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The Starlink satellites are shaped like flat panels. The flat sides facezenith and nadir during normal operations. Their brightness is determined bythe product of the solar illumination on the downward facing side of the panelmultiplied by the area of that side projected toward the observer on Earth.This geometry leads to a unique brightness function that is not shared by othersatellites. For example, the observed brightness is very sensitive to the solarelevation angle. There are circumstances where sunlight only illuminates theupward facing side of the satellite rendering it invisible to Earth-basedobservers. A brightness model depending on the solar aspect and the observeraspect of the flat panel, in addition to the satellite distance, is described.Absolute brightness is the only free parameter of the model, and it is taken tobe that at a distance of 1,000 km when the solar and observer factors areunity. This model has been successfully fitted to a set of observed magnitudes.The absolute visual magnitude of a Starlink satellite as determined from thisfitting is 4.1 +/- 0.1. The model could be used to determine the absolutemagnitude of the Starlink satellite known as Dark Sat which has a speciallow-albedo coating.

A Flat-Panel Brightness Model for the Starlink Satellites and Measurement of their Absolute Visual Magnitude