Diagram of earthshine formation

A not-to-scale cartoon of the sun-earth-moon system viewed from the pole of earth's orbit. In the top panel, the earth's topocentric phase angle, α, with respect to BBSO is defined. The plot also shows the moon's selenographic phase angle, θ, with respect to one of the fiducial points (Grimaldi) used in the observations made from BBSO (also indicated). β is the angle between the sunlight that is incident somewhere on the earth and reflected, as earthshine, to Grimaldi. θ0 ( = β - α) is the angle between the earthshine that is incident, and reflected from the moon. The path of the earthshine is indicated by the broken lines. θ0 is of order 1°, or less. In the lower panel the same diagram is drawn for a negative lunar phase angle, and extra features like the moon's orbit around the earth are indicated. On both panels the aspect of the moon as would be seen from BBSO is also indicated in a box. The light-shaded areas of the earth indicate the approximate longitudes range that contributes to the earthshine. Note how for positive lunar phases (top panel) the earthshine contribution comes from longitudes east of BBSO while for negative phase (lower panel) angles it comes from longitudes west of BBSO.
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Image credit: Earthshine Project, BBSO.