Empirical Earth · The Sky — Sun, Moon & Stars

Moon phases are sunlight on a sphere — including the Moon you see by day

Phases are the sunlit half of a sphere seen from a moving Earth; the lit edge always faces the Sun (even by day), and earthshine lights the rest.

The claim

“Phases prove the Moon is self-luminous, or hidden by a shadow object.”

What is true

The Moon does have a dark portion, and the phases run exactly as described.

What is measured

Phases are the sunlit half of a sphere viewed from a moving Earth, predictable to the minute, with the lit edge always toward the Sun (visible even by day), the dark part faintly Earth-lit, and the horizon “size change” a measured illusion. None of it needs a self-luminous Moon or a shadow object.

What would show this is wrong

a lunar phase that does not match the Moon’s measured angle from the Sun, or a daytime Moon whose lit side points somewhere other than the Sun, neither of which has ever been observed.

Sources

  1. Moon phases and the daytime Moon. Phases arise from the Sun lighting half the Moon while it orbits Earth over a ~29.5-day synodic month; the Moon is visible by day at every phase except new link
  2. Earthshine. The unlit part of the Moon glows faintly from sunlight reflected off Earth (“the old Moon in the new Moon’s arms”), showing the dark portion is merely unilluminated, not absent. link
  3. The Moon illusion. The Moon appears larger near the horizon, but its measured angular size (~0.5°) is unchanged between horizon and zenith; the effect is a perceptual illusion, not a change link
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