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Orbital tilt of the Moon

Why isn’t there an eclipse every month?

The Moon’s path around Earth is tipped about 5° to Earth’s path around the Sun. Most months the full Moon rides above or below Earth’s shadow, and the new Moon rides above or below the Sun. An eclipse needs the Moon at a crossing, not just at a phase.

Earth’s shadow is a real cone past the Moon’s orbit. A 5° tip is enough for the full Moon to miss that cone most months. The drawing of the tip is exaggerated so you can see it.

Phase is not position on the tilt

Full moon means the Moon is opposite the Sun as seen from Earth. It does not mean the three bodies are in a straight line in space. The extra dimension is the 5° tip. Only when a full or new moon happens near a node — a crossing of the two planes — do you get an eclipse.

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Why this site’s orrery does not draw the shadow

A top-down Sun–Earth–Moon picture flattens the 5°. If it also drew Earth’s umbra as a geometric cone in that plane, the full Moon would sit in the shadow every month and the picture would teach the wrong eclipse. The live simulator says so, and this page is the reason.

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On the hub: Sun–Earth–Moon simulator · Moon

Related questions

Why does the Moon change shape? The Moon makes no light of its own. One half is always lit by the Sun. Phases are your changing view of that lit half as the Moon goes around Earth — about 29.5 days from full moon to full moon.

What is a synodic month? A synodic month is the time from one full moon to the next, about 29.5 days. It is longer than the Moon’s trip around Earth relative to the stars, because Earth has moved and the Moon must catch up to the same angle with the Sun.

What causes tides? The Moon’s gravity pulls harder on the water nearer to it than on the Earth’s centre, and harder on the centre than on the far-side water. The ocean stretches into two bulges. Earth then turns under those bulges, so most coasts see two high tides a day.

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