It may be boiling hot across large swathes of Turtle Island, but the underlying reality has not changed: Today marks the first day of fall in the Northern Hemisphere.
At precisely 10:21 a.m. on Thursday September 22, the sun “crosses precisely over Earth’s equator as it heads south for the season,” as Sky & Telescope puts it.
“If you were watching the scene from far away in space, you'd see Earth's Northern Hemisphere tipped sunward during our hot months, and tipped away from the Sun when Earth is on the opposite side of its orbit: winter,” Sky & Telescope says. “The spring and fall equinoxes are the instants halfway between, when the sun shines equally on both hemispheres.
Ancient Indigenous Peoples did not need fancy instruments to tell them this, of course. They did not necessarily need to know that the seasons happen because Mother Earth’s axis is tilted at a 23.5-degree angle to the sun, and thus get different levels of light depending on where the planet is in its orbit. They merely observed the world around them, harvested the crops and knew that winter was coming. There’s plenty of evidence for that, as Earthsky.org points out, starting with the shortening days.
“For us in the Northern Hemisphere, the sun is rising later now, and nightfall comes sooner,” Earthsky.org says.
While Equinox means “equal” and thus implies that the day and night are the same length on this day in spring and fall, other factors intrude, and this is not quite the case.
“Our star rises due east and sets due west today,” notes Astronomy.com. “If the Sun were a point of light and Earth had no atmosphere, everyone would get 12 hours of sunlight and 12 hours of darkness. But the presence of air and the finite size of our star make today a few minutes longer than 12 hours.”
Either way, the shortening of days will soon accelerate, the air will get a chill, and we will start to think about coats, boots and sweaters.