Solar Eclipses: How They Work and Why Totality Is Rare

A solar eclipse happens when the Moon passes between Earth and the Sun. Learn the types, why totality is rare, the Saros cycle, and how to watch one safely.

Category: Planetary Science · Created: 2026-10-04 · Updated: 2026-10-04 · 4 min read

A solar eclipse occurs when the Moon moves between the Sun and Earth and casts its shadow on part of our planet. In a total eclipse the Moon covers the Sun completely for a few minutes, the sky darkens, and the Sun's faint outer atmosphere, the corona, becomes visible. Total eclipses are possible on Earth because of a cosmic coincidence: the Sun is about 400 times wider than the Moon but also about 400 times farther away, so the two look almost the same size in our sky.

Why do solar eclipses happen?

The Moon circles Earth about every 29.5 days and passes between us and the Sun at every new moon. Yet we do not see an eclipse each month, because the Moon's orbit is tilted by about 5 degrees compared with Earth's orbit around the Sun. Most months the Moon's shadow passes above or below Earth. An eclipse needs a new moon that happens at the same time as the Moon crosses the plane of Earth's orbit, and that alignment comes in two eclipse seasons each year, about six months apart. The geometry follows from the orbital rules described in Kepler's laws of planetary motion.

What are the types of solar eclipse?

Total: the Moon fully covers the Sun. It is visible only from inside the narrow path of totality, typically about 100 to 250 kilometres wide.
Annular: the Moon is near the far point of its orbit and looks slightly smaller than the Sun, so a bright ring of sunlight remains around it.
Partial: observers outside the path of totality see the Moon take a bite out of the Sun, though it never covers it fully.
Hybrid: the eclipse changes between annular and total along its path, because of Earth's curved surface.

The dark central shadow is called the umbra, the lighter outer shadow the penumbra. Anyone standing in the penumbra sees a partial eclipse.

How long does totality last?

The longest totality that is physically possible is about 7 minutes 32 seconds, and most last only two to four minutes. The total eclipse of 8 April 2024, which crossed Mexico, the United States, and Canada, lasted up to about four and a half minutes. The eclipse of 12 August 2026 crossed Greenland, Iceland, and northern Spain. The next, on 2 August 2027, passes over southern Spain, North Africa, and Egypt, and lasts more than six minutes at its peak. On 22 July 2028, totality crosses Australia and New Zealand.

How often do total eclipses happen?

Somewhere on Earth there is a total eclipse about every 18 months, but at any one place the average wait is roughly 360 to 410 years. Eclipses repeat in nearly identical form after the Saros cycle of 6,585.3 days, about 18 years and 11 days, a pattern first recognised by Babylonian astronomers. The extra one-third of a day shifts the path a third of the way around the globe. The Greek Antikythera mechanism carried a dial to predict eclipses using this very cycle.

What have eclipses taught us?

In 1868, astronomers examining the Sun's atmosphere during a total eclipse in India saw a bright yellow spectral line that matched no known element. It was helium, named after the Greek word for the Sun, found in the Sun before it was found on Earth.
In 1919 expeditions led by Arthur Eddington photographed stars near the eclipsed Sun, and their positions appeared shifted by about the amount that general relativity predicts for bent starlight, which helped make Einstein world-famous. The same theory predicts gravitational waves.
The corona is millions of degrees hot while the visible surface is only about 5,800 kelvin. Why the corona is so much hotter is still an open question, and eclipse observations remain one of the ways to study it.

Is the Moon moving away?

Yes, by about 3.8 centimetres a year, as measured by lasers bounced off reflectors that the Apollo astronauts left on the Moon. Tidal forces slowly transfer energy from Earth's spin to the Moon's orbit (see tidal locking). In roughly 600 million years the Moon will appear too small to cover the Sun, and total eclipses will end. We live in a rare era where the match is almost perfect.

How can you watch an eclipse safely?

Never look at the Sun, even when most of it is covered, without proper eye protection. The damage, solar retinopathy, can be permanent and does not hurt while it happens. Use eclipse glasses certified to the ISO 12312-2 standard or a pinhole projector. Ordinary sunglasses are not enough, and neither are unfiltered cameras, binoculars, or telescopes. The only time you may look without a filter is during the brief totality, when the Sun is completely covered, and you must put the filter back on as soon as the first bright sliver of sunlight reappears.