Empirical Earth · The Sky — Sun, Moon & Stars

Libration — the Moon rocks, and we see 59%

We see ~59% over time because the Moon librates (rocks ~±8°/±7°); a fixed disc cannot, a tidally-locked sphere on an ellipse does.

The claim

“We see one fixed face — a disc that never turns.”

What is true

We do always see the same face of the Moon. The near side is the only side anyone on Earth has ever seen with their own eyes.

What is measured

We see about 59% of the Moon’s surface over time as it rocks ~±8°/±7° ( libration ). A fixed flat disc could never show more than one face.

What would show this is wrong

the identical lunar hemisphere always presented with no rocking, revealing 50% or less of the surface over time.

Sources

  1. Lunar libration. The Moon appears to rock ~±8° in longitude and ~±7° in latitude, so ~59% of its surface is seen from Earth over time. libration . link
  2. Lunar libration — amplitudes and what it reveals. Longitude libration reaches ±7.9° (eccentric orbit, period 27.55 d) and latitude libration ±6.68° (the Moon’s 6.68° tilt, period 27.21 d); w link
  3. Diurnal libration. Because we observe the Moon from Earth’s rotating surface rather than its center, our viewpoint shifts by up to a full Earth-diameter between moonrise and moonset, letting link
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