Partial Solar Eclipse: Why Only Part of the Sun Is Covered and Who Gets to See It

Pictures of an eclipse in progress often show the Sun as a crescent, as if something had taken a bite out of it. That is a partial solar eclipse. It is the most common type in NASA's count of eclipses, and the part of the Sun left uncovered is still intense enough to harm the eye.

This guide explains what a partial eclipse is, why only part of the Moon's shadow reaches Earth, how to read the numbers that describe how much of the Sun is covered, and what NASA says about watching one safely. It ends with a task in the Alignment Bench.

What a partial solar eclipse is

NASA says a partial solar eclipse happens when the Moon passes between the Sun and Earth but the Sun, Moon and Earth are not perfectly lined up. Only part of the Sun appears covered, which gives it a crescent shape. NASA's glossary defines it as a solar eclipse seen from within the Moon's penumbra, where the Moon appears to block part of the Sun but not all of it.

There are two ways to meet one. In the first, the whole eclipse is partial. NASA's eclipse tables define this type as an eclipse in which the Moon's penumbral shadow crosses Earth while the umbral and antumbral shadows completely miss it. In the second, you are standing outside the narrow path of a total or annular eclipse. NASA says people outside the area covered by the Moon's inner shadow see a partial eclipse in those cases too.

Why only the penumbra touches Earth

NASA's geometry page says the Moon's shadow has two concentric cones: a dark inner shadow, the umbra, and a lighter outer shadow, the penumbra. Within the penumbra the Sun is only partially blocked. When the Moon and Sun are not perfectly aligned, only the Moon's penumbra hits Earth. The umbra misses our planet completely.

Why would they not line up? The Moon's orbit is tilted by about five degrees to the plane of Earth's orbit, according to NASA. At most New Moons the Moon passes above or below the Sun, and its shadow misses Earth. NASA's video caption puts it this way: sometimes the shadow is too high, sometimes too low, and other times it is just right. A partial eclipse sits at the edge of "just right." The shadow's axis misses Earth, but the outer shadow still touches it.

NASA's eclipse maps give a number for this. They call an eclipse central, meaning total, annular or hybrid, if the shadow axis passes within 0.997 Earth radii of Earth's center. For a partial eclipse the shadow axis misses Earth entirely. NASA adds that the place where the eclipse is greatest then lies on the day and night boundary, with the Sun on the horizon.

The drawing is the Bench's side view with the Moon at its near position and 1.2 degrees above the plane of Earth's orbit.

What you see, and where

NASA's eclipse guide says that on the day of a total eclipse a partial eclipse is visible across a much larger area. The penumbra is wide. NASA's maps note that its region of visibility often covers more than half of the daylight side of Earth. In the 1999 example on NASA's animation page, the penumbra was about 4,300 miles (6,900 kilometers) in diameter and took a little over five hours to sweep across Earth. Everyone inside it saw a partial eclipse, and outside it no eclipse was visible.

Two numbers describe how much of the Sun is hidden. NASA's glossary defines them:

• Eclipse magnitude is the fraction of the Sun's diameter covered by the Moon. By convention it is given at the instant of greatest eclipse.
• Eclipse obscuration is the fraction of the Sun's area covered by the Moon. NASA warns not to mix it up with magnitude.

On NASA's maps, the edge of the penumbra is a curve of eclipse magnitude 0.0. For a total eclipse, the edge of the umbra is a curve of magnitude 1.0. Every place between the two edges sees a partial eclipse of some size.

In NASA's count, partial eclipses are the most common type. The Five Millennium Canon of Solar Eclipses lists 4,200 partial eclipses among 11,898 solar eclipses from 2000 BCE to 3000 CE, which is 35.3 percent.

Watching safely

For a partial eclipse NASA gives no exceptions. There is no period of totality, so it is never safe to look directly at the eclipse without proper eye protection.

• Look through safe eclipse glasses or a safe handheld solar viewer at all times. NASA says regular sunglasses, no matter how dark, are not safe.
• Safe viewers are thousands of times darker and ought to comply with the ISO 12312-2 international standard. NASA does not approve any particular brand.
• Inspect the viewer first and discard it if it is torn, scratched or damaged. Always supervise children.
• Never look at the Sun through a camera lens, telescope, binoculars or other optics while wearing eclipse glasses. Those devices need a solar filter fixed over the front.
• Without glasses, use a pinhole projector with the Sun at your back. NASA says the circular holes of a colander project crescent shapes onto the ground during the partial phases. Never look at the Sun through the pinhole.

NASA's Goddard eclipse glossary adds a warning aimed at the crescent. Even when 99 percent of the Sun's surface is covered, the remaining crescent is intense enough to cause permanent retinal damage. NASA's Goddard eye-safety page says the light level can be comparable to twilight and the crescent can still burn the retina. The same page says retinal injury is painless and its effects may not show for at least several hours. The guide to watching eclipses safely has the full list.

Common mistakes

• Thinking a partial eclipse is safe to look at. NASA says it is never safe without proper eye protection.
• Trusting a dim Sun. Light levels comparable to twilight do not mean the retina is safe, according to NASA's Goddard eye-safety page.
• Mixing up magnitude and obscuration. Magnitude counts the Sun's width that is covered. Obscuration counts its area.
• Looking through the pinhole. The projector is for looking at the image, not at the Sun.
• Expecting a partial eclipse only near the center. The penumbral zone can span a thousand miles or more.
• Calling a crescent Sun a total eclipse. In a total eclipse the Sun is completely blocked. A crescent means the umbra is somewhere else, or nowhere.
• Setting the Moon too far above the plane in the Bench. If the Moon is too high, its shadow misses Earth completely and there is no eclipse at all. See why most New Moons bring no solar eclipse.

Try it in the instrument

Open the Alignment Bench and set the sky to a partial solar eclipse, with the Moon at its near position and above the plane. Three settings decide the result.

1. Phase: New Moon. Solar eclipses only occur at a New Moon, according to NASA.
2. Height: above the plane, but not by much. Fred Espenak of NASA's Goddard Space Flight Center writes that if the New Moon takes place within about 17 degrees of a node, a solar eclipse will be visible from some location on Earth. The Moon's orbit is inclined about 5.1 degrees, so near a node the Moon is close to the plane. Inside the central limit of 0.997 Earth radii, the shadow's axis hits Earth and you get a total, annular or hybrid eclipse. Beyond it the axis misses Earth. NASA's tables note a rare in-between case, called non-central, where the edge of the umbra or antumbra still grazes Earth. Farther out, only the penumbra touches Earth, and that is your partial eclipse.
3. Position: near. At the near position an aligned Moon would give a total eclipse, so height is the only thing that keeps the umbra off Earth.

If you go too far above the plane, the shadow misses Earth completely. The total solar eclipse is the same picture with the Moon on the plane. The Bench is a simplified model with a range of accepted heights, so it teaches the relation and does not forecast real eclipses.

Questions people ask

What is the difference between a partial and a total solar eclipse?

In a total eclipse the Moon's umbra reaches you and the Sun is completely blocked. In a partial eclipse you are in the penumbra, and the Moon covers only part of the Sun.

Why do people at the edge of a total eclipse see only a partial one?

They are outside the narrow umbra path but inside the wide penumbra. NASA says a typical umbral path may be less than 50 miles wide, while the penumbral zone can span a thousand miles or more.

Can I look at a partial eclipse with the naked eye?

No. NASA says it is never safe to look directly at a partial eclipse without proper eye protection, such as safe eclipse glasses or a safe handheld solar viewer, or to use an indirect method like a pinhole projector.

How do scientists say how much of the Sun is covered?

With eclipse magnitude, the fraction of the Sun's diameter covered by the Moon, and obscuration, the fraction of its area covered, according to NASA's glossary.

Sources

• Types of Solar Eclipses, NASA Science. https://science.nasa.gov/eclipses/types/. Opened November 9, 2026.
• Why Do Eclipses Happen?, NASA Science. https://science.nasa.gov/eclipses/geometry/. Opened November 9, 2026.
• Eclipse Viewing Safety, NASA Science. https://science.nasa.gov/eclipses/safety/. Opened November 9, 2026.
• Eclipse Glossary, NASA Science. https://science.nasa.gov/eclipses/glossary/. Opened November 9, 2026.
• What to Expect: A Solar Eclipse Guide, NASA Science. https://science.nasa.gov/feature/solar-eclipse-guide/. Opened November 9, 2026.
• Glossary of Solar Eclipse Terms, NASA Goddard Space Flight Center, Eclipse Web Site (Fred Espenak). https://eclipse.gsfc.nasa.gov/SEhelp/SEglossary.html. Opened November 9, 2026.
• Eye Safety During Solar Eclipses, NASA Goddard Space Flight Center, Eclipse Web Site (B. Ralph Chou, University of Waterloo School of Optometry). https://eclipse.gsfc.nasa.gov/SEhelp/safety2.html. Opened November 9, 2026.
• Key to Solar Eclipse Maps, NASA Goddard Space Flight Center, Eclipse Web Site (Espenak and Meeus). https://eclipse.gsfc.nasa.gov/SEcat5/SEmapkey.html. Opened November 9, 2026.
• Animation - Total Solar Eclipse of 1999 August 11, NASA Goddard Space Flight Center, Eclipse Web Site (Fred Espenak). https://eclipse.gsfc.nasa.gov/SEmono/TSE1999/T99animate.html. Opened November 9, 2026.
• Periodicity of Solar Eclipses, NASA Goddard Space Flight Center, Eclipse Web Site (Fred Espenak). https://eclipse.gsfc.nasa.gov/SEsaros/SEperiodicity.html. Opened November 9, 2026.
• Five Millennium Canon of Solar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Technical Publication TP-2006-214141 (Espenak and Meeus). https://eclipse.gsfc.nasa.gov/5MCSE/5MCSE-Text11.pdf. Opened November 9, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

Read next

• Total Solar EclipseA total solar eclipse is the Moon's darkest shadow, the umbra, landing on Earth.
• Annular Solar EclipseAn annular solar eclipse happens when the Moon is at or near its farthest point, so it looks smaller than the Sun and leaves a bright ring.