The Brightness of Starlink and OneWeb Satellites During Ingress and Egress from Terrestrial Eclipses

Abstract

A model that combines celestial geometry and atmospheric physics is used to calculate the dimming of artificial satellites as they enter and exit the Earth's shadow. Refraction of sunlight by the terrestrial atmosphere can illuminate a satellite while it is inside the eclipse region determined from geometry alone. Meanwhile, refraction combines with atmospheric absorption to dim the satellites for tens of km outside of that region. Spacecraft brightness is reduced more in blue light than in red because absorption of sunlight is stronger at shorter wavelengths. Observations from the MMT-9 robotic observatory are consistent with the model predictions. Tables of satellite brightness as functions of their location in the eclipse region are provided.