Wednesday, September 30, 2020

Nature in the City, Part II: Chicagohenge, 2020


After visiting my mother one day last week after work, I hopped aboard a bus which turned west on Randolph Street and this is what I saw. 

I'm not sure when the term "Chicagohenge" was coined, but it describes the phenomenon of the sun rising and setting close to the horizon at the vanishing point of the city's east-west streets during the vernal and autumnal equinoxes. It's not unique to, but is particularly spectacular in Chicago because of the canyon effect created by the skyscrapers in Chicago's Loop and the fact that the direction of our streets correspond almost directly to the cardinal points of a compass, unlike say the streets of Manhattan. 

The "henge" part as you can probably guess, is after Stonehenge, the prehistoric stone circle in Great Britain whose arrangement was very likely designed to line up with another significant astronomical moment of the year, Summer Solstice, the longest day of the year and the first day of summer. On this day in the Northern Hemisphere, the sun reaches its highest point in the sky before it begins its gradual descent which culminates with the Winter Solstice, the shortest day of the year roughly on the 21st of December, when the cycle begins anew. To the ancients, the reversal of the sun's downward slide became noticeable around December 25th, which became a day of great rejoicing to people who had no practical explanation for the motions of celestial objects, other than the will of the Gods. This day conveniently morphed into the day when much of Christendom celebrates the birth of Jesus. Credit for that goes to the Roman Emperor Constantine who in the process of converting his empire from paganism to Christianity, he likewise converted the day of celebration devoted to the figurative re-birth of the Sun into the day celebrating the literal birth of the Son. 

Smack-dab between the two solstices, the equinoxes are the two moments of the year marking respectively the beginning of spring and autumn, when the sun appears directly above the earth's equator. Because of the tilt of the earth's axis combined with the planet's continuous revolution about the sun, the solstices and equinoxes are moments which can be measured  to the second. This year, autumnal equinox occurred at 8:31 AM CDT on September 22, about 10 hours after this photograph was made. Had the setting of the sun in Chicago occurred precisely at the moment of the equinox, the sun would have appeared right on the horizon. Well not exactly because of the effect of refraction (bending) of light which makes the rising and setting sun and other celestial objects appear to be not where they actually are, but for the sake of argument, let's just assume that statement is true. 

The change of seasons is important to us modern day city slickers primarily because it marks the time when we change our wardrobe. Obviously the seasons are far more important to people who make their living off the land such as farmers. In this part of the world, it's harvest time and another astronomical event has for millennia, serendipitously aided farmers during this busy time for them. That event is Harvest Moon, which is the first full moon after the autumnal equinox. During Harvest Moon which will take place tomorrow, Thursday, October 1st, the particular relationship between the relative angles of the sun and the moon means that the lag time between moonrise from day to day is less than usual, resulting in the moon rising for several days, very close to the time of sunset. This provides extra illumination and is a boon to framers, giving them extra light to reap their harvest after the sun goes down. 

As an architectural photographer for many years, the significance of the equinoxes for me is that between them, the sun spends some of its time in the northern hemisphere, meaning that if I want to photograph a north facing facade of a building illuminated by the sun,  I can only do it in the time after sunrise or before sunset between roughly March 22 and September 20. The picture above marks the last day until next March when the sun will be visible between the tall buildings in the east-west streets of Chicago's Loop.

It was dumb luck, literally being at the right place at the right time, that allowed me to witness Chicagohenge first hand. I noticed dozens of photographers with serious gear snapping away as the sun made its last appearance above Randolph Street for six months.  

Strangely enough, my being aboard the bus gave me a slightly better vantage than the more prepared photographers as I was in the middle of the street and several feet higher than they were.

Life isn't always fair as they say and sometimes it's better to be lucky than good. At least it made up for missing that damn tornado lat month. 

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