Thursday, June 23, 2011

Soleri Bridge Summer Solstice (+1) Solar Noon, 2011

The sun, directly aligned with the pylons at solar noon

Business travel prevented me from wandering through the Soleri Bridge and Plaza with my parasol on the summer solstice itself. However, I am fortunate because the sun moves very little in the sky from one solar noon to the next around the day of the solstice, according to the Wikipedia article: "The days before and after the solstice, the declination speed is less than 30 arcseconds/day which is less than 1/60th of the angular size of the sun, or the equivalent to just 2 seconds of right ascension." Which is great if you happen to want to check it out on the day after the solstice, not so good if you are an astronomer, navigator, or calendar shaman trying to figure out the exact day and moment of the solstice. The difference is probably not discernible with the human eye, is what that means.

The formula for the solar angle at zenith on summer solstice is refreshingly simple, and gives the angle of a pylons of the bridge which were designed to match it: 90 - latitude + earth's axial tilt angle. Which gives a figure extremely close to 80 degrees, at this latitude, with current tilt.

On this blazing hot (112F eventually, a hot dry bicycle commute) day, only a few of us turned out for "Soleri Bridge Solstice + 1, 2011": me, and a few of Soleri's entourage who appeared to be there to talk about it in front of the camera (I may add the link here if I come across the video).

Pylon shadow a few minutes before solar noon

Shadow shrinking...

Almost gone...

Solar noon: when does a lack of shadow cause a smile of appreciation?

Photographer between the pylons, illustrating several solar phenomenon simultaneously

Camera tripod solar noon crosshairs


We are right to mark the passing of our too-few days, the changing of the seasons, the orbit of the earth around the sun, even the varying obliquity (axial tilt of the earth)(Wikipedia again): "ε = A + B sin(C(T + D)); with A = 23.496932° ± 0.001200°, B = − 0.860° ± 0.005°, C = 0.01532 ± 0.0009 radians/Julian century, D = 4.40 ± 0.10 Julian centuries, and T, the time in centuries from the epoch of 2000 as above. This means a range of the obliquity from 22° 38’ to 24° 21’, the last maximum was reached in 8700 BC, the mean value occurred around 1550 and the next minimum will be in 11800. This formula should give a reasonable approximation for the previous and next million years or so. Yet it remains an approximation in which the amplitude of the wave remains the same, while in reality, as seen from the results of the Milankovitch cycles, irregular variations occur. The quoted range for the obliquity is from 21° 30’ to 24° 30’, but the low value may have been a one-time overshot of the normal 22° 30’" 

In the year 11800, 9789 years from now, the angle of these Soleri Bridge pylons would differ enough from the zenith angle on the solstice due to the variance in obliquity that they will still throw a shadow at solar noon. To any blogging astronomer cyclist of that year who happens to want to visit the bridge to see the pylons in all their shadow-free momentary signifying glory, just beam over here earlier or later, when the solar noon is about 1 degree lower in the sky. Now, I don't actually have any idea at all if this bridge, bicycles, blogging, or Scottsdale itself will be around 9789 years from now. However, I do hope they still have parasols. Get up. Go ride.


  1. Thank for sharing... I learned several new things! And, I definitely need to get up and go ride.

  2. I have never stopped long enough to think about a day or moment when there could be no shadow cast. That is just totally cool.

  3. Finally!
    Oh yes, some bridge goodness!


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