Mary got us some solar-powered lights for the deck. They have a sensor which turns them on when it starts getting dark. This is related to when the sun sets, but is also affected by atmospheric conditions—overcast skies will cause it to get dark enough to trigger the lights earlier than on a clear evening. An additional layer of variability is added because we don’t always notice exactly when the lights come on.
The blue line shows official sunset time in Oakland. The plateau at the top of the curve was midsummer. The red line records when we notice that the lights have come on.
Thanks to MJF & Jim for assistance with data collection.