Overview

• The equinox is the exact moment the Sun’s center crosses Earth’s equatorial plane, starting astronomical autumn in the Northern Hemisphere and spring in the Southern Hemisphere.
• Only on the equinox does the Sun rise due east and set due west, offering an easy way to gauge true east–west at sunrise and sunset.
• Equal day–night (the equilux) typically follows the equinox by several days and varies by location due to atmospheric bending of sunlight and the way sunrise and sunset are defined.
• Space-weather forecasters note higher odds of geomagnetic activity around equinoxes, and stargazers report Saturn at its brightest near opposition with a partial lunar eclipse visible in far southern regions.
• Meteorologists define fall by calendar months beginning Sept. 1, a system that differs from the astronomical season tied to equinoxes and solstices.