Free Astronomy Magazine March-April 2023

MARCH-APRIL 2023 A bove, panorama of Los Angeles photographed in 2007 from the Griffith Park Observatory. The boxed area is the same as in the image below, taken in 1962. The exponen- tial growth, in 45 years, of light sources and the consequent skyglow is evident. [Getty Images] tion on the ground is very different because the main component of sky- glow is the light that propagates more or less horizontally and is dif- fused by the humidity in the air. Today, the familiar skyglow that forms the backdrop to our urban en- vironments can reach more than 30 kilometers beyond the last function- ing lamppost. Street lighting, bill- boards, shop signs and storefronts, sports venue floodlights, signal lights, and countless other energy- wasting sources contribute to the skyglow, which can only be measured by the human eye at its full extent. In this scenario, Kybe and colleagues focused their study on the January 2011-August 2022 period, where satellite and ground-based observa- tions are most reliable and abun- dant. Just over 51,000 estimates of naked eye limiting magnitude were selected from the Globe at Night database and integrated with satel- lite data using appropriate mathe- matical procedures. The result was an overview of the variation of the skyglow in the period considered. are, again, also the ones to which human eyesight is most sensitive in dark conditions. Paradoxically, if we evaluated the skyglow variations solely based on satellite data, we would discover that in recent de- cades the situation has not worse- ned at all and has, on the contrary, slightly improved. But our percep-