Repeat photography is the photographing of a scene repeatedly over a long period. It is usually applied to wilderness areas to document natural or human-caused changes over many decades. The U.S. Geological Survey has an ongoing repeat photography project involving mainly the deserts in southwest U.S. Some sets of photos from that program may be seen over at Wired.
Long before I learned that repeat photography was a formal procedure, I got interested in it on my own. My so far imprecise contributions may be seen in this post and this post.
To be most useful, a repeat photo of a scene should be taken from the same position and distance. But sometimes I learn of the existence of an older version of a picture I took when it's too late to correct for any differences in the composition. Here is an example.
Last June in Turkey, we visited the ruins of Priene. While we were at the theater, I photographed this goddess sitting in one of the front row seats that must certainly have been reserved for the notable.
Just last night, while skimming thru the Turkish archaeologist Ekrem Akurgal's Ancient civilizations and ruins of Turkey, I saw this picture of the theater of Priene.
They are the same seats photographed from a slightly different point. After going back and forth between the 2 photos several times, we decided that the seat marked with the red X in the Akurgal photo is the one in which the goddess is sitting in mine.
Akurgal's book was 1st published in 1969. The pictures in it may have been taken in the 1960s if not earlier. During the more than 40 years that separates the 2 pictures, the bottom right-hand corner of the goddess's seat broke off. We couldn't spot any other glaring differences between the 2 photos.