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How does a lunar eclipse happen?

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Quick answer

A lunar eclipse occurs when Earth moves between the Sun and the Moon, blocking sunlight and casting its shadow onto the Moon. This can only happen at full moon, when the three bodies are almost perfectly aligned. The Moon then darkens, and during a total eclipse it often glows red.

A lunar eclipse happens when Earth passes directly between the Sun and the Moon, so Earth's shadow falls on the Moon. This alignment occurs only at full moon, when the Sun, Earth and Moon line up in space.

Step by step

  1. The Sun shines steadily, lighting one half of the Moon at all times.
  2. The Moon orbits Earth and reaches its full phase, opposite the Sun.
  3. Earth casts a long, cone-shaped shadow into space away from the Sun.
  4. Usually the tilted orbit makes the Moon pass above or below that shadow.
  5. When the alignment is exact, the Moon enters the shadow and the eclipse begins.

Key things to understand

  1. The Sun always lights half of the Moon; the other half is in its own shadow.
  2. A lunar eclipse needs the Moon, Earth and Sun in a near-perfect straight line.
  3. Earth blocks sunlight and casts a cone-shaped shadow into space behind it.
  4. The Moon orbits Earth tilted about 5 degrees, so it usually misses the shadow.
  5. When the alignment is exact, the Moon drifts into Earth's shadow and dims.

Labelled diagram

  1. Sun

    The Sun shines in all directions, lighting half of Earth and half of the Moon.

  2. Earth

    Earth blocks sunlight and casts a cone-shaped shadow into space behind it.

  3. Moon outside shadow

    Most full moons pass above or below the shadow because the orbit is tilted.

  4. Moon inside shadow

    When aligned, the Moon enters the shadow and darkens during the eclipse.

How a lunar eclipse lines up

Real-world examples

  • During a total lunar eclipse, skywatchers across India, the US and Europe can see the Moon turn red at the same time.
  • A partial lunar eclipse looks like a dark bite taken out of the Moon's edge.
  • The total lunar eclipse of July 2018 was visible from Africa, the Middle East, India and Australia.
  • Ancient Greek astronomers used lunar eclipses to show that Earth is a sphere, because the shadow is always round.

Common mistake

✗ Myth: The Moon's monthly phases are caused by Earth's shadow falling on it.

✓ Fact: Phases come from seeing different portions of the Moon's sunlit half as it orbits. Earth's shadow reaches the Moon only during a lunar eclipse, which needs a full moon and near-perfect alignment.

People also ask

What causes a lunar eclipse?
A lunar eclipse is caused by Earth passing directly between the Sun and the Moon, so Earth's shadow falls on the Moon. This alignment only happens at full moon when the Moon's tilted orbit crosses Earth's orbital plane.
Why is the moon red during a lunar eclipse?
Earth's atmosphere bends sunlight into the shadow and scatters blue light away, so mostly red light reaches the Moon. The Moon reflects that red light back to us, making it look coppery or blood-red.
How often do lunar eclipses happen?
Lunar eclipses happen at least twice a year, but total ones are less common. Any eclipse is only visible from the night side of Earth, so a particular location sees fewer.
Why does the Moon turn red during a total lunar eclipse?
Earth's atmosphere bends sunlight into the shadow and scatters blue light away, so mostly red light reaches the Moon. The Moon reflects that red light back to us, giving it a coppery colour.
Why isn't there a lunar eclipse every full moon?
The Moon's orbit is tilted about 5 degrees relative to Earth's orbit around the Sun. Most full moons pass above or below Earth's shadow, so an eclipse happens only when the alignment is nearly perfect.
How long does a lunar eclipse last?
A total lunar eclipse can last over an hour, with the whole event from first dimming to full brightness taking several hours. The Moon moves slowly through Earth's shadow, so the change is gradual.
Is it safe to look at a lunar eclipse?
Yes, looking at the Moon during a lunar eclipse is completely safe with your eyes alone. Unlike a solar eclipse, the Moon is not bright enough to damage your eyes.

Part of the guide: Space & Astronomy →