On the beginning of the morning twilight based on sky brightness measurements

Abstract

We present the results of 83 measurements of night sky brightness with portable photometers for five locations, namely the Bosscha Observatory, Cimahi, Bandung, Yogyakarta, and Kupang during the moonless days from years 2011 to 2018. In the first place, instead of the zenith direction, we also evaluated the night sky brightness in various angles of the zenith distances. At the beginning of the morning twilight, solar dips were significantly affected by light and air pollution. It can cause a pseudo night effect, resulting from absorption by particles of low atmospheric height, which is due to air pollution. The duration of twilight is also influenced by the pollution level. A new tool to determine the boundaries of astronomical, nautical, and civil twilight using sky brightness difference was introduced. We also recommend a preferred solar dip of 18.5° for Islamic fajr prayer in Indonesia with twilight duration at 70.5 minutes.