The sky — astronomy

Solar Eclipse: How It Happens and How to Watch

A solar eclipse happens when, at new moon, the Moon passes directly between Earth and the Sun and casts its shadow on Earth. It can be watched safely only with certified eclipse glasses (ISO 12312-2) or indirect projection, and never with the naked eye, except during the brief totality of a total eclipse.

The Sun–Moon–Earth Alignment

A solar eclipse is a shadow event. When the Moon moves exactly between Earth and the Sun, its shadow falls on a small part of our planet. Anyone standing in that shadow sees the Moon’s disk covering the Sun’s.

Nature has arranged a remarkable coincidence here. The Sun is about 400 times wider than the Moon, and also about 400 times farther away, so from Earth both appear almost the same size in the sky, about half a degree across. That is why total eclipses look the way they do. For more on both bodies, see the Moon and the Sun.

A solar eclipse can only happen at new moon, when the side of the Moon facing us is dark. The cycle of phases is explained in lunar phases.

Total, Partial and Annular Eclipses

The kind of eclipse depends on exact geometry and on how far away the Moon is.

• Total eclipse. The Moon covers the whole solar disk. Observers inside the darkest part of the shadow (the umbra, from Latin for shadow) see a twilight sky in the middle of the day and the Sun’s corona, its outer atmosphere. Totality at any one place lasts at most about seven and a half minutes, and usually much less.
• Partial eclipse. The Moon covers only part of the Sun. This is what people in the lighter outer shadow (the penumbra) see on either side of the path of totality. Some eclipses are only partial, because the umbra misses Earth entirely.
• Annular eclipse. The Moon’s orbit is an ellipse, so its distance changes. If an eclipse happens when the Moon is farther away, it looks slightly smaller than the Sun and leaves a bright ring around its edge. The word annular comes from the Latin for ring. The Sun is never fully covered, so eye protection is needed the whole time.
• Hybrid eclipse. A rare type that shifts between annular and total along its path because of Earth’s curvature.

Totality is visible only along a narrow track, at most a few hundred kilometres wide, which is why people travel to see it. Upcoming eclipses are listed on NASA’s eclipse pages (see the sources). For example, a total solar eclipse is expected on 12 August 2026, with a path across Greenland, Iceland and Spain and a partial phase visible across much of Europe.

Why Not Every New Moon Brings an Eclipse

There is a new moon every month, yet eclipses are much rarer. The reason is the tilt of the Moon’s orbit, about 5 degrees to the plane in which Earth circles the Sun (the ecliptic). At most new moons the Moon passes slightly above or below the Sun, and its shadow misses Earth.

An eclipse is possible only when new moon occurs near one of the two nodes, the points where the Moon’s orbit crosses the ecliptic. These windows, called eclipse seasons, come about twice a year. As a result there are at least two and at most five solar eclipses in a calendar year. Read more about the nodes in the lunar nodes in astronomy.

Eclipses repeat in a pattern called the saros, about 18 years and 11 days long. After that interval the Sun, Moon and nodes return to almost the same relative positions, which is one reason eclipses can be predicted so precisely. A lunar eclipse follows the same geometry, but it looks different and is visible from the entire night side of Earth. See the lunar eclipse.

Watching Safely

Sunlight can damage the retina, often without any pain at the time. Scientific and health institutions agree on a few firm rules:

1. Eclipse glasses must meet the ISO 12312-2 standard. Ordinary sunglasses, however dark, are not enough. Check the glasses for scratches or holes before use.
2. Never look at the Sun through a camera, binoculars or a telescope unless a proper solar filter is fitted at the front of the instrument. Eclipse glasses do not make these safe, because the optics concentrate the light.
3. Indirect projection. The simplest safe method is a pinhole projector: let sunlight pass through a small hole in card onto a white sheet. Gaps between tree leaves do the same, casting many small crescents on the ground.
4. Only during totality of a total eclipse, and only while the Sun is completely covered, is it safe to remove your glasses and look at the corona. As soon as the first bright bead of sunlight returns, put them back on. During partial and annular eclipses, a filter is needed throughout.
5. If you are unsure about your equipment, or your eyes feel unusual after watching, consult an eye care professional.

For more practical observing tips, see how to watch the sky.

Eclipses in History

Eclipses have fascinated people for thousands of years. Ancient Chinese and Babylonian records mention them, and scientists still use such records to study how Earth’s rotation has changed over time. According to ancient tradition, the Greek thinker Thales predicted an eclipse in 585 BCE that stopped a battle between the Lydians and the Medes. The exact date and details are still discussed by historians.

Eclipses have also advanced science. In 1868, observers studying an eclipse noticed a spectral line in the Sun’s prominences that led to the discovery of helium, found on the Sun before it was found on Earth. In 1919, observations during a total eclipse were used to test the prediction of general relativity that gravity bends light. Today, eclipses still give researchers a rare chance to study the Sun’s corona.

A solar eclipse shows how well we understand the motions of the heavens: we can know years ahead when and where a rare, beautiful sight will appear. Bring certified glasses, some patience and hope for clear skies.

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