Penumbral Lunar Eclipse: Why the Faintest Eclipse of the Moon Is So Easy to Miss
A penumbral lunar eclipse happens when the Full Moon passes through only the faint outer part of Earth's shadow. The Moon dims so slightly that NASA says it can be difficult to notice.
Try it in the instrument
Set the sky to a penumbral lunar eclipse, with the Moon above the plane.
Also called
Penumbral eclipse, Total penumbral eclipse (rare variant where the whole Moon fits inside the penumbra, term used by NASA)
A simplified model
The drawings are computed from the numbers in the sources. They simplify the real sky. For exact times and dates, use official tables.
Picture a lunar eclipse that most people sleep through. The Full Moon crosses Earth's shadow, and the only sign is a slight dimming that comes and goes. NASA says the Moon dims so slightly that the eclipse can be difficult to notice. This is the penumbral lunar eclipse, the faintest of the three types.
This guide explains what the penumbra is, why eclipses in it are so hard to see, how astronomers measure how deep one goes, and why a few are rare "total penumbral" events. It ends with a short task in the Alignment Bench, where you place the Moon a little farther above the plane than for a partial eclipse.
What a penumbral lunar eclipse is
NASA describes a penumbral eclipse as one where the Moon travels through Earth's penumbra, the faint outer part of its shadow. NASA also adds a warning: if you do not know it is happening, you might miss it. The U.S. Naval Observatory's glossary calls the penumbra the area of partial shadow surrounding the umbra, the dark inner core.
Like every lunar eclipse, it happens at Full Moon. NASA's eclipse site sorts lunar eclipses into three types by how much of the Moon enters which part of the shadow.
| Type | Where the Moon goes | What NASA says you can see |
|---|---|---|
| Penumbral | Only into the outer shadow | A slight dimming that can be difficult to notice |
| Partial | Part of the disk enters the umbra | A dark bite, as if a semi-circle were cut from the Moon |
| Total | The entire disk enters the umbra | A red-orange Moon |
A penumbral eclipse has only two contact moments. P1 is when the Moon first touches the penumbra, and P4 is when it last leaves it. NASA's catalog says that neither of these events is observable.
Earth's outer shadow
The penumbra is a cone of partial shadow that widens as it stretches away from Earth, in the direction opposite the Sun. NASA's eclipse site explains that from inside this zone Earth blocks part of the Sun's disk, but not all of it. Some of the Sun's direct rays keep reaching even the most deeply shaded part of the Moon.
NASA puts a number on that. When one edge of the Moon is 9/10 of the way into the penumbra, about 10 percent of the Sun's rays still reach the Moon's deepest limb. Under those conditions the Moon stays relatively bright, with only a subtle gradient across its disk.
How big is the penumbra? NASA's Five Millennium Canon of Lunar Eclipses measures the band between the edge of the umbra and the outer edge of the penumbra. It ranges from 1,887.7 arcseconds when Earth is at aphelion to 1,951.9 arcseconds at perihelion. An arcsecond is 1/3,600 of a degree. The Moon itself spans 1,763.0 to 2,011.8 arcseconds, from apogee to perigee. So the penumbral band that rings the umbra is about as wide as the Moon.
The figure shows the Alignment Bench at Full Moon with the Moon 1.2 degrees above the plane of Earth's orbit. The Moon skims the outer band of the shadow and misses the umbra.
Why it is so hard to see
NASA's eclipse site is blunt about it. The contacts P1 and P4, along with the early and late stages of a penumbral eclipse, are completely invisible to the eye, with or without optical aid. The catalog adds that a penumbral eclipse cannot be detected visually unless its magnitude is greater than about 0.6.
The penumbral magnitude is the fraction of the Moon's diameter inside the penumbra. NASA says it ranges from 0.0004 to 1.0858 across the penumbral eclipses in its catalog, and that for most of them it is less than 1, so only part of the Moon's disk enters the penumbra. The shallowest ones are barely a trace. Even a Moon that is halfway in, at a magnitude of 0.5, falls below the 0.6 line.
There is a second threshold. NASA says a penumbral eclipse becomes readily apparent only when it is within about 0.05 magnitudes of becoming a partial eclipse. In other words, the Moon has to creep almost to the edge of the umbra before the shading is obvious.
How depth is measured
Because the Moon never touches the umbra in a penumbral eclipse, NASA lists its umbral magnitude as a negative number. The catalog explains that this number is the distance from the Moon's near edge to the edge of the umbra, in units of the Moon's diameter. A negative value means the Moon lies completely outside the umbra. The closer the number is to zero, the closer the Moon comes to a partial eclipse.
NASA's table of every lunar eclipse from 2021 through 2030 lists six penumbral eclipses. The Naval Observatory's list agrees with the two that have already happened.
| Date | Umbral magnitude | Where NASA's table says it is visible |
|---|---|---|
| May 5, 2023 | -0.046 | Africa, Asia, Australia |
| March 25, 2024 | -0.132 | Americas |
| February 20, 2027 | -0.057 | Americas, Europe, Africa, Asia |
| July 18, 2027 | -1.068 | East Africa, Asia, Australia, Pacific |
| August 17, 2027 | -0.525 | Pacific, Americas |
| December 9, 2030 | -0.163 | Americas, Europe, Africa, Asia |
Compare the three eclipses of 2027. The February 20 eclipse is the deepest, with an umbral magnitude of -0.057. That is close to NASA's 0.05 mark for a penumbral eclipse to look obvious. The July 18 eclipse, at -1.068, keeps the Moon more than a full Moon-width from the umbra, so it is the shallowest of the three.
In a few penumbral eclipses the Moon's whole disk fits inside the penumbra. NASA calls these total penumbral eclipses, and they occur when the penumbral magnitude reaches 1.0. They are rare. The Canon counts 141 of its 4,378 penumbral eclipses, or 3.2 percent. It explains why: the Moon cannot fit entirely inside the penumbral band when it is near perigee, because it looks too big. In the 21st century the Canon lists only five, on March 14, 2006 and then in 2053, 2070, 2082 and 2099.
Common mistakes
- Expecting a bite. A penumbral eclipse has none, because the Moon never reaches the umbra. A dark, curved edge means the Moon has reached the umbra, as in a partial lunar eclipse.
- Watching for the start. The first contact, P1, is completely invisible to the eye, so a clock is more useful than your eyes at that moment.
- Expecting a red Moon. NASA ties the red-orange glow to a total eclipse, when the Moon is inside the umbra and lit by sunlight filtered through Earth's atmosphere. See total lunar eclipse.
- Worrying about eye safety at the wrong eclipse. NASA says it is safe to look at the Moon with unprotected eyes or through a telescope during all types and all stages of a lunar eclipse, penumbral ones included. Solar eclipses are the ones that need care. NASA says you must never look directly at the Sun without proper eye protection, except during the brief totality of a total solar eclipse, and that regular sunglasses are not safe for viewing the Sun.
- Mixing up two penumbras. In a solar eclipse it is the Moon's shadow that has an umbra and a penumbra, and people standing in that penumbra see a partial solar eclipse. In a penumbral lunar eclipse the penumbra belongs to Earth, and the Moon is the one inside it.
Try it in the instrument
Open the Alignment Bench and set the sky to a penumbral lunar eclipse, with the Moon above the plane. Pick the Full Moon position and raise the Moon higher than you did for a partial eclipse, so it clears the umbra but still touches the outer shadow.
Here is the relation, in Moon-widths. NASA says the umbra is typically about 2.7 times the Moon's diameter, so its radius is about 1.35 Moon-widths. The Canon's penumbral band adds about one more Moon-width all the way around, which puts the penumbra's radius near 2.35. The Moon's own half-width adds 0.5 at each limit. So the Moon's center sits in the penumbra-only zone when it is between about 1.85 and about 2.85 Moon-widths from the middle of the shadow.
Now convert to the bench's units. NASA's Moon Fact Sheet gives the Moon's apparent diameter as 1,896 arcseconds, about 0.53 degrees, so one Moon-width is about half a degree. The penumbra-only zone then runs from about 1 degree to about 1.5 degrees above or below the plane. A target near 1.2 degrees lands in the middle. Compare that with NASA's August 17, 2027 eclipse, which has an umbral magnitude of -0.525, meaning the Moon's near edge stays about half a Moon-width from the umbra.
Keep going and the Moon leaves the shadow entirely. Past about 1.5 degrees even the penumbra misses it, and there is no eclipse at all, which is the story of why most Full Moons pass without an eclipse. The Moon's orbit is tilted about 5 degrees (NASA lists 5.145), so its height can reach that far, but the shadow covers only a small part of that range. These numbers are rough arithmetic from NASA's typical values, not official limits, and the bench uses a simplified version.
Questions people ask
When is the next penumbral lunar eclipse?
NASA's list of upcoming lunar eclipses names February 20, 2027 (UTC) as penumbral. NASA's decade table lists it as visible from the Americas, Europe, Africa and Asia, while the Moon page on NASA Science adds western Australia. Two more penumbral eclipses follow on July 18 and August 17, 2027.
How long does a penumbral eclipse last?
NASA's decade table leaves the duration blank for penumbral eclipses. The Canon says nine penumbral eclipses in its 5,000-year span last 292 minutes or more, close to 5 hours. All of them happen near apogee and within several weeks of aphelion, when Earth's penumbra is largest and the Moon looks small and moves slowly.
How common are penumbral eclipses?
They are the most common type. The Canon counts 36.3 percent of its 12,064 lunar eclipses as penumbral. NASA's table for 2021 through 2030 lists six penumbral eclipses among its 22 lunar eclipses. The Canon also says every calendar year has two to five lunar eclipses of all types.
Will binoculars or a telescope help?
Not for the faintest ones. NASA says a penumbral eclipse cannot be detected visually, with or without optical aid, unless its magnitude is greater than about 0.6. Looking through them is safe, though, because NASA says the Moon is safe to view with a telescope during every stage of a lunar eclipse.
Sources
- Eclipses and the Moon - NASA Science, NASA Science. https://science.nasa.gov/moon/eclipses/. Opened November 9, 2026.
- Eclipses - NASA Science, NASA Science. https://science.nasa.gov/eclipses/. Opened November 9, 2026.
- Eclipse Viewing Safety - NASA Science, NASA Science. https://science.nasa.gov/eclipses/safety/. Opened November 9, 2026.
- NASA - Visual Appearance of Lunar Eclipses, NASA Eclipse Web Site (Goddard Space Flight Center). https://eclipse.gsfc.nasa.gov/LEcat5/appearance.html. Opened November 9, 2026.
- NASA - Lunar Eclipse Figures, NASA Eclipse Web Site (Goddard Space Flight Center). https://eclipse.gsfc.nasa.gov/LEcat5/figure.html. Opened November 9, 2026.
- NASA - Enlargement of Earth's Shadows, NASA Eclipse Web Site (Goddard Space Flight Center). https://eclipse.gsfc.nasa.gov/LEcat5/shadow.html. Opened November 9, 2026.
- NASA - Lunar Eclipses: 2021 - 2030, NASA Eclipse Web Site (Goddard Space Flight Center). https://eclipse.gsfc.nasa.gov/LEdecade/LEdecade2021.html. Opened November 9, 2026.
- Five Millennium Canon of Lunar Eclipses: -1999 to +3000 (2000 BCE to 3000 CE), NASA Technical Publication TP-2009-214172 (Espenak and Meeus). https://eclipse.gsfc.nasa.gov/5MCLE/5MCLE-Text10.pdf. Opened November 9, 2026.
- Glossary, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/asa_glossary. Opened November 9, 2026.
- Eclipses of the Sun and Moon, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/data/UpcomingEclipses. Opened November 9, 2026.
- Moon Fact Sheet, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. 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.
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