking southward since the first day of summer, when it shone from its highest point in the sky. At the fall equinox, it crosses the celestial equator, an imaginary circle in the sky that’s an extension of Earth’s equator into the heavens.
Earth’s equator is an imaginary circle with a latitude of 0° located midway between the poles. Stretch the equator out into the sky and it becomes the celestial equator. NASA
If you’re standing somewhere along Earth’s equator, say not far from the city of Singapore, the celestial equator begins at the due east point of the eastern horizon, passes directly overhead and meets the horizon again at the due west point. Picture it as a giant invisible hoop stretching from one horizon to the next. It continues below the western horizon around the back side of the planet until it meets up again at your ...