Three things in a line
What Happens in an Eclipse?
Sun, Moon and Earth line up. Somebody ends up in somebody's shadow.
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Step 1 of 5
Anything lit makes a shadow.
Block a lamp with your hand and there is a dark patch behind it. Space works the same way — the Sun is the lamp, and every planet and moon trails a shadow.
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Step 2 of 5
Solar eclipse: the Moon's shadow lands on us.
The Moon's shadow is only about a hundred kilometres wide where it lands. When the Moon passes exactly between the Sun and Earth, its shadow falls onto the ground. Stand in that shadow and daytime goes dark.
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Step 3 of 5
Lunar eclipse: our shadow lands on the Moon.
Earth's shadow is vastly wider, so half the planet sees this one at once. When Earth is in the middle, our shadow swallows the Moon. It does not vanish — it turns dim red, lit only by light bent through all our sunrises at once.
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Step 4 of 5
The Sun and Moon look the same size — by luck.
That coincidence is why a total eclipse fits so perfectly. The Sun is 400 times wider than the Moon and also happens to be about 400 times further away. So they appear almost exactly the same size in our sky.
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Step 5 of 5
So why not one every month?
A five-degree tilt is the difference between a monthly event and a rare one. The Moon's orbit is tilted about 5 degrees. Most months it passes a bit above or below the line, and the shadow misses entirely.
Totality lasts only a couple of minutes because the Moon's dark shadow patch is barely 100 km across.
The short version
The Sun is 400 times wider than the Moon and 400 times further off, so they look the same size and a total eclipse fits exactly. A 5-degree tilt in the Moon's orbit is why this is not monthly.
Try it yourself
Hold a coin in front of a lamp and move it toward your eye. At the right distance it covers the lamp exactly. That is a total eclipse.