A penumbral eclipse is a lunar eclipse where the Moon passes through Earth’s outer, partial shadow (the penumbra) without entering the darker central shadow (the umbra). It’s the subtlest type of lunar eclipse, often so faint that most people wouldn’t notice it happening. Of the 228 lunar eclipses occurring during the 21st century, 86 of them (about 38%) are penumbral eclipses, making them the most common type.
How Earth’s Two Shadows Create Different Eclipses
Earth casts two distinct shadows into space. The umbra is the deep, central cone where Earth blocks all direct sunlight. Surrounding it is the penumbra, a much larger, lighter shadow where Earth blocks only some of the Sun’s light. The penumbra exists because the Sun isn’t a single point of light. It’s a disk spanning about half a degree in the sky, so its rays reach around Earth’s edges at slightly different angles, creating that gradient of partial shadow.
During a total lunar eclipse, the Moon passes through the penumbra and then fully into the umbra, producing the dramatic red-orange “blood moon.” A partial eclipse sends part of the Moon into the umbra. A penumbral eclipse, by contrast, keeps the Moon entirely within that outer shadow. The Moon never touches the umbra at all, which is why the visual effect is so mild.
Why Penumbral Eclipses Are Hard to See
Penumbral eclipses are pale and difficult to observe. The earliest and latest stages are completely invisible to the eye, even with binoculars or a telescope. This is because the penumbra only partially blocks sunlight. Even when the Moon’s edge is 90% of the way into the penumbral shadow, roughly 10% of the Sun’s rays still reach that deepest point. The result is a Moon that stays relatively bright, with only a subtle shading gradient across its surface.
The effect only becomes noticeable when the eclipse magnitude gets close to 1.0, meaning the Moon is nearly deep enough in the penumbra to start entering the umbra. NASA notes the shading becomes “readily apparent” only when the eclipse is within about 0.05 magnitudes of becoming a partial eclipse. In practical terms, most penumbral eclipses come and go without anyone on the ground realizing the Moon looks any different.
Compare this with a partial eclipse, where the boundary between the bright penumbra and the dark umbra creates a sharp, obvious line across the Moon’s face. That umbral edge typically appears very dark or black, making partial eclipses easy naked-eye events. Penumbral eclipses offer nothing that crisp.
What Causes a Penumbral Eclipse
A lunar eclipse of any type can only happen during a full Moon, when the Moon is on the opposite side of Earth from the Sun. But not every full Moon produces an eclipse. The Moon’s orbit is tilted about 5 degrees relative to Earth’s orbit around the Sun, so most months the Moon passes above or below Earth’s shadow entirely.
Eclipses happen when the full Moon falls near a “node,” one of the two points where the Moon’s orbital path crosses the plane of Earth’s orbit. Whether the result is a penumbral, partial, or total eclipse depends on how closely the three bodies line up. A penumbral eclipse occurs when the alignment is close enough for the Moon to clip Earth’s outer shadow but not precise enough for it to reach the umbra. The more off-center the alignment, the shallower the penumbral eclipse, and the less visible it becomes.
Penumbral Eclipse Magnitude
Eclipse magnitude measures how deeply the Moon dips into a given shadow, expressed as a fraction of the Moon’s diameter. For penumbral eclipses during the current century, magnitudes range from as low as 0.0004 (the Moon barely grazing the penumbra’s edge) to as high as 1.0858 (the Moon completely engulfed in the penumbra). A magnitude above 1.0 means the entire Moon sits within the penumbral shadow, sometimes called a “total penumbral eclipse.” These are the only penumbral eclipses that produce a visually noticeable dimming across the whole lunar disk, and they’re relatively rare.
How to Watch One
Unlike solar eclipses, lunar eclipses are completely safe to watch with your bare eyes. No special glasses or filters are needed, because you’re looking at reflected sunlight on the Moon’s surface, not at the Sun itself. Binoculars or a small telescope can help you spot the subtle shading, but they aren’t required.
If you want to catch the effect, your best strategy is to watch during the middle of the eclipse, when the Moon is deepest in the penumbra. Look for a slight darkening or tea-stain quality along one edge of the Moon’s face. Comparing the Moon’s brightness to how it looked an hour before the eclipse began can help your eye detect the difference. Photographing the Moon at regular intervals and comparing the images afterward often reveals the shading more clearly than real-time observation.
Upcoming Penumbral Eclipses
Penumbral eclipses occur several times per decade. Recent ones include May 5, 2023 and March 25, 2024. Looking ahead, 2027 features an unusually busy stretch with three penumbral eclipses: February 20, July 18, and August 17. Visibility depends on which side of the Earth faces the Moon during the event, so not every penumbral eclipse is observable from any given location. Local astronomy groups and NASA’s eclipse page list visibility maps for each event.