APOD: Chicagohenge: Equinox in an Aligned City (2024 Sep 22)

[APOD Robot]

Chicagohenge: Equinox in an Aligned City

Explanation: Chicago, in a way, is like a modern Stonehenge. The way is east to west, and the time is today. Today, and every equinox, the Sun will set exactly to the west, everywhere on Earth. Therefore, today in Chicago, the Sun will set directly down the long equatorially-aligned grid of streets and buildings, an event dubbed #chicagohenge. Featured here is a Chicago Henge picture taken during the equinox in mid-September of 2017 looking along part of Upper Wacker Drive. Many cities, though, have streets or other features that are well-aligned to Earth's spin axis. Therefore, quite possibly, your favorite street may also run east - west. Tonight at sunset, with a quick glance, you can actually find out.

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[JimT]

But the sun is not on the horizon in this picture, and because it sets at an angle to the right, by the time it touches the horizon it would likely be behind the buildings on the right. What am I missing here?

[gmPhil]

Today, and every equinox, the Sun will set exactly to the west, everywhere on Earth.

"Everywhere" on Earth? Just a minute... if you're on the North Pole, then every direction you look is due south, so ... (And vice versa for the South Pole.)

[tab]

But the sun is not on the horizon in this picture, and because it sets at an angle to the right, by the time it touches the horizon it would likely be behind the buildings on the right. What am I missing here?

Which means, simply, that something about this picture is a lie. Either it wasn't taken on the equinox or Wacker Drive is not, in fact, exactly east-west. Perhaps Chicago just wants to "be cool" like New York (see the link in the description to Manhattanhenge -- where the sun IS on the horizon and in the middle of the street).

[johnnydeep]

But the sun is not on the horizon in this picture, and because it sets at an angle to the right, by the time it touches the horizon it would likely be behind the buildings on the right. What am I missing here?

Which means, simply, that something about this picture is a lie. Either it wasn't taken on the equinox or Wacker Drive is not, in fact, exactly east-west. Perhaps Chicago just wants to "be cool" like New York (see the link in the description to Manhattanhenge -- where the sun IS on the horizon and in the middle of the street).

I guess it depends on where exactly on Wacker Drive this picture was shot. Only one part of it is aligned exactly East-West. Another large part is slightly South East:

[Chris Peterson]

But the sun is not on the horizon in this picture, and because it sets at an angle to the right, by the time it touches the horizon it would likely be behind the buildings on the right. What am I missing here?

Which means, simply, that something about this picture is a lie. Either it wasn't taken on the equinox or Wacker Drive is not, in fact, exactly east-west. Perhaps Chicago just wants to "be cool" like New York (see the link in the description to Manhattanhenge -- where the sun IS on the horizon and in the middle of the street).

I guess it depends on where exactly on Wacker Drive this picture was shot. Only one part of it is aligned exactly East-West. Another large part is slightly South East:


wacker drive chicago.jpg

The image was shot from Wacker Place, not Wacker Drive (but is looking down Wacker Drive, which is a few blocks away), just east of Michigan Avenue, with the Carbide and Carbon Building just on the left. Both streets run east/west, but there is a slight offset between them, so I'm not sure it's possible from where this shot was taken to look perfectly west and see right down the middle of the road this way. It might require having the camera rotated very slightly to the north, which would be consistent with the position of the Sun here when it's a couple of solar diameters above the horizon still.

[Cousin Ricky]

I saw Chicagohenge in September of 2021. Regretably, I didn’t get a photograph. Chicagohenge didn’t occur to me until I was riding the El shortly before sunset, and I am slow getting my camera out.

[johnnydeep]

But the sun is not on the horizon in this picture, and because it sets at an angle to the right, by the time it touches the horizon it would likely be behind the buildings on the right. What am I missing here?

Which means, simply, that something about this picture is a lie. Either it wasn't taken on the equinox or Wacker Drive is not, in fact, exactly east-west. Perhaps Chicago just wants to "be cool" like New York (see the link in the description to Manhattanhenge -- where the sun IS on the horizon and in the middle of the street).

I guess it depends on where exactly on Wacker Drive this picture was shot. Only one part of it is aligned exactly East-West. Another large part is slightly South East:


wacker drive chicago.jpg

Serves me right for relying on "Wacker Drive" being mentioned exclusively, with no mention of "Wacker Place" to be found (is it on the "X" page, which I don't have an account for?). But seriously, how did you determine this? Just familiarity with Chicago streets in the area?

And here's a better map:

[gmPhil]

In fact (further to my earlier observation), equinox at the poles should be quite interesting... you'd see the sun roll 360 degrees around the horizon!

[Chris Peterson]

Which means, simply, that something about this picture is a lie. Either it wasn't taken on the equinox or Wacker Drive is not, in fact, exactly east-west. Perhaps Chicago just wants to "be cool" like New York (see the link in the description to Manhattanhenge -- where the sun IS on the horizon and in the middle of the street).

I guess it depends on where exactly on Wacker Drive this picture was shot. Only one part of it is aligned exactly East-West. Another large part is slightly South East:


wacker drive chicago.jpg

Serves me right for relying on "Wacker Drive" being mentioned exclusively, with no mention of "Wacker Place" to be found (is it on the "X" page, which I don't have an account for?). But seriously, how did you determine this? Just familiarity with Chicago streets in the area?

And here's a better map:


wacker drive v wacker place.jpg

I saw the name on the building at the left (Carbide and Carbon) and the street sign for Michigan Avenue. Combine that with Google Maps and it becomes pretty clear where the picture was taken from. (Given the vertical angle, I might guess the Stetson overpass, or maybe even farther back.)

[johnnydeep]

I guess it depends on where exactly on Wacker Drive this picture was shot. Only one part of it is aligned exactly East-West. Another large part is slightly South East:


wacker drive chicago.jpg

Serves me right for relying on "Wacker Drive" being mentioned exclusively, with no mention of "Wacker Place" to be found (is it on the "X" page, which I don't have an account for?). But seriously, how did you determine this? Just familiarity with Chicago streets in the area?

And here's a better map:


wacker drive v wacker place.jpg

I saw the name on the building at the left (Carbide and Carbon) and the street sign for Michigan Avenue. Combine that with Google Maps and it becomes pretty clear where the picture was taken from. (Given the vertical angle, I might guess the Stetson overpass, or maybe even farther back.)

My, what impressive observation and deduction skills you have there Sherlock!

[Chris Peterson]

Serves me right for relying on "Wacker Drive" being mentioned exclusively, with no mention of "Wacker Place" to be found (is it on the "X" page, which I don't have an account for?). But seriously, how did you determine this? Just familiarity with Chicago streets in the area?

And here's a better map:


wacker drive v wacker place.jpg

I saw the name on the building at the left (Carbide and Carbon) and the street sign for Michigan Avenue. Combine that with Google Maps and it becomes pretty clear where the picture was taken from. (Given the vertical angle, I might guess the Stetson overpass, or maybe even farther back.)

My, what impressive observation and deduction skills you have there Sherlock!

It's elementary, my dear Watson!

[johnnydeep]

I saw the name on the building at the left (Carbide and Carbon) and the street sign for Michigan Avenue. Combine that with Google Maps and it becomes pretty clear where the picture was taken from. (Given the vertical angle, I might guess the Stetson overpass, or maybe even farther back.)

My, what impressive observation and deduction skills you have there Sherlock!

It's elementary, my dear Watson!

The same approximate street view:

And here is a direct link to Google Maps

[Aries]

Just some guessing here. Something is definitely wrong. The description just says mid Sept. so?? I'm sure that the street is laid out east west and the picture is taken from some floors up in the building built over the street below and the sun at infinity is pretty much west. Therefore what fits time wise with the 1/2 degree dia. sun up about 3 diameters, is that it is actually Sept. 19th at 6:39PM when the sun's altitude there is 1.4 deg. and declination is 270 or due west. The sun will go down to the right at an angle of approximately the latitude of 41 deg., 9 minutes later. Of course this is all just a possible explanation! Would be nice to know. Actually many supposed equinox photos show the sun up like this while in the supposed east west alignment at sunrise or sunset!

[Chris Peterson]

Just some guessing here. Something is definitely wrong. The description just says mid Sept. so?? I'm sure that the street is laid out east west and the picture is taken from some floors up in the building built over the street below and the sun at infinity is pretty much west. Therefore what fits time wise with the 1/2 degree dia. sun up about 3 diameters, is that it is actually Sept. 19th at 6:39PM when the sun's altitude there is 1.4 deg. and declination is 270 or due west. The sun will go down to the right at an angle of approximately the latitude of 41 deg., 9 minutes later. Of course this is all just a possible explanation! Would be nice to know. Actually many supposed equinox photos show the sun up like this while in the supposed east west alignment at sunrise or sunset!

If you're doing any calculations based on solar diameter, note that the Sun as seen in this image is almost certainly subtending a much larger angle than 1/2 degree because it is so overexposed. We can probably figure its actual size from the much less bright diffraction spikes off of the lens's iris.
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Chicagohenge_Artese_1320_sun.jpg (52.64 KiB) Viewed 2485 times

[fredturtle42@comcast.net]

This cannot possibly be Wacker Drive. The Chicago River runs along the north side of the east/west portion of Wacker Drive. There are no skyscrapers on the north side of the street.

[Chris Peterson]

This cannot possibly be Wacker Drive. The Chicago River runs along the north side of the east/west portion of Wacker Drive. There are no skyscrapers on the north side of the street.

Please read the discussion above.

[Aries]

Thanks Chris,
I didn't think of that and using your ray lines on my print came to more like 4.5 solar diameters high in photo. And I just used a solar dia. of 1/2 degree when it really was 31.83' or .53 deg. Yes I am using a program for local ALT/AZ for Chicago and changing the time by seconds to get the Sun really close to 270 AZ for each day. These are the times and theoretical solar diameters (.5 deg.) in ALT for the sun west: SEPT 16 at 6:34:58PM = 6.8 DIA; SEPT 17 at 6:36:21PM = 5.6 DIA; SEPT 18 at 6:37:43PM = 4.48 DIA; SEPT 19 at 6:39:06PM = 3.3 DIA; SEPT 20 at 6:40:29PM = 2.13 DIA; SEPT 21 at 6:41:52PM = 1 DIA and on SEPT 22 at 6:43:09PM = -.18 DIA. So using your new solar diameter, my new estimate is that this photo was taken Sept. 18 near 6:38PM. I wonder what else I'm assuming wrong? Any more ideas. I'm just trying to have some fun.

A side Note: I have a large accurate sundial laid out on my front drive and enjoy seeing the solar spot travel down the equinox line on these days. The line was laid out over many years so it is an average of the fluctuations in when the equinox occurs during the day. Actually the spot moves a little off line during the day as the declination changes.

[Chris Peterson]

Thanks Chris,
I didn't think of that and using your ray lines on my print came to more like 4.5 solar diameters high in photo. And I just used a solar dia. of 1/2 degree when it really was 31.83' or .53 deg. Yes I am using a program for local ALT/AZ for Chicago and changing the time by seconds to get the Sun really close to 270 AZ for each day. These are the times and theoretical solar diameters (.5 deg.) in ALT for the sun west: SEPT 16 at 6:34:58PM = 6.8 DIA; SEPT 17 at 6:36:21PM = 5.6 DIA; SEPT 18 at 6:37:43PM = 4.48 DIA; SEPT 19 at 6:39:06PM = 3.3 DIA; SEPT 20 at 6:40:29PM = 2.13 DIA; SEPT 21 at 6:41:52PM = 1 DIA and on SEPT 22 at 6:43:09PM = -.18 DIA. So using your new solar diameter, my new estimate is that this photo was taken Sept. 18 near 6:38PM. I wonder what else I'm assuming wrong? Any more ideas. I'm just trying to have some fun.

A side Note: I have a large accurate sundial laid out on my front drive and enjoy seeing the solar spot travel down the equinox line on these days. The line was laid out over many years so it is an average of the fluctuations in when the equinox occurs during the day. Actually the spot moves a little off line during the day as the declination changes.

In line with a comment I made earlier, it may be a mistake to assume we're looking perfectly west down that street. The imager may not have been exactly on the street's centerline. Try using the foreground curbs to construct a vanishing point... which should be true west. When I do this, I end up with the Sun almost one full solar diameter south of true west. I haven't played with the numbers like you have, but obviously we'd expect the Sun to be a little south of west shortly before setting on the equinox (possibly further adjusted by recognizing that the equinox isn't simply that date, but a specific time, and the Sun is low enough that atmospheric refraction may be starting to alter its apparent position).

[JimT]

Yikes! What have I started here?
Thank you all for your considerable efforts in addressing my question.
JimT