AAVSO: American Association of Variable Star Observers

Sky Brightness at Zenith During the January 2019 Total Lunar Eclipse

Volume 47 number 1 (2019)

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Jennifer J. Birriel
Department of Mathematics and Physics, Morehead State University, 150 University Boulevard, Morehead, KY 40351; j.birriel@moreheadstate.edu
J. Kevin Adkins
Department of Mathematics and Physics, Morehead State University, 150 University Boulevard, Morehead, KY 40351; jkadkins@moreheadstate.edu

Abstract

Lunar eclipses occur during the full moon phase when the moon is obscured by Earth’s shadow. During these events, the night sky brightness changes as the full moon rises and then passes first into the penumbral and then the umbral shadow. We acquired sky brightness data at zenith using a Unihedron Sky Quality Meter during the 20–21 January 2019 total lunar eclipse as seen from Morehead, Kentucky. The resulting sky brightness curve shows an obvious signature when the moon enters the umbral (partial) eclipse phases and the total eclipse phase. During the total eclipse phase, the brightness curve is flat and measures 19.1 ± 0.1 mag / arcsec2. The observed brightness at totality is close to typical new moon in January night at our location, which measures 19.3 ± 0.1 mag / arcsec2. The partial eclipse phase is symmetric on either side of totality. The penumbral phase is more difficult to identify in the plot, without comparison to a typical full moon night. There is a clear asymmetry in the curve just before and just after the umbral phase. This asymmetry is probably due to changes in terrestrial atmospheric conditions, such as high altitude clouds.