Photometric comparison of images taken during Earth Hour with the day prior.

Average ADU for comparison of combined exposures between Friday and Saturday. 5 images were taken between 8:00pm and 9:00pm MST (local time) then cropped to the middle 640x480. Data from ImagesPlus 3.0 Cross-Hair Analysis of the combined images, middle 640x480 (to eliminate influence of surrounding residential lighting).

Friday Saturday Delta
Lum 17022.9 16520.3 2.95%
Red 17018.5 16517.5 2.94%
Green 15433.2 14891.9 3.51%
Blue 13781.8 13254.0 3.83%
Tot/avg 63256.4 61183.7 3.28%

Image data:
Images were taken in RAW mode, with in-camera noise reduction, using remote shutter release, on tripod, from Peralta Trails clubhouse patio (telescope pad), facing directly east.

Exposure data: 20 sec., f/3.5, 18mm, ISO 400
Flat and bias frames were applied.
Processed using ImagesPlus 3.0 (Processing mode: At Capture WB - Nikon D50)
Stretch Parameters: 7000, 0.75, 7000, 1.0

An attempt was made to compare images taken on Saturday, before Earth Hour (7:45pm) with one taken during the hour. Although a reduction was seen, it was attributable to declining twilight since a similar reduction was noted when images were taken at the same times on Friday, the day before the test. It should be noted that much of the light we observe from this location emanates from Mesa, which to my knowledge did not participate in the test.