The writer carried out sky brightness measurements at zenith on clear days beginning in March 2023. A Sky Quality Meter manufactured by Unihedron was used for all measurements. In all cases, sky brightness values are in units of stellar magnitude/square arc-second. A Celestron neutral density filter with a transmission of 6.4 % was used for daytime measurements since the instrument cannot measure high levels of brightness. The goals of this study are to better understand the twilight arc of Venus and to collect brightness measurements of the daytime sky. These measurements can then be used as a baseline for measuring the sky brightness during solar eclipses. Some of the conclusions of this study are: The mean sky brightness for sunrise and sunset are 7.27 (0.099) and 7.45 (0.143) magnitudes/arc-second, respectively. The standard deviations are in parentheses. The mean sky brightness at the boundary of civil and nautical twilight before sunrise and after sunset are 13.68 (0.21) and 13.94 (0.20), respectively. In both cases, the difference between sunrise and sunset are statistically significant at the 95 % confidence level using the t-Test. It is also concluded that the summer haze brightens the sky by several tens of percent. The sky brightens in a nearly linear manner for solar zenith angles between 30° and 80°. This linear relationship breaks down for zenith angles higher than 80°. My summer measurements are consistent with a sky brightness of 1.9 (0.7) at a zenith angle of 0°. Finally, it is concluded that the time it takes for the sky to darken by a factor of 10 compared after sunset (or brighten by this same amount before sunrise) ranges from 16 minutes near summer solstice to 14 minutes at equinox. If Venus’ sky behaves in a similar manner then this would have to be taken into account when measuring its twilight arc.


The is grateful for a faculty development grant in 2014 that enabled him to purchase and SSP-4 photometer.

This document is currently not available here.