Question 1

Why does the Moon have phases?

Accepted Answer

Half the Moon is always lit by the Sun. The phases are our changing view of that lit half as the Moon orbits Earth every 29.5 days — not Earth's shadow. Earth's shadow only touches the Moon during a lunar eclipse.

Question 2

Why doesn't the Moon fall down?

Accepted Answer

It is falling — continuously, toward Earth. But it also moves sideways at about 1 km/s, so the Earth curves away beneath it as fast as it falls. That endless falling-and-missing is what an orbit is.

Question 3

How far away is the Moon?

Accepted Answer

On average 384,400 km, ranging from about 356,000 km at perigee to 406,000 km at apogee. We know the distance to within millimetres by bouncing laser pulses off retroreflectors left on the surface by Apollo — a round trip of about 2.6 seconds.

Question 4

Why do we only ever see one side of the Moon?

Accepted Answer

The Moon rotates exactly once per orbit — every 27.3 days — so the same face always points at Earth. This tidal locking is the result of Earth's gravity braking the Moon's spin billions of years ago.

Question 5

Why does the Moon look upside down in the southern hemisphere?

Accepted Answer

Observers in the two hemispheres stand head-to-head on a sphere, so they see the same Moon rotated 180° relative to each other. The Moon has no true 'right way up' — only observers do.

Question 6

How does the Moon cause tides?

Accepted Answer

The Moon's gravity pulls hardest on the ocean nearest it and weakest on the far side, stretching Earth's water into two bulges. As Earth rotates through both, most coasts get two high tides roughly every 24 hours and 50 minutes.

Question 7

How do eclipses happen?

Accepted Answer

Eclipses are alignments. A solar eclipse is the Moon's shadow touching Earth at new moon; a lunar eclipse is Earth's shadow covering the Moon at full moon. They don't happen monthly because the Moon's orbit is tilted about 5 degrees.

The Moon, in your pocket