Astronomy · Rung 3 of 12

The Moon

The Moon always has half its face lit by the Sun. What changes is how much of that half faces you, and that depends on the angle between the Sun, the Earth and it: new when it is on the Sun's side, full when it is opposite. The cycle lasts 29.5 days, slightly more than its real 27.3-day orbit, because meanwhile the Earth has moved along too. It spins on itself in exactly the time it takes to go around, so it always shows us the same face. And there is no eclipse every month because its orbit is tilted 5°: almost always it passes above or below the shadow.

Where this sits

Comes after
The sky from here

Exercises from this rung

Generated by the app, from easiest to hardest

  1. Why do we always see the same face of the Moon?

    • It takes as long to spin as to orbit us
    • Earth holds it with its magnetic field
    • It does not spin at all
    • Its other face is always in darkness

    Why Synchronous rotation: it spins on its axis in 27.3 days, exactly its orbital period. That is what tides do over time, brake the spin until it locks. The far side is not dark: it gets as much sun as the near side, only when it is new moon here it is midday there.

    easier

  2. There are two high tides a day, not one. Where does the second come from?

    • The water on the far side is left behind
    • It bounces off the continents
    • The Sun produces the other one
    • The Earth turns twice per tide

    Why The Moon pulls harder on the near-side water than on Earth's centre, and harder on the centre than on the far-side water. The result is a stretch both ways: one bulge towards the Moon and another on the opposite side, left behind. As the Earth turns, every point passes through both each day. The Sun modulates the height, it does not create the second bulge.

    in between

  3. The Moon recedes 3.8 cm a year. Where does that energy come from?

    • From the braking of Earth's rotation
    • From the Sun losing mass
    • From the expansion of the universe
    • From the solar wind

    Why Tides rub against the sea floor and brake Earth's spin: the day lengthens about 1.8 milliseconds per century. That angular momentum does not vanish, it transfers to the Moon's orbit, which rises. Six hundred million years ago the day lasted 21 hours, and you can read that in the growth bands of fossil corals.

    harder