It’s officially fall, y’all.
Break out your favorite cardigan because, at approximately 2:21 pm CDT today, the autumn equinox will occur thus signaling the beginning of the fall season in the Northern Hemisphere. [Featured image: @jamielinkphotography]
Remind me what an equinox is again?
Seasons start and end due to a series of astronomical phenomenon. Winter follows the winter solstice, spring the vernal equinox, summer the summer solstice, and fall the autumn equinox.
The reason we have seasons is due to the fact that the earth is tilted on an axis toward the sun. During the times of the year when the North Pole is angled toward the sun, we in the Northern Hemisphere experience warmer weather, while those in the Southern Hemisphere get a cooler climate. When the North Pole tilts away from the sun is when we find ourselves where we are now: fall, and later, winter.
Equinoxes occur on March 21 (the vernal equinox) and September 22 (the autumn or September equinox). The vernal equinox occurs when the sun crosses the celestial equator — an abstract projection of the Earth’s equator into outer space — from the south to the north, while the sun crosses the celestial equator from the north to the south for the autumn equinox. Equinoxes are the times at which days and night are approximate in length — in Latin, equinox translates to “equal night.”
Which means a solstice is what?
While both equinox and solstice are based on the relationship between the sun and the earth, they are not quite the same. The solstice ushers in the summer and winter seasons, and occur upon the sun’s highest and lowest points in the sky. As such, the solstice indicates the longest (June 20) and shortest day (December 21) of the year. Furthermore, daylight lasts longer during the summer months, while its shorter in winter months with longer and colder nights.
What does the autumn equinox mean for Chicago?
As you may have seen on your commute in the city the last few days, the phenomenon of Chicagohenge corresponds with the autumn equinox. In the week running up to each equinox, the sun aligns perfectly with Chicago’s east-west facing streets for both sunrise and sunset.
Because Chicago was built on a grid system, that is, with streets arranged at right angles into the shape of a grid — with streets lining up near perfectly on compass north to south and east to west — residents can witness the astrological phenomenon.