A solar eclipse is the Moon photobombing the Sun. When the Moon moves directly between us and the Sun, it blocks the light and throws a shadow onto Earth. If you happen to be standing in the dark middle of that shadow, the Sun disappears for a few minutes in the middle of the day. It is rare because the Moon's path is tilted, so most of the time it passes a little above or below the Sun and the shadow misses us. Slide the Moon in the simulator and line up the shadow.
Most people think a solar eclipse should happen at every new moon. In fact the Moon's orbit is tilted about 5 degrees, so its shadow usually passes above or below Earth, and only near the nodes does it line up.
What's actually happening
It seems like it should happen every month. The Moon laps the Earth roughly every 29.5 days, and once each lap it passes between us and the Sun (that is a new moon). So why isn't there an eclipse every month? Because the Moon's orbit is tilted about 5° relative to the flat plane Earth travels in. Most new moons, the Moon crosses the sky a little high or a little low, and its shadow sails harmlessly past Earth into empty space.
An eclipse needs two things to line up at once: a new moon, and the Moon sitting near one of the two points (the nodes) where its tilted orbit crosses Earth's path. When both happen together, the Moon's shadow cone touches the planet. That cone has two parts: a narrow dark core called the umbra, and a broader, softer fringe called the penumbra. If you stand under the penumbra you see a partial eclipse, a bite taken out of the Sun. If you stand in the umbra's track, a strip usually only about 150 kilometres wide racing across the ground, you get totality, the Sun blotted out entirely.
None of this would be so dramatic without a cosmic fluke. The Sun is roughly 400 times wider than the Moon, but it is also roughly 400 times farther away, so by pure coincidence the two look almost exactly the same size in our sky. That is why the Moon can cover the Sun's bright disc so precisely that its faint outer atmosphere, the corona, flares into view. The match is not permanent: the Moon drifts about 3.8 cm farther from Earth each year, so in a few hundred million years it will be too small to cover the Sun, and total eclipses will end forever.
A total eclipse needs a 400-to-400 size-and-distance fluke and a node crossing, which is why it is so rare and so brief.
- 1On a sunny day, hold a small coin at arm's length and a larger plate nearer your face, then close one eye and line the coin up over a bright object.
- 2Move the coin until it just covers the object: a tiny coin can blot out something huge simply by being closer, the same trick the Moon plays on the Sun.
- 3Now shift the coin slightly up or down. It stops covering the object: that small miss is exactly why the tilted Moon usually fails to eclipse the Sun.
Common questions
The Moon's orbit is tilted about 5° to Earth's path, so most new moons its shadow sails above or below the planet. An eclipse needs the Moon near one of the two nodes where the orbits cross at the same time as a new moon.
The corona, the Sun's million-degree outer atmosphere, is about a million times fainter than the bright disc. Only when the Moon hides the disc exactly does the corona flare into view, for a few minutes invisible at any other time.
No. The Moon drifts about 3.8 cm farther from Earth each year, so in roughly 600 million years it will look too small to cover the Sun completely, and Earth will see its last total solar eclipse.