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📘 A fingernail's growth, measured in space

The Moon is not parked at a fixed distance from Earth. Its average distance increases by about 3.8 centimeters, or 1.5 inches, each year. That is roughly the pace at which a fingernail grows, so no one can watch the gap open from one night

3
lessons
~15 min
to learn
Adults
level
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What you’ll learn

  1. The Moon is moving, but very slowlyExplain how lunar laser ranging establishes that the Moon's orbit is expanding.A tiny annual change becomes measurable when scientists time laser pulses over decades.
  2. Earth's tides hand over angular momentumConnect tidal bulges, friction, Earth's rotation, and the Moon's expanding orbit.A lagging ocean tide lets Earth transfer angular momentum into the Moon's orbit.
  3. The rate is real, but it is not a forever ruleInterpret the present recession rate carefully across geological and astronomical timescales.The trend is measured today, while its size and far-future consequences depend on a changing system.

Questions this course answers

How do scientists measure the Moon's changing distance?

Laser ranging times light sent to and returned from reflector arrays on the Moon, revealing a long-term distance trend.

What gives the Moon its present outward push?

Earth's rotating, lagging tidal bulge pulls the Moon forward along its orbit.

What happens to Earth's rotation as angular momentum moves into the Moon's orbit?

The tidal interaction transfers angular momentum from Earth's spin, making the length of day increase over long timescales.

Why should 3.8 centimeters per year not be projected unchanged for all of history?

Tidal dissipation depends on the physical configuration of the Earth-Moon system, which has changed through geological time.

What does tidal locking mean for the Moon?

Synchronous rotation makes one lunar rotation take the same time as one orbit around Earth.

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