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.