APOD: Chicagohenge: Equinox in an Aligned City (2018 Mar 20)

[APOD Robot]

[img]https://apod.nasa.gov/apod/calendar/S_180320.jpg[/img] Chicagohenge: Equinox in an Aligned City

Explanation: Sometimes, in a way, Chicago 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 last 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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[wondering]

This pic seems to make its rounds around equinoxes. Perhaps, you can clarify something which always bothered me about it. If we are looking true west wouldn't the sunset imply the sun is actually on the horizon (or half of the Sun is on the horizon)? This looks like the Sun is a few degrees above and will actually set north of the road shown.

[Guest]

The funny thing is that it simply is not true that the sun rises exactly east and sets exactly west on equinox day. It is not true because of the refraction of light in the Earth's atmosphere. The center of the unrefracted disk of the sun rises in the east, that's correct. But at that time the center of the refracted sun is already half a degree (more than its diameter!) above the horizon. And it has risen at a more northerly direction, in moderate northern latitudes by around 4 minutes earlier and 0.5-1.5 degrees more northerly.
Uli Bastian, Heidelberg, Germany

[Guest]

But even more: The instant of sunrise/sunset in most countries of the world is not defined as the rise of even the refracted cener of the sun's disk, but as the rise of its refracted upper edge. This adds another quarter degree to the discrepancy between the geometric/theoretical sunrise and the conventional sunrise. Only the former is due east in equinox day, the latter is elsewhere, as explained above.
Summary: At the time of the conventional sunrise, the unrefracted center of he sun's disk is about three quarters of a degree, or 1.5 diameters below the horizon. Same holds for the sunset.
Uli Bastian, Heidelberg, Germany

[Guest]

Two more little remarks:

1) Exactly on the equator, the sun indeed rises due East and due West on equinox day, because there the refraction effect and the apparent motion of the sun are parallel: both straight up. But in all other places the sun does not rise due east. And at the north pole, it has risen (just once!) already days before spring equinox and will set (just once!) only days after autumn equinox. Both in a direction due south !

2) Despite all the above (I mean my three postings together), the Chicago APOD needs not be a fake. If the street indeed points due east/west, the sun will actually be above the horizon on the west. And in order to be so far above the horizon as in the picture, the street has to gently slope westwards by about 1 degree in addition. Only then, but then indeed, the picture can show due west on equinox day. Chicago citizens may check for the slope ...

Uli Bastian, Heidelberg, Germany

[De58te]

@ wondering. Technically the vernal equinox happens today at 11:15 a.m. Chicago time, so the sun wouldn't be setting, but high above around high noon position. If you wanted to see the sun set at equinox you'd have to be east of Chicago, say out in the Atlantic Ocean somewhere. For the most part though the few hours after doesn't matter to most people where the sun sets since the difference is so minute it only matters to sticklers of mathematical precision.

[Whiskybreath]

Not a good configuration for drivers. I bet there are many more accidents than there would otherwise be at such times.

[wondering]

@Guest, @d58te Thank you for the explanation. Would this be an accurate summary? Equinox by definition is when the Sun is on the celestial equator. When that occurs, the Sun may not be rising in Chicago (in the case of this APOD) and may rise 0-12 hours later. Since there are 365 days in a year and 360 degrees in a circle, this could imply the Sun has moved beyond the Equinox point by ~1/2 deg in 24 hours. This is ~1x diameter of the Sun discrepancy. Further, given the light travel time, we tack on 4 minutes delay or about 1/8th of a degree (that does not seem that important). And, the refraction of Sun's light in Earth's atmosphere changes its apparent position on the sky by a few minutes. This should only be ~0.25 diameter of Sun if it really is just a few minutes. But, lets be generous and allow ~1x for refraction. The latitude of the observer should not factor here except for the position of the Sun at noon.
If I look at the APOD, I see a discrepancy of ~3x diameter of Sun, which seems too large for the above factors. What am I doing wrong?

[suicidejunkie]

Not a good configuration for drivers. I bet there are many more accidents than there would otherwise be at such times.

It is going to line up with straight roads twice a year anyways in most places. At least with due east/west it is a well-known day.

Unless you make your roads bend so there are always buildings to block the setting sun. But that is not cool.

[Jdx]

Bet you guys are tons of fun at parties.

[heehaw]

Not a good configuration for drivers. I bet there are many more accidents than there would otherwise be at such times.

It would have been MUCH better if the Streets all parallel and perpendicular to the Avenues (all parallel) had been not North-South but rather Northeast-southwest, in which case no one would EVER be driving straight into the rising or setting sun. Sigh!

[Chris Peterson]

Not a good configuration for drivers. I bet there are many more accidents than there would otherwise be at such times.

It would have been MUCH better if the Streets all parallel and perpendicular to the Avenues (all parallel) had been not North-South but rather Northeast-southwest, in which case no one would EVER be driving straight into the rising or setting sun. Sigh!

True. But in Chicago, on the solstices the rising and setting Sun are only 12° from those roads, about two palm widths. So if you were right downtown, the buildings would probably block it. But over the vast majority of the city, it wouldn't be much of an improvement.

Maybe they should just build the whole city on a turntable! (Of course, in the not too distant future most of the cars will be driving themselves, so it won't matter anymore.)

[neufer]

Not a good configuration for drivers. I bet there are many more accidents than there would otherwise be at such times.

It is going to line up with straight roads twice a year anyways in most places. At least with due east/west it is a well-known day.

Unless you make your roads bend so there are always buildings to block the setting sun. But that is not cool.

Roads that bend are even more dangerous during sunrise/sunset:

The dazzling sunsets that kill 36 drivers in 12 months: Glare contributes to 3,000 accidents and is particularly dangerous at this time of year

By Ray Massey for the Daily Mail
Published: 19:03 EDT, 15 October 2013

<<The glare of the setting sun during rush hour is to blame for 36 deaths a year, a report from the AA warns today. The danger is particularly high now as autumn turns to winter and the sun sets at around 6pm – just as most drivers are heading for home.

It plays a part in nearly 3,000 accidents every year with drivers at risk of being temporarily ‘blinded’ by the dazzle of the sun on the windscreen.

The biggest danger is on minor roads.

AA president Edmund King said most drivers were sensible enough to slow down when they are dazzled. ‘But where a dazzling sunset gets particularly nasty is when the road turns unexpectedly into it or the glare appears from behind trees or buildings or by reflection,’ he said. ‘Drivers can’t gamble that it will change quickly – in the couple of hundred yards that takes to happen, there may be a pedestrian, cyclist or jogger. Likewise, overtaking into low sunlight when the road ahead is obscured is risking disaster.’

The AA also points out that the rate of head-on crashes involving lorries nearly quadruples in twilight conditions. ‘If the vehicle casts a long shadow in front of it, it is very likely that oncoming drivers and those coming out of turnings will have difficulty seeing it,’ says the report.

The AA says that the danger of sunset coinciding with the rush hour poses different challengers for different road users.

It urges drivers to ‘watch the backs’ of joggers, dog walkers and pedestrians as these groups are almost twice as likely to be killed or seriously injured in road accidents if they have their backs to the vehicles. The AA says its own research based on official 2004 pedestrian casualty statistics, shows that 10.8 per cent of the 5,566 pedestrians killed or seriously injured when in the road were walking or running with their backs turned away from the traffic. This compares with 5.9 per cent of casualties who were directly facing oncoming cars.
----------------------------------------------------------------------------------------

• • AA RULES FOR SUNSET DRIVING
  Keep your windscreen clean – including the inside.
  
  If blinded, slow down immediately. It is tempting to carry on regardless to let the glare pass but by then it may be too late.
  
  If driving at sunset, anticipate the effects of glare on you and other drivers. Drivers heading west or through terrain where the sun may appear suddenly need to expect to travel more slowly than usual.

[Craig Willford]

If you have ever done celestial navigation aboard a boat at sea, you will remember that you have to estimate the height of the sextant above the water at the time of the shot. After all, the higher you are, the farther over the horizon you can see. Draw some exaggerated drawings of a globe and a sailor and then draw lines from his eyes to the sun and you'll see that he is not looking parallel to a tangent to the Earth.

Note here that is up around the third story of the office buildings, about twice the height of the stop signs. That would convert to a similar picture, taken at the same moment but at street level, with the sun much lower. Imagine lowering the line from camera to sun without tilting as you lower, until the camera is at street level. That will account for much of the apparent elevation.

[Sa Ji Tario]

In these conditions of dawn and dusk you have to be careful with the moment when the angle of light is made of total reflection that is very blinding and there are many possibilities of accidents with vehicles. Even accidents were recorded in the flight of the birds.
If you are in the equator and in a street that runs from east to west there you will see the sun rise and "fly" over it during the equinoxes, in the high north and south latitudes you will see it moving north in the south and vice versa in the north, what I said about the total reflection is that I worked as a transit officer in my country.

[Craig Willford]

Let me say that one can make an estimate of the angle of the sun as viewed from the ground, not up three stories high, as I had estimated in my previous post that I thought the camera might have been. There is a clue in the center of the picture: a pedestrian dangerously in the crosswalk against the green light for east-west traffic.

I assume the pedestrian to be a female wearing a skirt or dress. If we estimate the distance from the street to the hem of the skirt to be 2.5 feet and the shadow cast of that same distance to be on level ground and estimate that distance to be 30 feet, then the arctan of (2.5/30) is 0.08314 radians or 4.76 degrees. All of this is estimate and presumption and the result came up with a flawed answer, because that amounts to more than 9 sun diameters and clearly it is less than that. But at least the process is available and fun to experiment with.

Lastly, if we can find each other at parties, you bet the contributors to this "Discuss" forum are "fun".

[Alan King]

The photo shows the sun perfectly aligned with the street, but clearly above the horizon. If the photo was taken on equinox day, it should be exactly at the horizon, whether east for sunrise or west for sunset. I can suggest two explanations:
1) The photo was taken either a few days after the spring equinox, or a few days before the fall equinox;
2) The streets were laid out according to magnetic north, rather than to the axial north pole.
I suspect the latter, both for Chicago and for Washington, D.C., where (77 degrees west longitude) I've expected to see the sun due south at solar noon -- 12:08 pm standard time, 1:08 pm daylight time. But it doesn't happen. Hence, my guess that Washington is laid out according to magnetic north.
That's the best I can do. I'd welcome advice. My motto: "I can always be corrected by facts."

[neufer]

The photo shows the sun perfectly aligned with the street, but clearly above the horizon. If the photo was taken on equinox day, it should be exactly at the horizon, whether east for sunrise or west for sunset. I can suggest two explanations:

1) The photo was taken either a few days after the spring equinox, or a few days before the fall equinox;

2) The streets were laid out according to magnetic north, rather than to the axial north pole.
I suspect the latter, both for Chicago and for Washington, D.C., where (77 degrees west longitude) I've expected to see the sun due south at solar noon -- 12:08 pm standard time, 1:08 pm daylight time. But it doesn't happen. Hence, my guess that Washington is laid out according to magnetic north.

That's the best I can do. I'd welcome advice. My motto: "I can always be corrected by facts."

Is the Illinois/Indiana border also based upon magnetic compass lines

• https://tinyurl.com/yb5qd9tq

[Yup]

Bet you guys are tons of fun at parties.

Star parties are a gas! (Especially when observing nebulae).