sabato, novembre 10, 2007

Airglow fluctuations?

I recently bought a Sky Quality Meter and began making measurements of the night sky brightness. My favourite dark site, Casera Razzo (in the Alps, Italy), marked 21.45 (magnitudes per square arc second) one month ago, and 21.27 a week ago. However none of the two times was as dark as I recall, earlier in spring and summer.
I discovered that airglow, one of the most important contributions to natural skyglow may vary a lot, depending an solar activity, "gravity waves" in the atmosphere, and other causes. Variations are on time scales of minutes (auroras), hours, months and the 11 years Sun cycle. It seems that airglow may vary as much as nearly ten imes (TBC).
I had a look to the database of Unihedron SQM readings and did a little of data mining.
The picture shows the readings at the Telescopio Nazionale Galileo (Canary Island), Glaubenberg (Switzerland) and Grand Junction (USA). I think there are signs of a trend, at least for the three sites. The last months look having brighter skies: on average 0.3-0.4 magnitude per square arc seconds.