During a total lunar eclipse, the Moon should be completely dark, but it usually glows deep red instead because some sunlight is bent through Earth’s atmosphere into Earth’s shadow and softly illuminates the lunar surface. If Earth had no atmosphere to bend and filter that light, the Moon would indeed go completely black while it sits in the deepest part of the shadow.

The basic shadow setup

  • A lunar eclipse happens when the Sun, Earth, and Moon line up so that Earth sits directly between the Sun and the Moon.
  • Earth then casts a long, cone-shaped shadow into space, and the Moon passes into this shadow.
  • The darkest central part of this shadow is called the umbra; when the Moon is fully inside it, you get a total lunar eclipse.

Why the Moon seems to go dark

  • From Earth, the bright full Moon suddenly fades because direct sunlight can no longer hit the side facing us; Earth is blocking the Sun.
  • The contrast is dramatic: the Moon goes from brilliant white to a much dimmer, eerie disk, which is why people often describe it as “going dark,” even though it usually does not vanish completely.

Why it glows red instead of black

  • Earth’s atmosphere bends (refracts) sunlight into the umbra, so the Moon still receives a little light that has skimmed through the thin ring of air around Earth.
  • In that journey, shorter blue wavelengths are scattered away (Rayleigh scattering), while longer red and orange wavelengths survive, so the Moon is bathed in a faint reddish glow—hence the “blood moon.”

When would it be truly black?

  • If Earth had no atmosphere at all, no sunlight would be bent into the shadow cone, and the Moon would become nearly invisible during totality, a truly black disk against the sky.
  • Extremely dusty or polluted atmospheres (for example, after a major volcanic eruption) can also make eclipses much darker than usual, because even the red light gets more strongly absorbed and scattered.

Quick Scoop summary (for your post)

  • Earth blocks direct sunlight from reaching the Moon during a total lunar eclipse, plunging the Moon into the darkest part of Earth’s shadow (the umbra).
  • The Moon usually doesn’t turn completely black because Earth’s atmosphere bends a small amount of red sunlight into the shadow, softly lighting the lunar surface.
  • Only in a hypothetical airless Earth—or in extremely dusty atmospheric conditions—would the Moon appear almost completely dark during totality.

Information gathered from public forums or data available on the internet and portrayed here.