by David Kingsley
Did the fog block the valley lights?-Leonard
Last night's valley fog clearly reduced the usual light domes from San Jose and San Francisco. The winter Milky way was clearly visible all the way from Cassiopia across the sky through Monocerus and Canis Major, the kind of light dome I like when observing!
I bought a Sky Quality Meter when they first came out this summer for about $80, and now routinely use this standardized photometer to take quick quantitative measurements of sky brightness on different nights. The Sky Quality Meter is about the size of a deck of cards, has a clear window that faces the sky, and makes an average sky brightness reading based on about an 80 degree swatch of overhead sky (roughly 40 degrees off zenith in all directions when pointed directly overhead, see http://www.unihedron.com/projects/ darksky/). My readings last night at Montebello were about half a magnitude darker than my last trip to Montebello on a fogless night in early December (background sky brightness of 20.6 magnitudes per square arcsecond vs. 20.1 on December 6th). That corresponds to roughly a one color block darkening in the light pollution maps shown here: http://cleardarksky.com/lp/Montebello_CA.html.
Even with last night's fog, Montebello is still a near town location. For comparison, the darkest readings I have ever recorded meter were between 21.7 and 22.0 magnitudes per square arcsecond (Timor Cottage in Australia). That's at least one to one and half magnitudes darker than Montebello last night, even with the valley fog.
After all the rain we have been having, it felt wonderful to get out with a telescope. Happy holidays to everyone and best wishes in 2006,
Posted on sf-bay-tac Dec 24, 2005 19:00:30 PT
Converted by report.pm 1.2 Mar 16, 2006 22:53:49 PT