On the east coast, fall arrives at 10:21 this morning. It will mark the second time this year that the Earth is neither tilted toward nor away from the sun. The first was the vernal equinox on March 20.
There are two equinoxes each year: one that denotes the arrival of spring, and one the arrival of autumn. The length of day, position of the sun, and strength of the sun are approximately the same each equinox. Of course, the difference between the spring equinox and today’s is that we are now losing daylight, whereas in March, we were rapidly gaining it.
The day of an equinox, the entire planet receives roughly 12 hours of daylight and 12 hours of darkness. It’s not exactly 12 because of the way sunrise and sunset are calculated. A true 12-hour day and 12-hour night usually occurs just before or just after the equinox and is called the equilux. The fall equilux occurs on Sunday this year—that’s when the sun will both rise and set at 6:35. The chart below shows the amount of daylight for Sunday, as well as throughout the entire year.
Did you know that meteorologists and climatologists don’t consider fall to begin on the same day? Those of us in the weather community believe the first day of autumn to be September 1, the first day of winter December 1, the first day of spring March 1, and the first day of summer to beJune 1. These dates are when the meteorological seasons begin and are based on seasonal temperatures, not the position of the sun.
Astronomical seasons are different and based on where the Earth is during its orbit around the sun. The time of the equinoxes, as well as the first day of astronomical summer and winter, generally move around the calendar by about a day. However, in the past and future, the first day of fall has come and can come as early as the 21st of September or as late as the 24th, which it did back in 1931 and won’t do again until 2303. In 2092 and 2096, the autumnal equinox will occur on the 21st.
In the southern hemisphere, today marks the start of spring. Daylight will continue to increase there for another three months and peak just as we are beginning winter.
The sun will rise due east and set due west tomorrow, the first full day of autumn. After that, the sun will continue to rise and set farther and farther south of this direction. The sun’s smallest path across the sky will occur on the first day of winter.
One myth you might hear about the equinox is that you can balance an egg on a broom on this day. This isn’t true. If you can do it today or on the first day of spring, you can do it any other time of the year. You also happen to be very talented.
Last thing: Boston and the surrounding area will lose about another three hours of daylight between now and the end of autumn. If that depresses you, know that it’s only 12 more weeks until the days start getting longer again!