I've added a CdS photocell to my network of home temperature sensors. I'm reading it as an analog input using the WebServer Arduino code that goes with the Ethernet shield.
I'm calling this a Twilight Sensor because the sensor range I happened to choose covers that half-light of mornings and evenings. I've cooked up a special FLOT graph that traces equi-temporal contours from day to day.
http://c2.com/~ward/arduino/twilight.cgi?hours=360
If you study this long enough you will see that, yes, the days are getting longer.