Partial Lunar Eclipse: How Earth's Shadow Takes a Curved Bite Out of the Full Moon
A partial lunar eclipse happens when only part of the Full Moon passes through the dark inner part of Earth's shadow. The Moon sits a little off the center of the shadow, so a curved bite appears and then slides away.
Try it in the instrument
Set the sky to a partial lunar eclipse, with the Moon above the plane.
Also called
No other names found in our sources.
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.
Sometimes a Full Moon rises with a piece missing. A dark, rounded bite creeps in from one side, reaches its deepest point and then slides away, while the rest of the Moon stays bright. The Moon is at its fullest phase the whole time, so the bite is not a phase. It is Earth's shadow.
This guide explains what a partial lunar eclipse is, why the shadow's edge is curved, how astronomers measure the bite and how long it lasts. It ends with a short task in the Alignment Bench, where you place the Moon a little above the plane of Earth's orbit.
What a partial lunar eclipse is
NASA describes a partial lunar eclipse as the result of an imperfect alignment of the Sun, Earth and Moon. The Moon passes through only part of Earth's umbra, and the shadow grows and then recedes without ever entirely covering it. The umbra is the dark inner core of Earth's shadow. Around it lies the faint outer shadow, the penumbra, and the Moon crosses that as well.
Like every lunar eclipse, it happens at Full Moon. There are three types. NASA's Five Millennium Canon of Lunar Eclipses counted all 12,064 lunar eclipses between 2000 BCE and 3000 CE and sorted them this way.
| Type | Share of those eclipses | Umbral magnitude (NASA) |
|---|---|---|
| Penumbral | 36.3 percent | Less than 0 |
| Partial | 34.9 percent | More than 0 and less than 1 |
| Total | 28.8 percent | 1 or more |
A partial eclipse has four contact moments. P1 and P4 mark the Moon entering and leaving the penumbra. U1 and U4 mark the Moon's edge entering and leaving the umbra, which are the start and end of the partial phase. The U.S. Naval Observatory's diagram guide uses the same labels. NASA also marks the instant of greatest eclipse, when the Moon passes closest to the shadow's axis.
| Moment | What it means | Can you see it? |
|---|---|---|
| P1 | The Moon touches the penumbra | No. NASA says it is completely invisible to the eye, with or without optical aid |
| U1 | The Moon's edge enters the umbra | Yes. The bite begins |
| Greatest eclipse | The Moon is closest to the shadow's axis | Yes. The bite is deepest |
| U4 | The Moon's edge leaves the umbra | Yes. The bite ends |
| P4 | The Moon leaves the penumbra | No. Invisible, like P1 |
Why the shadow looks curved
The edge of the bite is a smooth curve. NASA's eclipse site explains that the umbra is typically about 2.7 times as wide as the Moon, so it looks as if a semi-circular bite has been taken out of the Moon. The part of the Moon inside the umbra usually looks very dark or black. NASA says this is mostly a contrast effect, because the rest of the Moon, still in the penumbra, may be brighter by a factor of about 500.
The curve also carries a famous lesson. NASA's eclipse site credits Aristotle (384 to 322 BCE) with using the curved shadow at partial eclipses to show that Earth is round. He noticed that Earth's shadow was round no matter where the eclipse took place, whether the Moon was high in the sky or low near the horizon. He reasoned that only a sphere casts a round shadow from every angle.
Do not expect a razor edge. The Canon says there is no abrupt boundary between the umbra and the penumbra, because the shadow's density varies continuously. It changes fastest near the theoretical edge of the umbra.
The figure shows the Alignment Bench at Full Moon with the Moon 0.7 degrees above the plane of Earth's orbit. The Moon is not centered on the shadow, so only part of it gets into the umbra.
Measuring the bite
Astronomers measure the bite with the umbral magnitude, the fraction of the Moon's diameter inside the umbra at the moment of greatest eclipse. For a partial eclipse it is always more than 0 and less than 1. A value of 0.5 means the umbra covers half of the Moon's width.
NASA's table of every lunar eclipse from 2021 through 2030 lists seven partial eclipses.
| Date | Umbral magnitude | Length of the partial phase |
|---|---|---|
| November 19, 2021 | 0.974 | 3 hours 28 minutes |
| October 28, 2023 | 0.122 | 1 hour 17 minutes |
| September 18, 2024 | 0.085 | 1 hour 3 minutes |
| August 28, 2026 | 0.930 | 3 hours 18 minutes |
| January 12, 2028 | 0.066 | 56 minutes |
| July 6, 2028 | 0.389 | 2 hours 21 minutes |
| June 15, 2030 | 0.502 | 2 hours 24 minutes |
The pattern is clear: the deeper the bite, the longer the partial phase. The August 28, 2026 eclipse covered 0.930 of the Moon's diameter and lasted 3 hours 18 minutes. The one listed for January 12, 2028 covers only 0.066 and lasts 56 minutes.
The Moon's speed matters too. The Canon lists seven partial eclipses of 209 minutes or more, about 3.5 hours. All of them happen with the Moon close to apogee, its farthest point from Earth, so its slow trajectory through the umbra tends to prolong the partial phase.
NASA's catalog also gives every eclipse a gamma value. Gamma is the closest distance between the Moon's center and the axis of the umbra, in units of Earth's equatorial radius, and it is positive or negative depending on whether the Moon passes north or south of that axis. The bench shows the same idea as a height above or below the plane of Earth's orbit.
Common mistakes
- Thinking the bite is a Moon phase. A lunar eclipse happens only at Full Moon, when the whole face is lit. The dark bite is Earth's shadow.
- Assuming "partial" means small. NASA's table lists the August 28, 2026 eclipse at an umbral magnitude of 0.930, which is nearly the whole width of the Moon.
- Waiting for the penumbral stage to show. The Moon enters the penumbra first, but NASA says that first contact, P1, is completely invisible to the eye. Skilled observers need a penumbral magnitude of about 0.6 to detect even faint shading. The obvious show is the umbral bite, which begins at U1.
- Expecting a razor-sharp edge. The shadow fades gradually, so two observers may mark the edge in slightly different places.
- Treating a partial lunar eclipse like a partial solar 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. A partial solar eclipse is different. NASA says you must look through safe solar viewing glasses or a safe handheld solar viewer at all times, and that viewing the Sun through binoculars or a telescope without a special-purpose solar filter will instantly cause severe eye injury. NASA's eclipse safety page has the details.
Try it in the instrument
Open the Alignment Bench and set the sky to a partial lunar eclipse, with the Moon above the plane. Pick the Full Moon position, then raise the Moon until the shadow covers only part of it.
Here is the relation. The Moon's orbit is tilted about 5 degrees to the plane of Earth's orbit (NASA's Moon Fact Sheet lists 5.145 degrees), so the Moon's height changes during every trip around Earth. NASA says lunar eclipses occur when the Moon is near one of the two points where its orbit crosses the plane. At Full Moon the middle of Earth's shadow lies on that plane, because the shadow points directly away from the Sun, and the Sun's apparent path is that same plane, according to the Naval Observatory. A Moon at height 0 crosses the middle and can fit entirely inside the umbra. Lift it, and its center drifts away from the middle of the shadow, so more of the disk sticks out.
Now some rough numbers. NASA's fact sheet gives the Moon's apparent diameter as 1,896 arcseconds, about 0.53 degrees. An umbra about 2.7 times that wide spans roughly 1.4 degrees as seen from Earth. By that arithmetic, the first bite appears when the Moon's center is about 0.97 degrees from the middle of the shadow. Below roughly 0.45 degrees the whole Moon fits inside, and you get a total eclipse. Between the two you get a partial eclipse. At about 0.7 degrees, roughly half of the Moon's width is in the umbra, similar to the June 15, 2030 eclipse (umbral magnitude 0.502). These are back-of-the-envelope results from NASA's typical numbers, not official limits, and the bench uses a simplified version.
Raise the Moon even higher and it slips out of the umbra completely. Then only the faint outer shadow reaches it, which is a penumbral lunar eclipse. Lower it toward 0 and you are back to a total lunar eclipse.
Questions people ask
How much of the Moon does the shadow cover?
Anywhere from a sliver to nearly all of its width. In NASA's table for 2021 through 2030, the umbral magnitude of partial eclipses runs from 0.066 to 0.974. That means the umbra covers between about 7 percent and about 97 percent of the Moon's diameter.
When is the next partial lunar eclipse?
The most recent one in NASA's table was on August 28, 2026, and the Naval Observatory's list of recent eclipses also marks it as partial. NASA lists it as visible from the eastern Pacific, the Americas, Europe and Africa. Three penumbral eclipses come in 2027. After them, the next partial eclipse in the table is on January 12, 2028, visible from the Americas, Europe and Africa, followed by July 6, 2028.
Do I need a telescope or special glasses?
No. NASA calls partial eclipses easy naked-eye events. Binoculars or a telescope can improve the view, according to NASA's page on the March 2026 eclipse, but they are optional.
How is a partial eclipse different from a penumbral one?
In a penumbral eclipse the Moon never touches the umbra, so its umbral magnitude is negative and the dimming is so slight that it can be hard to notice. NASA says skilled observers can detect faint shading only when the penumbral magnitude passes about 0.6. A partial eclipse always has the dark umbral bite.
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 - Periodicity of Lunar Eclipses, NASA Eclipse Web Site (Goddard Space Flight Center). https://eclipse.gsfc.nasa.gov/LEsaros/LEperiodicity.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.
- Eclipses of the Sun and Moon, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/data/UpcomingEclipses. Opened November 9, 2026.
- Explanation of Lunar Eclipse Diagrams, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/downloads/eclipses/lun_ecl_expl_web.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.
- Moon Fact Sheet, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Opened November 9, 2026.
- March 2026 Total Lunar Eclipse: Your Questions Answered, NASA Science. https://science.nasa.gov/solar-system/moon/march-2026-total-lunar-eclipse-your-questions-answered/. 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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