For a few minutes during a total solar eclipse, day turns into an eerie twilight. The temperature drops, birds fall silent, stars appear, and around the black disk of the Moon the Sun's pearly outer atmosphere, the corona, glows in the sky. People who have experienced totality often describe it as one of the most moving sights of their lives. Lunar eclipses are gentler but no less beautiful, as the full Moon slowly darkens and turns a deep copper red. Both are the result of a simple alignment of the Sun, Earth and Moon, yet they happen only occasionally. This explainer describes how eclipses work, why they are rare and how to watch them safely.
The basic idea: shadows in space
An eclipse happens when one celestial body moves into the shadow of another.
- A solar eclipse happens when the Moon passes between the Sun and Earth, and the Moon's shadow falls on Earth. It can only occur at new moon.
- A lunar eclipse happens when Earth passes between the Sun and the Moon, and Earth's shadow falls on the Moon. It can only occur at full moon.
Every shadow cast by a sphere lit by a large light source has two parts:
- The umbra, the dark central part of the shadow, where the light source is completely blocked.
- The penumbra, the lighter outer part, where the light source is only partly blocked.
Which part of the shadow you are in determines what kind of eclipse you see.
A remarkable coincidence
Total solar eclipses are possible on Earth because of a happy coincidence. The Sun is about 400 times wider than the Moon, but it is also about 400 times farther away. As a result, the Sun and the Moon appear almost the same size in our sky, each covering about half a degree. When they line up perfectly, the Moon can just cover the bright surface of the Sun.
This coincidence will not last forever. The Moon is slowly moving away from Earth, by about 1.5 inches (3.8 centimeters) per year. In several hundred million years, it will appear too small to cover the Sun completely, and total solar eclipses will no longer happen.
Types of solar eclipses
Total solar eclipse
When the Moon completely covers the Sun's bright disk, observers inside the Moon's umbra see a total eclipse. The umbra is small when it reaches Earth, usually only about 100 miles (160 kilometers) wide or less. As the Moon moves along its orbit and Earth rotates, the umbra sweeps across Earth's surface, creating a narrow path of totality. Totality at any one place lasts at most about 7½ minutes, and usually only two to four.
During totality, the sky darkens, bright planets and stars appear, and the Sun's corona becomes visible. Just before and after totality, observers may see Baily's beads, points of sunlight shining through valleys on the Moon's edge, and the diamond ring, a final bright flash of light.
Partial solar eclipse
Outside the narrow path of totality, people in the Moon's penumbra see a partial eclipse, in which the Moon covers only part of the Sun. Partial eclipses are visible over a much wider area. Some eclipses are only partial everywhere on Earth, when the umbra misses Earth entirely.
Annular solar eclipse
Because the Moon's orbit is elliptical, its distance from Earth varies. When a solar eclipse happens while the Moon is near its farthest point, it appears slightly smaller than the Sun and cannot cover it completely. A bright ring of sunlight remains around the Moon. This is called an annular eclipse, from the Latin "annulus," meaning ring. People sometimes call it a "ring of fire."
Hybrid eclipse
Occasionally, an eclipse is annular in some locations and total in others, because of the curvature of Earth. These rare events are called hybrid eclipses.
Types of lunar eclipses
Total lunar eclipse
When the entire Moon passes into Earth's umbra, we see a total lunar eclipse. Unlike a solar eclipse, it can be seen from anywhere on the night side of Earth, so many more people can watch it. Totality can last more than an hour and a half.
Why the Moon turns red
During a total lunar eclipse, the Moon does not disappear. Instead, it usually turns a coppery red, orange or brown. This happens because Earth's atmosphere bends, or refracts, some sunlight into the shadow. As sunlight passes through the atmosphere, blue light is scattered away, and mostly red light continues. In effect, the Moon is lit by the light of all the sunrises and sunsets happening around the edge of Earth at that moment. This is why total lunar eclipses are sometimes called "blood moons."
The exact color depends on conditions in Earth's atmosphere. After large volcanic eruptions, which put dust and aerosols high into the atmosphere, eclipses can be very dark.
Partial and penumbral lunar eclipses
In a partial lunar eclipse, only part of the Moon enters the umbra, and a dark bite appears on the Moon. In a penumbral eclipse, the Moon passes only through Earth's penumbra, causing a subtle dimming that many people do not notice.
Why eclipses do not happen every month
If the Moon passes between Earth and the Sun every new moon, why is there not a solar eclipse every month? The reason is that the Moon's orbit is tilted by about 5 degrees relative to the plane of Earth's orbit around the Sun, called the ecliptic.
Most months, the new moon passes slightly above or below the Sun as seen from Earth, and its shadow misses Earth. The full moon similarly passes above or below Earth's shadow. Eclipses happen only when a new or full moon occurs close to one of the two points where the Moon's orbit crosses the ecliptic, called the nodes. These alignments happen during "eclipse seasons," which occur roughly every six months and last a few weeks.
Each year, there are at least four eclipses, usually two solar and two lunar, and at most seven. But most of them are partial or penumbral, and any particular location on Earth sees a total solar eclipse only rarely, on average once every few hundred years.
Predicting eclipses
Astronomers can predict eclipses centuries in advance with great precision, using detailed models of the motions of Earth and the Moon. Ancient astronomers also discovered the saros cycle, a period of about 18 years, 11 days and 8 hours, after which similar eclipses repeat. Babylonian astronomers used it to predict eclipses more than two thousand years ago.
Eclipses and science
Eclipses have contributed to important scientific discoveries:
- In 1868, astronomers studying the solar corona during an eclipse discovered a new element in the Sun's spectrum. It was later named helium, after the Greek word for the Sun, and was found on Earth decades afterward.
- In 1919, measurements of starlight bending around the Sun during a total solar eclipse provided early support for Einstein's general theory of relativity, which predicted that gravity bends light.
- Eclipses still allow scientists to study the corona and the Sun's magnetic activity.
How to watch a solar eclipse safely
Looking directly at the Sun can cause permanent eye damage, called solar retinopathy, even during a partial eclipse when most of the Sun is covered. The damage can occur without pain.
Safe ways to watch:
- Eclipse glasses: Use glasses or handheld viewers that meet the international safety standard ISO 12312-2. Inspect them for scratches or damage, and buy them from reputable sources.
- Solar filters: Telescopes, binoculars and cameras need special solar filters mounted on the front. Never look through them with eclipse glasses alone, as concentrated sunlight can burn through the glasses and injure your eyes instantly.
- Pinhole projection: Make a small hole in a piece of cardboard and let sunlight pass through it onto another surface. You will see a projected image of the eclipsed Sun. Even the gaps between leaves of a tree act as natural pinholes, creating many little crescents on the ground.
Never use: sunglasses, even very dark ones, smoked glass, exposed film or homemade filters. They do not block enough harmful light.
During totality only: When the Moon completely covers the Sun inside the path of totality, it is safe to remove eclipse glasses and look at the corona with your naked eyes. As soon as the bright Sun begins to reappear, put your glasses back on.
Lunar eclipses are completely safe to watch with the naked eye, binoculars or a telescope.
Questions readers ask
Why are total solar eclipses so rare at any one place? The Moon's umbra is small, so the path of totality is narrow. Any specific place experiences a total eclipse only once every few hundred years on average.
Why can more people see a lunar eclipse than a solar eclipse? A lunar eclipse is visible from the entire night side of Earth, while a total solar eclipse is visible only along a narrow path.
What is the difference between an annular and a total eclipse? In an annular eclipse, the Moon is farther from Earth and appears smaller than the Sun, leaving a ring of sunlight. In a total eclipse, it covers the Sun completely.
Can I use sunglasses to watch a solar eclipse? No. Only certified eclipse glasses or proper solar filters are safe.
Why does the Moon turn red in a lunar eclipse? Earth's atmosphere bends sunlight into Earth's shadow and filters out blue light, leaving red light that illuminates the Moon.
In short
Eclipses happen when the Sun, Earth and Moon line up and a shadow falls on Earth or the Moon. Solar eclipses occur at new moon, when the Moon's shadow touches Earth; they can be total, partial, annular or hybrid. Lunar eclipses occur at full moon, when Earth's shadow falls on the Moon, often turning it red. Because the Moon's orbit is tilted, eclipses happen only during eclipse seasons. Watch solar eclipses only with certified eclipse glasses or projection, and enjoy lunar eclipses freely. Both are reminders of the beautiful precision of the solar system.