Passive polarization clock!

Most polarization sundials are far more accurate, but they need the user to turn one piece and match grey levels. This one you just hang in a south-facing window.

Noon

3:30 pm

5:00 pm

At the moment, it runs counterclockwise (so I should have used the north window) and does a 180 degree turn rather than 360, but I know how to fix both of these problems.

I found this powerpoint presentation after I made the above, which describes both the design above and the "active" half-shade analyzer.