How Do Solar Eclipses Work? Why the Moon Fits the Sun Perfectly

Few sights move people like a total solar eclipse. In the middle of the day the light turns strange and silvery, the temperature drops, birds fall silent and, suddenly, the Sun disappears, leaving a black disc surrounded by a shimmering crown of light. How does this happen, and why can we see it at all?

The Moon gets in the way

A solar eclipse happens when the Moon passes directly between the Sun and Earth. The Moon’s shadow then falls on part of our planet. That can only happen at new moon, when the Moon lies on the Sun’s side of Earth.

So why isn’t there an eclipse every month? Because the Moon’s orbit is tilted by about 5 degrees relative to Earth’s orbit around the Sun. Most months, the new moon passes a little above or below the Sun in our sky. Only when a new moon happens close to the points where the two orbits cross, about twice a year, do we get an eclipse season.

A perfect fit by coincidence

The Sun is about 400 times wider than the Moon. By pure chance, it is also about 400 times farther away. So in our sky the two discs look almost exactly the same size, and the Moon can cover the bright face of the Sun precisely, while leaving its faint outer atmosphere visible.

This is not permanent. The Moon is slowly moving away from Earth, by about 3.8 centimetres a year. In several hundred million years it will look too small to cover the Sun completely, and total eclipses will end.

Types of solar eclipse

• Total: the Moon covers the Sun completely. This is only seen from inside the path of totality, the strip where the Moon’s darkest shadow, the umbra, sweeps across Earth. It is usually less than 200 kilometres wide.
• Annular: the Moon’s orbit is slightly oval. When it is at its farthest from Earth, it looks a little too small and leaves a bright “ring of fire” around it.
• Partial: outside the central path, observers see the Moon take only a bite out of the Sun. A partial eclipse is visible over a much wider area.

What happens during totality

As totality approaches, shadows sharpen and the light fades. Just before the Sun disappears, beads of sunlight shine through valleys on the Moon’s edge, called Baily’s beads, and a final flash creates the diamond ring. Then the corona, the Sun’s outer atmosphere, millions of degrees hot but normally hidden by glare, spreads out in pearly streamers. Bright planets and stars appear. Totality lasts from a few seconds to at most about 7.5 minutes.

Eclipses have made scientific history. During the total eclipse of 1919, astronomers measured how starlight was bent by the Sun’s gravity, confirming Einstein’s general theory of relativity.

How to watch safely

Never look directly at the Sun, even when most of it is covered. Use eclipse glasses that meet the ISO 12312-2 standard, or project the Sun’s image through a pinhole onto paper. Ordinary sunglasses are not enough. Only during the few minutes of totality, when the Sun is completely covered, is it safe to look with the naked eye.

Upcoming eclipses

A total eclipse crossed Greenland, Iceland and Spain on 12 August 2026. The next big one, on 2 August 2027, will cross southern Spain and North Africa; near Luxor in Egypt totality will last about 6 minutes 23 seconds. Turkey last saw a total eclipse on 29 March 2006, along the Antalya coast.

In short

• A solar eclipse happens when the Moon passes between the Sun and Earth.
• It needs a new moon, but the Moon’s orbit is tilted about 5°, so not every month.
• The Sun is 400 times bigger but 400 times farther than the Moon, so they look the same size.
• Eclipses can be total, annular or partial.
• Totality reveals the corona and lasts at most about 7.5 minutes.
• Always use ISO 12312-2 eclipse glasses; never look at the Sun unprotected.