An eclipse happens when one celestial body moves into the shadow of another. On Earth, we experience two kinds: solar eclipses, where the Moon passes between Earth and the Sun, and lunar eclipses, where Earth passes between the Sun and the Moon. Both depend on a precise alignment of all three bodies, and the specific type of eclipse you see depends on where you are and how the geometry lines up.
Why Eclipses Don’t Happen Every Month
Since the Moon orbits Earth roughly once a month, you might expect an eclipse at every full moon and every new moon. The reason that doesn’t happen comes down to a slight tilt. The Moon’s orbit is angled about 5 degrees relative to Earth’s orbit around the Sun. Most months, the Moon passes a bit above or below the Sun’s position in the sky, and no eclipse occurs.
The Moon’s tilted orbit crosses Earth’s orbital plane at two points called nodes. An eclipse can only happen when the Moon is near one of these crossing points at the same time it’s in its new or full phase. These windows, sometimes called eclipse seasons, shift gradually over time, which is why eclipses are relatively rare at any given location. In a typical year, Earth sees two to five solar eclipses and zero to three lunar eclipses globally, though most people won’t be in the right spot to witness any particular one.
The Cosmic Coincidence Behind Solar Eclipses
Solar eclipses are possible because of an extraordinary fluke of geometry. The Sun is about 400 times larger in diameter than the Moon, and it also happens to sit roughly 400 times farther away from Earth. This makes the two objects appear almost exactly the same size in our sky. When the Moon slides directly in front of the Sun, it can cover the solar disk almost perfectly.
This match isn’t exact every time, though, because both the Moon’s distance from Earth and Earth’s distance from the Sun vary slightly throughout their orbits. Those small changes in distance produce different types of solar eclipses.
Types of Solar Eclipses
The Moon casts a shadow with distinct zones, and which zone sweeps over your location determines what you see.
- Total solar eclipse: The Moon is close enough to Earth to completely cover the Sun. If you’re standing within the darkest core of the shadow (called the umbra), the Sun’s bright surface vanishes entirely for a few minutes. The sky darkens dramatically, and the Sun’s faint outer atmosphere, the corona, becomes visible as a pale halo.
- Annular solar eclipse: The Moon is a bit farther from Earth, so it appears slightly smaller than the Sun and can’t cover it completely. Instead, a bright ring of sunlight remains visible around the Moon’s silhouette. This happens when you’re within a zone of the shadow that extends beyond the darkest core.
- Partial solar eclipse: The Moon’s lighter outer shadow (the penumbra) crosses your location, but you’re outside the path where the Moon lines up centrally with the Sun. You see the Moon take a bite out of one side of the Sun’s disk.
A total eclipse follows a narrow path across Earth’s surface, typically only about 100 miles wide. People outside that path but still under the penumbra see a partial eclipse. People farther away see nothing unusual at all.
What a Total Solar Eclipse Looks and Feels Like
A total solar eclipse unfolds in stages. First, the Moon’s edge makes contact with the Sun’s edge, and a small notch appears on one side of the solar disk. Over the next hour or so, the Moon gradually covers more and more of the Sun. The light takes on an eerie quality, temperatures can drop noticeably, and shadows on the ground sharpen.
In the final seconds before totality, the thin remaining crescent of sunlight breaks into tiny bright points called Baily’s beads, caused by sunlight streaming through valleys and craters along the Moon’s uneven edge. As only one bright point remains, it produces a burst of light on one side of the Moon while the corona begins to glow faintly on the opposite side. This creates what observers call the diamond ring effect: a single brilliant spark set into a pale luminous ring. Princeton professor Henry Norris Russell, who first described the phenomenon, noted it’s partly an optical illusion experienced by the naked eye.
Then totality arrives. The Sun’s surface disappears completely, the corona shimmers outward in delicate streamers, and stars or bright planets may become visible. Totality lasts anywhere from a few seconds to about seven and a half minutes, depending on the geometry. When it ends, the sequence reverses: a diamond ring flares on the opposite edge, Baily’s beads reappear briefly, and the crescent Sun begins to grow again.
Types of Lunar Eclipses
A lunar eclipse works in the opposite direction. Earth blocks sunlight from reaching the Moon. Because Earth is much larger than the Moon, its shadow is wide enough to cover the entire lunar disk, so lunar eclipses are visible from anywhere on the night side of the planet (unlike solar eclipses, which require you to be in a narrow path).
A total lunar eclipse occurs when the Moon passes entirely into Earth’s darkest shadow. A partial lunar eclipse happens when only part of the Moon enters that dark shadow. There’s also a penumbral lunar eclipse, where the Moon passes through Earth’s faint outer shadow. Penumbral eclipses are subtle enough that most people don’t notice them.
Why the Moon Turns Red
During a total lunar eclipse, the Moon doesn’t go black. Instead, it glows a deep red or copper color, which is why total lunar eclipses are sometimes called “blood moons.” The explanation is the same physics that makes sunsets red.
Sunlight is made up of all visible colors, each with a different wavelength. Blue light has a short wavelength and scatters easily when it hits molecules in Earth’s atmosphere. That scattering is why the daytime sky looks blue. Red light has a longer wavelength and passes through the atmosphere more easily. During a total lunar eclipse, the only sunlight reaching the Moon has been filtered and bent through the thickest parts of Earth’s atmosphere around its edges. The blue light gets scattered away, and what’s left is red and orange. NASA describes it as if all the world’s sunrises and sunsets were being projected onto the Moon at once. This filtering process is called Rayleigh scattering.
The exact shade varies from eclipse to eclipse. Heavy cloud cover or volcanic ash in Earth’s atmosphere can make the Moon appear darker, almost brownish. Clear skies tend to produce a brighter, more vivid red.
Viewing Solar Eclipses Safely
Looking directly at the Sun during a partial or annular solar eclipse can cause serious eye damage. Regular sunglasses, even very dark ones, are not safe for solar viewing. You need dedicated eclipse glasses or a handheld solar viewer that complies with the ISO 12312-2 international safety standard. These filters are thousands of times darker than ordinary sunglasses.
The one exception is during the brief minutes of totality in a total solar eclipse, when the Moon completely covers the Sun’s bright surface. At that point, and only at that point, it’s safe to look without a filter. As soon as any sliver of the Sun reappears, you need your eclipse glasses back on. Lunar eclipses, by contrast, are perfectly safe to watch with the naked eye since you’re just looking at reflected light on the Moon.
The Next Major Eclipse
The next total solar eclipse occurs on August 12, 2026. Its path of totality will cross the Arctic Ocean, Greenland, Iceland, the Atlantic Ocean, Portugal, and northern Spain. For anyone in Europe or nearby regions, this will be the most accessible total solar eclipse in years.