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ExploreSpace and Earth

The world, a little closer

Earth and Moon.

The Moon changes shape every night, drags the oceans about, and now and then blots out the Sun. One orbit under one light explains all three.

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01 / The experiment

One orbit, under one light.

Step through the month and watch the lit face turn. Switch on the tides, then reveal the shadows.

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Sunlight arrives from the right. The Moon's orbit is drawn tilted by 14 degrees, nearly three times its real 5, so the gap past the shadows shows at this squeezed distance. Sizes and distances are not to scale.The tidal bulge is exaggerated many thousands of times. Real tides are a metre or so on an ocean thousands of kilometres deep.

02 / One orbit, three effects

Phases, tides, eclipses.

All three come from where the Moon is and where the light comes from.

Every 29.5 days01

Phases

The Moon is always half lit. Its position in the orbit decides how much of that half faces us.

Cause
Our angle on the lit half
Rhythm
A month
We see
Crescent to full and back
Twice a day02

Tides

The near ocean is pulled toward the Moon; the far ocean is left behind. Each coast passes both bulges daily.

Cause
The Moon’s uneven pull
Rhythm
12 h 25 min
We see
Two bulges the Earth turns through
A few each year03

Eclipses

The orbit’s 5° tilt makes the Moon miss the shadows most months. Eclipses happen only where the orbits cross.

Cause
A body crossing a shadow
Rhythm
Only at nodes
We see
A darkened Moon or a hidden Sun
The big idea

The Moon is always half lit. Phases, tides and eclipses are all about where it is in its tilted orbit relative to the Sun.

29.5 days

Half lit, always

The Moon does not shine. It is a grey rock lit by the Sun, and like any ball in sunlight it is always exactly half lit and half dark. What changes through the month is not the Moon but our view of it. As it travels round the Earth we see the lit half from different angles: edge-on at the start, so a thin crescent; face-on two weeks later, so a full disc.

That cycle of phases takes 29.5 days, which is where the month comes from. At new moon the Moon lies between us and the Sun, and its lit side faces away. At full moon it is on the far side of the Earth from the Sun, and its lit side faces us. In between it waxes, growing from crescent to half to gibbous, then wanes back again.

Try it: step through New, First quarter, Full and Last quarter, and watch which side of the Moon is bright each time.

The same face

The Moon spins exactly once for every orbit it makes, so it always shows the Earth the same face. This is not a coincidence: the Earth’s gravity, pulling slightly harder on the near side over billions of years, slowed the Moon’s spin until the two locked together. The far side was unseen by anyone until a spacecraft photographed it in 1959.

Tides

The Moon pulls on the Earth, and pulls hardest on the side nearest to it. The ocean facing the Moon is drawn up into a bulge. The ocean on the far side is pulled least, and is left behind as the solid Earth is drawn away from it, so it bulges too. Two bulges, on opposite sides. As the Earth turns beneath them, each coast passes through both in a day: two high tides and two low tides, a little over twelve hours apart.

The Sun raises tides as well, about half as strong. When Sun and Moon line up, at new and full moon, their bulges add and the tides run highest.

Shadows and eclipses

Reveal the shadows and each body trails a cone of darkness away from the Sun. When the Moon passes through the Earth’s cone, we see a lunar eclipse: the full Moon dims to a dull red. When the Moon’s much smaller cone touches the Earth, people standing in it see a solar eclipse, the Sun blotted out for a few minutes.

If the Moon’s orbit lay flat, this would happen every month. It does not, because the orbit is tilted by about five degrees, and most months the Moon passes above or below the cones. Only when new or full moon falls where the tilted orbit crosses the Earth’s plane do the shadows meet. That is why eclipses come in seasons, a few each year, and why a total solar eclipse at any one place is a once-in-centuries event.

Try it: reveal the shadows and choose Full. The Moon passes above Earth’s shadow and stays bright. Now switch on Eclipse season: the crossing line swings round to point at the Sun, the full Moon slides into the shadow and turns a dull coppery red, and at New the Moon’s small shadow falls on the Earth.