The autumnal equinox occurs tonight, September 22, at precisely 10:29 PM Eastern time. It’s a milestone celestial event, one that marks the official end of summer and start of fall. It’s also got a few other fun and useful implications.
First some quick background. The equinox is defined as the precise moment when the sun is directly above the Equator. That is, if you were standing on the Equator on the equinox, the sun would be directly overhead (90 degrees up in the sky) when it reaches its highest point.
For the six months preceding the fall equinox, the sun has been shining directly overhead at locations in the northern hemisphere. Beginning at the spring equinox in March, the point where this occurs moves gradually northward from the Equator until the summer solstice, when the sun shines directly overhead along the Tropic of Cancer (latitude 23° 26′ N). It then moves south again to recross the Equator at the fall equinox. This pattern is caused by the tilt of Earth’s axis from the sun, which is (not coincidentally) tilted 23° 26′ from vertical.
Fun Fact #1: On the fall (and spring) equinox, the sun rises due east and sets due west everywhere on Earth.
It’s the only time of year this occurs, and a useful tidbit of information if you’re navigating by the sun. At other times of year, the relative position of sunrise and sunset vary depending on your latitude.
Fun Fact #2: There is still more daylight than darkness on the equinox.
A common misconception is that the equinox marks the time when there are equal amounts of day (when the sun is up) and night (sun is down). This isn’t actually true, as you can readily see by looking at sunrise and sunset times (on today’s equinox, for example, sunrise in Boston is at 6:31 AM and sunset at 6:41 PM).
The reason? Sunrise is defined as when the top of the sun’s disc first rises above the horizon, and sunset when the final sliver disappears below the horizon. That means that even though the center of the sun is up for exactly 12 hours on the solstice, the upper portion of the sun is actually up for longer, hence the time difference. (Day and night become equally roughly four days after equinox.)