Chicagohenge on the Autumnal Equinox

Chicagohenge – an equinox sunset.

Twice a year, during the autumnal and vernal equinoxes, the sun puts on a spectacular show as it rises and sets amidst the soaring skyscrapers of Chicago’s east/west streets. I tried to capture this image during the previous two equinoxes without success. The first time I tried it I totally misjudged the sunset angle and time, while the second time I tried it the sky was completely cloudy.  This year I was finally able to capture Chicagohenge during the autumnal equinox. I snapped the above photo on Sunday, September 22 at 6:36pm.

During the Equinox, the tilt of the Earth’s axis is inclined neither away from, nor towards the sun, so the center of the sun is in the same plane as the Earth’s equator. In short, this means the sun sets right at 270° – perfectly West.  Now, if you’re thinking to yourself, “if the sun sets directly in the West, shouldn’t it be right at the the horizon in your above photo?” you’d be right. But it turns out that Chicago’s street grid is off by a fraction of a degree. So when you’re looking down the ‘concrete canyon’ you’re actually turned ever so slightly southwest, so by the time the sun meets the horizon on the autumnal equinox, it may be partially blocked by the buildings on the north side of the street.

This is both a blessing and a curse, to me anyway. A blessing because it makes for a really bright and unique sunset on the actual equinox; but a curse because the effect of the sun setting at the horizon, in between the buildings, doesn’t always happen on the actual equinox. Knowing the sun would continue it’s march south, I did what any obsessive photographer/skywatcher would do; I went back out to try and catch it at the horizon. The below shot was taken on Wednesday, September 25, three days after the autumnal equinox. Admittedly, I think the 9/24 would have been the ideal date, but this gives you a pretty good idea of what it could look like if timed perfectly.

Long story short, there are varying degrees of “Chicagohengey-ness” in the few days before and after each equinox that make each occurrence unique. In spring, the ‘henge starts to become visible a day or two before the equinox as the sun moves from south to north. Conversely, Chicagohenge may peak a day or two after the vernal equinox as the sun moves north to south. The equinox varies by a few days from year-to-year (here’s why), so the exact sunset times and angles vary slightly as well, making certainties tough to pin down. What was ‘perfect’ one year may be off by a few hours or degrees the next.  Your best bet is to get out there and witness it for yourself.

(9.25.13)-More Chicagohenge

Technical details: 

Shooting directly into the sun is tricky so here are some numbers to help you out. The top photo was shot at 1/8000, f/5.6, ISO 320, at 200mm. The below image was taken at 1/125, f/5.6, ISO 200, at 200mm. It’s pretty spectacular to see just how much the sun’s light is diffused as it approaches the horizon. That and the fact that it is partially obscured by buildings meant a much slower shutter speed was needed. Additionally, I used The Photographers Ephemeris to help me judge sunset times and the sun’s actual angle.