Empirical Earth · The Sky — Sun, Moon & Stars

Earthshine — the Earth lighting the Moon

That is earthshine — sunlight bounced off the daylit Earth onto the lunar night side; brightest at crescent, exactly two-sphere geometry.

The claim

“The faint glow on the Moon’s dark part proves the Moon makes its own light.”

What is true

The dark part of the Moon is not perfectly black. There is a faint glow there, and anyone with binoculars can see it.

What is measured

The faint glow on a crescent Moon’s dark side is sunlight bounced off Earth. From the Moon a “full Earth” shines ~50× brighter than our full Moon.

What would show this is wrong

an ashen glow that did not track Earth’s illuminated phase as seen from the Moon, brightest at new Moon here.

Sources

  1. Earthshine (Da Vinci glow). Sunlight reflected off the daylit Earth lights the Moon’s night side; explained by Leonardo da Vinci (~1510). From the Moon a full Earth is ~50× a full Moon. plan link
  2. Earthshine measures Earth’s albedo — and its slow changes. Big Bear Solar Observatory photometry of the Moon’s ashen light gives a mean terrestrial albedo of 0.297 ± 0.005 (Goode et al. 2001 link
  3. Earth’s spectrum in earthshine. Spectra of earthshine — sunlight reflected off Earth onto the Moon’s night side and back — show Earth’s own biosignatures, including the molecular-oxygen band link
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