Total Lunar Eclipse: Why the Full Moon Turns Red Inside Earth's Shadow
A total lunar eclipse happens when the entire Full Moon passes into the darkest part of Earth's shadow. Sunlight bent through Earth's air still reaches the Moon, so it glows red instead of vanishing.
Also called
Blood Moon (nickname used on NASA's Moon eclipse page)
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.
Most nights the Full Moon is a bright, steady disk. Once in a while a dark bite appears on one edge, grows across the face, and then the whole Moon turns a dim coppery red. The Moon does not go dark. It stays visible, lit by sunlight that has taken a detour around the planet.
This guide explains what makes a lunar eclipse total, why the color is red, how the phases unfold and how long totality lasts. It ends with a short task in the Alignment Bench, the instrument that shows the Sun, Earth and Moon from the side.
What makes a lunar eclipse total
A lunar eclipse can only happen at Full Moon. NASA says lunar eclipses occur during the full moon phase, and the U.S. Naval Observatory defines one as an eclipse in which the Moon passes through the shadow cast by the Earth. NASA adds that each lunar eclipse is visible from half of Earth.
NASA's pages name three kinds: total, partial and penumbral. The difference is how much of the Moon enters the umbra, the dark inner core of Earth's shadow. In a total eclipse, the Moon's entire disk passes through the umbra. In a partial lunar eclipse only part of the disk enters it, and in a penumbral lunar eclipse the Moon crosses only the faint outer shadow.
Astronomers sort eclipses with the umbral magnitude. The Naval Observatory defines magnitude as the fraction of the Moon's diameter covered by Earth's shadow at the greatest phase. NASA's eclipse tables turn that idea into a simple rule for each type.
| Type | What the Moon does | Umbral magnitude (NASA) |
|---|---|---|
| Total | The whole disk is inside the umbra | 1 or more |
| Partial | Part of the disk is inside the umbra | More than 0 and less than 1 |
| Penumbral | The disk stays out of the umbra | Less than 0 |
Earth's shadow has two parts
Earth casts a shadow in two layers. NASA's eclipse site explains that from inside the outer layer, the penumbra, Earth blocks part but not all of the Sun's disk. In the inner layer, the umbra, the planet's mass blocks all direct sunlight.
How wide is the umbra? NASA's eclipse site says it is typically about 2.7 times the Moon's diameter. That is why a Full Moon can sink completely inside it. NASA's Five Millennium Canon of Lunar Eclipses, a catalog of eclipses from 2000 BCE to 3000 CE, notes that the umbra is largest when Earth is at aphelion, its farthest point from the Sun. So 2.7 is a typical value and not a fixed one.
The figure shows the Alignment Bench at Full Moon with the Moon at a height of 0 degrees. The Moon rides on the plane of Earth's orbit and heads straight into the middle of the umbra. That is the picture to keep in mind for a total eclipse.
Why the Moon turns red
The red color is Earth's atmosphere at work. NASA explains that sunlight not blocked by our planet is filtered through a thick slice of air on its way to the Moon. The blues and violets scatter away, while the oranges and reds get through. NASA compares the result to every sunrise and sunset in the world being projected onto the Moon at once. Its eclipse site adds that the lower air and the stratosphere act together like a ring-shaped lens that bends reddened sunlight into the shadow.
The color is not the same every time. NASA's eclipse site says the Moon can range from bright orange to very dark grey during totality. The French astronomer A. Danjon proposed a five-point scale to rate it.
| Danjon value | How NASA describes the eclipse |
|---|---|
| 0 | Very dark; the Moon is almost invisible, especially at mid-totality |
| 1 | Dark, gray or brownish; details are hard to make out |
| 2 | Deep red or rust-colored; very dark center, brighter outer umbra |
| 3 | Brick-red; the edge of the umbra is usually bright or yellow |
| 4 | Very bright copper-red or orange; the edge looks bluish and bright |
Two NASA pages treat dust differently, so the sources do not fully agree. The Moon page on NASA Science says that the more dust or clouds in Earth's atmosphere during the eclipse, the redder the Moon appears. The NASA eclipse site says that after major volcanic eruptions in 1963, 1982 and 1991, eclipses were unusually dark. One in December 1992 left the Moon a dull gray disk that was hard to see with the naked eye. The pages do not explain how to reconcile the two statements. Both agree that the color depends on what is in Earth's air at the time.
The phases, step by step
NASA names six moments in an eclipse that turns total. The "P" moments mark the penumbra and the "U" moments mark the umbra. NASA's page on the March 3, 2026 total lunar eclipse gave the times in UTC, the world's common time standard, and they show the order well.
| Moment | Time on March 3, 2026 (UTC) | What NASA says happens |
|---|---|---|
| P1: penumbral eclipse begins | 8:44 | The Moon starts to dim, but the effect is subtle |
| U1: partial eclipse begins | 9:50 | A bite seems to be taken out of the Moon; the part in the umbra looks very dark |
| U2: totality begins | 11:04 | The entire Moon is in the umbra and tinted a coppery red |
| U3: totality ends | 12:03 | The red fades as the Moon leaves the umbra |
| U4: partial eclipse ends | 13:17 | The whole Moon is in the penumbra, and the dimming is subtle again |
| P4: penumbral eclipse ends | 14:23 | The Moon leaves the penumbra and the eclipse ends |
NASA's decade table lists the same eclipse with an umbral magnitude of 1.151, partial phases that last 3 hours 27 minutes and a total phase of 58 minutes. That matches the timeline above to within a minute.
Totality is the short part, and its length varies a lot. In NASA's table of every lunar eclipse from 2021 through 2030, totality runs from 15 minutes on May 26, 2021 (umbral magnitude 1.009) to 1 hour 42 minutes on June 26, 2029 (umbral magnitude 1.844). The whole stretch from the first bite to the last, totality included, stays between 3 hours 7 minutes and 3 hours 40 minutes. In that table, totality tends to be longer when the umbral magnitude is higher. The deeper the Moon sinks into the umbra, the longer it stays inside.
The Moon's path matters too. The Canon calls a total eclipse central when the Moon passes through the center line of the umbra. It counts 59.6 percent of its 3,479 total eclipses as central. The rest miss the center line but still fit inside the umbra.
The longest totalities in the Canon last about 106 minutes. It says they happen when the Moon is near apogee, its farthest point from Earth, so it looks small and moves slowly through a shadow that is at its largest. The most recent one was on July 16, 2000.
Common mistakes
- Thinking the Moon disappears. During totality the only light reaching the Moon is heavily filtered by Earth's atmosphere. The Moon is dim, not gone, and NASA describes colors from bright orange to very dark grey.
- Expecting the same red every time. Eclipses differ, which is why the Danjon scale exists. NASA suggests judging the Moon near the middle of totality.
- Reaching for eclipse glasses. You do not need them for the Moon. 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 solar eclipse is different. NASA says you must never look directly at the Sun without proper eye protection, except during the brief period of totality in a total solar eclipse. Regular sunglasses are not safe for viewing the Sun, and NASA's eclipse safety page explains safe viewers and indirect viewing.
- Calling the whole event "totality." Totality is only the middle. In the March 3, 2026 timeline, the partial phases alone lasted 3 hours 27 minutes.
- Assuming every Full Moon brings an eclipse. It does not. Read why most Full Moons pass without an eclipse.
Try it in the instrument
Open the Alignment Bench and set the sky to a total lunar eclipse. Choose the Full Moon position, with the Moon on the far side of Earth from the Sun, and bring the Moon's height to about 0 degrees.
Height is the key. 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). NASA says lunar eclipses occur when the Moon is near one of the two points where its orbit crosses that plane, so its path then makes an angle of about 5 degrees with the plane. The plane of Earth's orbit is also the Sun's apparent path, according to the Naval Observatory's glossary, and NASA says Earth's shadow points in the anti-solar direction, straight away from the Sun. So at Full Moon the middle of the shadow sits on the plane, and the Moon's height above or below it is the same as its distance from the middle of the shadow.
Now the rough arithmetic. 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. For the entire Moon to fit inside, its center has to stay within about 0.45 degrees of the middle. That is a back-of-the-envelope result from NASA's typical numbers, not an official limit, and the bench uses a simplified version. Aim for the middle, and you will match NASA's rule for a total eclipse: an umbral magnitude of 1 or more.
Then raise the Moon a little. Only part of the disk stays in the umbra, and the bench will no longer call it total. That is the partial lunar eclipse.
Questions people ask
How long does a total lunar eclipse last?
Totality lasts from 15 minutes to 1 hour 42 minutes in NASA's table for 2021 through 2030. The span from the start of the partial phase to its end, which includes totality, runs from 3 hours 7 minutes to 3 hours 40 minutes in the same table.
Can I watch without special equipment?
Yes. NASA's March 2026 page says all you need is a line of sight to the Moon. A dark place away from bright lights makes the view more dramatic, and binoculars or a telescope can improve it. For photos, NASA suggests a tripod and exposures of at least several seconds.
When is the next total lunar eclipse?
NASA's table of lunar eclipses for 2021 through 2030 lists the next one on December 31, 2028. It has an umbral magnitude of 1.246 and about 1 hour 11 minutes of totality, and NASA lists it as visible from Europe, Africa, Asia, Australia and the Pacific. Two more total eclipses follow in 2029, on June 26 and December 20. The table gives the calendar date of greatest eclipse, so check your local time.
Can everyone see the same eclipse?
Not quite. NASA says each lunar eclipse is visible from half of Earth. Its visibility maps show that the entire eclipse is visible only from the unshaded zone. Near the edges, the Moon may rise or set partway through the event.
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.
- 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.
- Moon Fact Sheet, NASA Space Science Data Coordinated Archive. https://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Opened November 9, 2026.
- Glossary, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/asa_glossary. 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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