Solar activity and atmospheric attenuation effects on the visibility of stars and planets during twilight

By use of the values of the sky twilight brightness deduced at sea level at Abu Simbel in 1980 during the high solar activity period, the visibility of stars and planets of magnitudes less than 5.5 during twilight are obtained. The results are given in charts for each one degree of Sun's depression below the horizon. These charts can be applied for different altitudes above the horizon and different bearing angles from direction of sunset. Tables of corrections for different values of atmospheric attenuation and solar activity are given.