Lesson 4 of 4

Eclipses and tides.

About 30 minutes · 7 practice questions

Read the full article: Earth and Moon

Why not every full moon brings a lunar eclipse, when a solar eclipse happens, and how the gravity of the Moon and the Sun lifts the ocean: two high tides a day, spring and neap tides, and why the Sun's tide is small.

Step 1 of 7 · Every full moon?

Step 1 of 7

Every full moon?

Look inside draws each body’s shadow: a cone pointing away from the Sun. At full moon the Moon is on the side of the Earth opposite the Sun, exactly where the Earth’s shadow lies.

The Moon’s orbit is tilted to the plane in which the Earth goes round the Sun. In the Scene the tilt is exaggerated, $25^\circ$ ⁠ instead of $5^\circ$ ⁠, because its distances are squeezed hard: the real Moon is $60$ ⁠ Earth radii away. Predict, then the Scene will open the shadows.

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Step 2 of 7

Eclipse season

The tilted orbit crosses the Earth’s plane at two points, and the line through them keeps roughly the same direction in space, much like the Earth’s axis. Twice a year that line points at the Sun, and then new and full moons fall exactly on the line from the Earth to the Sun. This is an eclipse season.

Eclipse season is on now, and the full moon moves into the Earth’s shadow and turns red: a lunar eclipse. What happens two weeks earlier, at new moon?

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Step 3 of 7

Into the shadow

It is first quarter in an eclipse season now. The Moon is off to the side, far from both shadows. Move it along its orbit.

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Step 4 of 7

Two bulges

The Moon’s gravity pulls on the Earth too, and above all on its water. In the Lessons on gravity you saw that the pull falls off like $\frac{1}{r^2}$ ⁠. So the Moon pulls the ocean on the near side harder than the Earth’s centre, and the centre harder than the ocean on the far side. What matters for the tide is that difference.

Switch on Show tides. The bulge in the Scene is greatly exaggerated: in the open ocean it is only about half a metre.

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Step 5 of 7

Tide after tide

The Earth turns beneath the two bulges. But the Moon moves too, $12.2^\circ$ ⁠ a day, so a shore takes a little more than a day to come back under the Moon.

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Step 6 of 7

Spring and neap tides

The Sun raises two bulges as well, along the line to the Sun. It is first quarter now: the Moon is at a right angle to the Sun, and so are their bulges.

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Step 7 of 7

The Sun’s tide

One thing is still odd: the Sun is $27$ ⁠ million times as massive as the Moon, and its pull on the Earth is $180$ ⁠ times the Moon’s. It is what holds the Earth in its orbit. Yet its tide is only about half the Moon’s.

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Interactive scene

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Each step sets up the Scene for you, and a control marked “fixed” stays put for that step.

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Practice · Predict, then look

A full moon in an ordinary month. Will the Moon pass into the Earth’s shadow, that is, will there be a lunar eclipse?

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Practice · Predict, then look

In the eclipse season the orbit’s crossing line points at the Sun. Which eclipse happens at new moon, on day $0$ ⁠?

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Practice · Reach the target

The eclipse season goes on. Change Day of the month until the Moon enters the Earth’s shadow, then press Check.

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Practice · Choose

How many times a day does the water rise to high tide on an open seashore?

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Practice · Calculate

The Moon returns to the same place in the sky every 24 h 50 min, because meanwhile it moves along its orbit. How long is it from one high tide to the next? Write it in minutes.

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Practice · Predict, then look

The Sun raises a tide too, a weaker one. It is first quarter now. What happens to the gap between high and low tide when the Moon reaches new moon?

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Practice · Choose

The Sun pulls on the Earth about $180$ ⁠ times harder than the Moon does. So why is its tide only about half the Moon’s?