# Using a shadow to estimate the date a photo was taken

This post might be a bit nerdy, but at least it runs in the family: My dad was recently trying to find out when this photo inside a church had been taken.

Photo of the inside of the “Maria Magdalena” church in Hamburg Klein-Borstel, taken some time before its official opening in 1938. (Picture used with permission; HAA_ORh_028.9_(0567) of the Hamburgisches Architekturarchiv)

Since some parts of the altar aren’t finished yet, we knew it had been taken some time before the church was opened on December 11th, 1938. We also know that the church runs east-west, so in the picture above, we are facing east, south is to the right.

Zooming in on the shadows the benches make on the floor, we see that they are close to parallel to the joints in the floor, hence the sun must be pretty much south.

Detail of the photo above. (Picture used with permission; HAA_ORh_028.9_(0567) of the Hamburgisches Architekturarchiv)

Now we can estimate the height of the benches and the length of the shadows. Actually, we only need the ratio.

Detail of the photo above. (Picture used with permission; HAA_ORh_028.9_(0567) of the Hamburgisches Architekturarchiv)

Using all the random bits of trigonometry we can remember, we can calculate the angle of the sun in the sky.

Then, we can use that angle to go to a page like  http://www.sonnenverlauf.de to find the date on which the sun is at that angle when also standing directly in the south.

Ignoring details like summer/winter times and the small angle we see between the shadow and the joints, it turns out that the picture was taken in early October.

Or at least that’s what I first thought, assuming that the church will not have been ready in early March if it only officially opened in December. Then my dad pointed out that you can see numbers of the songs up on the wall: Number 213, 391, 392, and 394.

Photo of the inside of the “Maria Magdalena” church in Hamburg Klein Borstel, taken some time before its official opening in 1938. (Picture used with permission; HAA_ORh_028.9_(0567) of the Hamburgisches Architekturarchiv)

In 1938, that church most likely used the “Hamburgisches Gesangbuch, Einheitsgesangbuch der Evang.-luth. Landeskirche in Schleswig-Holstein-Lauenburg, Hamburg, Mecklenburg-Schwerin, Lübeck, Mecklenburg-Strelitz, Eutin” from 1930. Now if we had that song book, we could probably see whether the picture was taken in spring or fall.

But since I did all the heavy lifting with the trigonometry, it is now my dad’s turn to hunt down the song book and find out. I will report back! :-)