The Effect of the Total Energy of the Stars and the Presence of the Moon Above the Horizon on the Onset of True Dusk and True Dawn

Abstract

This study is concerned with determining the altitude of the sun under the horizon to the morning and the evening of the night sky including the true dawn and dusk in cases moonless and moon light conditions. True dusk and dawn appear only after the total energy of the stars appears or disappears as an inevitable result of twilight light. The measured of the night sky are done for four months (from May to August, 2018) in Malaysia (3.82° N, and 100.8° E). The Sky Quality Meter (SQM) was used to measure the brightness of the night sky. The measurements were taken when the device was directed to the zenith position during the entire monitoring period. The two methods that calculate the threshold for eye visibility are M and M1 in the case of the cloudless conditions. The determination of true dusk and true dawn is based on the concept of the onset of the eye's visual threshold of magnitude (m, mag/arcsec2) as M= ∆m = 1.3m (which is the absence or appearance of the total energy of the stars relative to the celestial background), M1 =NELM=6m and the relationship to the sun vertical depression of Do (degree). At the Moonless, the depression of true dusk ranges as 11.2° ≤ Do ≤14.8°, while Do of the true dawn ranges as 12.2˚ ≤ Do ≤14.5˚. In case the full moon, the beginning of distinguishing dusk and dawn light from moonlight is Do ≈ 9.5˚ at 17.3m. The mean values of the light minimum of a full-moon-case, for morning and evening twilight is Do ≈12.5˚ at 17.9m and elongation ≈171˚. The average light level magnitude throughout the night is 20.61m (scotopic vision) and is at Do ≈15˚ in the beginning of night (or ends).