Here in the Northern Hemisphere, leaves on the trees are starting to turn colors. There’s a chill in the air in the mornings and evenings. This Wednesday marks the first day of astronomical fall (autumn) in the Northern Hemisphere. According to the meteorological definition, the seasons begin on the first day of the months that include the equinoxes and solstices. In the Northern Hemisphere, for example, spring runs from March 1 to May 31; summer runs from June 1 to August 31; fall runs from September 1 to November 30; and winter runs from December 1 to February 28 (February 29 in a leap year). However, the astronomical definition uses the dates of equinoxes and solstices to mark the beginning and end of the seasons.
Today, we know each equinox and solstice is an astronomical event. It’s caused by Earth’s tilt on its axis and orbit around the sun, and that means Earth’s Northern and Southern Hemispheres trade places throughout the year in receiving the sun’s light and warmth most directly. During an equinox, the Earth’s axis is tilted neither toward nor away from the Sun, resulting in nearly equal amount of daylight and darkness at all latitudes. The 2021 autumnal equinox for the Northern Hemisphere (spring equinox for the Southern Hemisphere) happens on Wednesday, September 22, at 19:21 UTC (Coordinated Universal Time). Converted, this time in the continental US is 3:21 PM EDT, 2:21 PM CDT, 1:21 PM MDT (for us here in Colorado), and 12:21 PM PDT. At this special moment – the instant of the September equinox – the sun is straight overhead, as seen from Earth’s equator. That’s the meaning of an equinox. The September equinox sun crosses the sky’s equator, going from north to south.
Over the next few weeks, as we go into October, the Sun will rise noticeably south of due east and will set noticeably south of due west. Translated, that means shorter days and longer nights for the Northern Hemisphere (and longer days, shorter nights in the Southern Hemisphere). Migrating birds in the Northern Hemisphere will travel south towards warmer climates, Arctic sea ice will begin to freeze again, and Antarctic ice will start melting.
Fun Fact: The staircases at the main Maya pyramid, El Castillo, at Chichen Itza, Mexico are built at a carefully calculated angle which makes it look like a snake of sunlight slithers down the stairs when the equinoxes occur.
Image: U. Horodyskyj