Tuesday, July 04, 2006

A modern Stonehenge

It was not in my first summer in Frankfurt that one evening in late June, just before sunset, I made a quite stunning observation: When walking home from the old physics institute, I saw, just by chance, a very remarkable shadow, which was cast by the low sun onto one of the bank towers in downtown Frankfurt. The shape of this shadow puzzled me. Then, I realised that it was cast by the tower of the Marriott hotel, a building with a very characteristic combination of three tall wings with different heights. I was fascinated, and stayed where I was, just watching the shadow wandering along the front of the bank tower. Before the shadow could leave this screen, the sun had disappeared below the horizon.

That was amazing. It seemed that just by some urbanistic coincidence, the shadow of the Marriott building could be used to indicate remarkably well the time of the summer solstice: Only around this time of the year, sunset is far enough in the Northwest, so that the shadow can reach the bank tower. Or was there a plan? Anyway, I felt some kind of archaic awe in view of this spectacle of nature.

Since that evening, I have tried to spot this event at least once each summer. Of course, the shadow appears on the bank tower not only at the exact day of the solstice, but during about three weeks around the longest day. However, I am convinced that it could be used easily to establish a calendar exact within a few days.

Last Monday was a very bright, sunny evening, and I decided to have my annual look at the spectacle, and to try to document it on photos. Sunset at the co-ordinates of Frankfurt (50.12 North, 8.68 East) is calculated by this JavaScript Sun Table Calculator to be at 21:38 MESZ (daylight savings time - 9:38 pm) on the 3rd of July. However, due to the range of the Taunus hills in the Northwest, it actually occurs 20 Minutes earlier than that, at about 21:20. At that time, the sun's altitude at is 1.4 degrees, and its azimuth is 305.3 degrees, in contrast to the nominal 308.7 degrees at sunset, which is calculated for the standard altitude at sunset of -0.83 degrees.

This azimuth is indeed in complete agreement with the angle that can be extracted from Frankfurt city maps:

Here, the upper left mark indicates the Marriott building, the lower right mark the location of the bank tower, at a distance of about 800 meters or half a mile. The line shows the direction of the shadow at sunset, with an azimuth of 306 degrees.

When I came home shortly after 8 pm on monday, I could witness another nice shadow display: About one hour before sunset, the Frankfurt Messeturm casts its characteristic pencil-shaped shadow on the very same bank tower that later shows the Marriott shadow:

You may recognise the street: the location of the photo is quite exactly where Bee was swaying the German colours two weeks ago.

Finally, one hour later, there it was, the spectacle of the Marriott shadow at midsummer:

With the last rays of sunlight, the complete shadow of the Marriott building seemed to fit just on the front of the bank tower.

Then, it was gone, together with the sun.

Sometimes, one hears the regret that living in a big city implies the loss of all authentic experience of nature. Fundamental awareness of the succession of seasons, for example, which was so important to our ancestors who built places like Stonehenge to fix these observations, would be forgotten in modern urban environments. I would argue that the contrary is true: Big cities with their skyscrapers and tall buildings offer manifold landmarks that allow to track the characteristic motion of the sun over the sky in the course of the year. Such events as the Marriott shadow may be overlooked easily in busy every-day city life, and, as it was the case for me, you may need some chance to spot them in the first place. But they are there.

You just have to open your eyes.


  1. Wow! Frankfurt -
    A Modern Stone Henge. Great title

  2. this is amazing. I absolutely love google earth, it's the greatest invention ever. Takes 2 seconds to fly from santa barbara to toronto.

  3. one can get some orientation in a city by looking to the parabolic plates

    That's a cool observation! I never took notice of that before, but yes, wiht hindsight, it's obvious ;-)

    Best regards, Stefan


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