Happy Autumnal/Vernal Equinox!
At 9:54 pm Eastern Daylight Time on Saturday, September 22, the Equinox occurs, and autumn officially begins in the Northern Hemisphere, and spring in the Southern Hemisphere. Here’s why…
At any given time, from our point of view here on Earth, the sun is situated in front of the distant background stars (although we can’t see them due to its bright glare). As Earth completes one orbit of the sun every year, the sun appears to shift eastward through those fixed stars, tracing a great circle around the sky. That circle is called the Ecliptic. Because the Earth’s orbit is in more or less the same plane as all the other planets, the major Solar System bodies are always positioned near that imaginary track around the sky.
Now, imagine a second circle positioned directly over the Earth’s equator, and painted as a strip around the sky, and dividing the celestial sphere into two great bowls, the Northern Hemisphere stars and the Southern stars. Since the Earth’s equator is south of observers located in the Northern Hemisphere, that Celestial Equator circle around the sky is always in the southern part of the sky, and runs from the eastern horizon to the western horizon.
Because the Earth’s axis of rotation is tilted 23.5° away from the Ecliptic, the equator circle and the ecliptic circle behave like two hula hoops with the same center, but with one tilted so that they intersect at only two spots. At the moment of the Autumnal Equinox, the sun is situated at one of the intersection points — and its apparent motion through the stars is carrying it into the southern bowl of the sky — as if it is “stepping over” the equator. Six months from now, on the Vernal Equinox, the sun will again cross the Celestial Equator at the other intersection point — this time heading into the northern bowl of the sky. At that moment, Spring will begin for the Northern Hemisphere.
The Equinox triggers a few interesting effects. First of all, for the next six months, the sun will spend all of each day in front of the southern hemisphere stars, and also high overhead of the lucky folks who live there! With the sun higher in their sky, they experience more daylight hours and receive more highly concentrated solar radiation, producing warmer weather. (Compare the intensity of a flashlight’s light when it’s beamed straight at a wall versus obliquely at the wall. The bright circle of the beam gets weaker when it spreads into an oval.) At the same time, North Americans, Europeans, and Asians have to accept shorter, colder days and longer nights (which are great for warmly dressed astronomers). On the day of the Equinox, everyone worldwide experiences 12 hours each of daytime and night-time. This is where the term, Latin for “equal night”, comes from.
The nights around the equinoxes offer better chances to see the aurorae at high northern and southern latitudes, too. Just as two bar magnets lined up with their poles in the same direction repel one another strongly, the Earth’s magnetic field repels the sun’s field. At the equinoxes, the Earth’s axis tilts neither towards nor away from the Sun, so the two “magnets” aren’t lined up as well, reducing Earth’s ability to deflect the Sun’s field and the charged particles that trigger aurorae in our upper atmosphere.
A final aside: The sun’s path along the Ecliptic is where the Zodiac originated. Early sky watchers noted that, in the course of one year, the sun travelled through twelve (well, 13 actually) constellations, passing through the same constellation every year on the same range of dates. Over the centuries, the dates have shifted somewhat. Nowadays, every year in the third week of September, the sun sits among the stars of the constellation Virgo (the Maiden).