Why Most Full Moons Pass Without a Lunar Eclipse: The Tilt of the Moon's Orbit

Every month the Moon swings around to the far side of Earth from the Sun, and every month it is full. If the Moon's path lay flat in the same plane as Earth's orbit, it would slide into Earth's shadow each time, and a lunar eclipse would arrive with every Full Moon. It does not work that way. NASA says the reason is a tilt in the Moon's orbit.

This guide shows how that tilt works, what an eclipse season is and why a Full Moon has to fall in the right place to bring an eclipse. It uses the 13 Full Moons of 2026 as a check and ends with a short task in the Alignment Bench, where the Moon sits 2 degrees below the plane.

A Full Moon is necessary but not enough

NASA says lunar eclipses occur during the full moon phase, and its Five Millennium Canon of Lunar Eclipses states that eclipses of the Moon can only occur at Full Moon. The reverse does not hold. A Full Moon is the right setting, but the Moon also has to pass through Earth's shadow, which points straight away from the Sun.

NASA's Space Place page puts it simply. The Moon goes around Earth every month, but it does not always get in Earth's shadow. The Moon can be behind Earth and still get hit by light from the Sun.

NASA's Moon page counts the result: anywhere from four to seven eclipses a year, solar and lunar together. The Moon Fact Sheet lists the cycle of phases as 29.53 days, so there are more Full Moons in a year than eclipses.

The tilt of the Moon's orbit

Think of Earth's orbit around the Sun as a flat sheet, which astronomers call the ecliptic plane. NASA's page on why eclipses happen says the Moon does not orbit in that plane. Instead, the Moon's orbit around Earth is tilted, or inclined, by about five degrees. NASA's technical eclipse pages give the figure as 5.1 degrees, and the Moon Fact Sheet lists 5.145.

Five degrees sounds small, but the shadow is small too. NASA says Earth's umbra is typically about 2.7 times the Moon's diameter. The fact sheet gives the Moon's apparent diameter as 1,896 arcseconds, about half a degree. By that arithmetic the umbra is only about 1.4 degrees wide as seen from Earth, so a Moon a couple of degrees above or below the plane sails right past it.

The tilted orbit crosses the plane at two points called nodes. NASA's technical page says the Moon's orbit crosses the ecliptic at two points, and the Naval Observatory defines a node as either of the points where the plane of an orbit intersects a reference plane. Near a node the Moon is level with the plane, and that is where lunar eclipses occur. NASA says the Moon is then near one of its nodes, moving at an angle of about 5 degrees to the ecliptic. Everywhere else along its orbit the Moon is above or below the plane.

The figure shows the Alignment Bench at Full Moon with the Moon 2 degrees below the plane. Earth's shadow sits on the plane, so the Moon passes underneath it.

Eclipse seasons

How close to a node must the Full Moon be? NASA's page on the periodicity of lunar eclipses gives a rule. If Full Moon takes place within about 17 degrees of a node, a lunar eclipse will be visible from a portion of Earth. The page adds that the actual value ranges from 15.3 to 17.1 degrees because of the shape of the orbits. This angle is measured along the Moon's orbit, not as a height above the plane.

That gives each node an eclipse zone 34 degrees wide. The Sun appears to cross a zone in 34.5 days, according to the Canon, and NASA's Eclipses page says each season lasts about 35 days. One NASA page prints the number as 345 days, which looks like a missing decimal point. The same page says the Sun moves 0.99 degrees a day, and 34 degrees at that speed takes about 34 days.

The Sun passes both nodes in a year, so there are two eclipse seasons a year. NASA says each one repeats just short of six months later. The midpoints of the seasons are 173.3 days apart, a little less than half a year, because the nodes slowly shift westward, by 19.3 degrees a year.

Inside a season, the Full Moon can line up with the shadow. A lunar cycle is shorter than a season, so the Canon concludes that there will always be one, and possibly two, lunar eclipses in each season. Over a whole calendar year, it counts two to five.

NASA's table shows the rhythm. In 2026 the lunar eclipses fall on March 3 and August 28, about six months apart. In 2027 there are three: February 20, July 18 and August 17. The last two sit about a month apart, which fits inside a single season of about 35 days.

A year of Full Moons: 2026

The U.S. Naval Observatory's data service lists 13 Full Moons in 2026, in Universal Time. NASA's table of every lunar eclipse from 2021 through 2030 lists only two in that year.

So 11 of the 13 Full Moons of 2026 bring no eclipse. The Naval Observatory's own list of eclipses agrees on the two that did happen, a total eclipse on March 3 and a partial one on August 28. Only Full Moons that fall inside an eclipse season can bring an eclipse, and the two seasons of 2026 produced one lunar eclipse each.

Common mistakes

• Expecting an eclipse every month. NASA says the tilt of the Moon's orbit prevents monthly solar and lunar eclipses.
• Picturing the shadow as huge. At about 2.7 Moon-widths across, the umbra is a narrow target for a Moon that can be as much as about 5 degrees above or below the plane.
• Mixing up the 17-degree rule and the height. The 17 degrees is an angle along the orbit, measured from a node. The bench's height is how far the Moon sits above or below the plane. The tilt links the two, but they are different numbers.
• Thinking an eclipse season is a single night. A season lasts about 35 days, long enough for one or two lunar eclipses.
• Assuming all eclipses are equally safe to watch. NASA says it is always safe to look at a lunar eclipse with the naked eye, unlike a solar eclipse. The Canon notes that every lunar eclipse has a solar eclipse within about two weeks, so the difference matters. 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 its eclipse safety page explains eclipse glasses and indirect viewing.

Try it in the instrument

Open the Alignment Bench and set the sky to a Full Moon with the Moon 2 degrees below the plane. The bench will report no eclipse, because the Moon slides under Earth's shadow.

Here is why, with NASA's numbers. Earth's shadow points straight away from the Sun, and the Sun's apparent path is the plane of Earth's orbit, according to the Naval Observatory. So at Full Moon the middle of the shadow sits on the plane, and the Moon's height is its distance from the middle of the shadow. The Moon is about 0.53 degrees wide (1,896 arcseconds), the umbra is about 2.7 times that wide, and the Canon says the penumbra adds a band about as wide as the Moon. Adding it up, the Moon's disk first touches even the faint outer shadow when its center is about 1.5 degrees from the plane. At 2 degrees, about one Moon-width of clear sky separates the Moon's edge from the shadow.

Raise the Moon step by step and watch the result change.

Treat the table as rough arithmetic from NASA's typical numbers, not as official limits. The bench uses a simplified version. It does agree with NASA's 17-degree rule. At 17 degrees from a node, a 5.1-degree tilt lifts the Moon by about 1.5 degrees (5.1 times the sine of 17 degrees), which is where the table ends.

Questions people ask

How many lunar eclipses happen in a year?

Two to five, according to the Canon. Exactly two is the most common: it describes 70.8 percent of the calendar years in the Canon's 5,000-year count. Three eclipses occur in 17.7 percent of years, four in 10.8 percent and five in 0.7 percent.

Does a lunar eclipse always come with a solar eclipse?

Within about two weeks, yes. The Canon says the same Sun-node alignment that produces a lunar eclipse always results in a solar eclipse within a fortnight, before or after it. The reason is that the mean time from New Moon to Full Moon, 14.77 days, is less than half the length of an eclipse season. NASA lists four types of solar eclipse: total, partial, hybrid and annular. It says they are visible from such a small area of Earth each time that they are much rarer to encounter than lunar eclipses. Read NASA's eclipse safety guidance before you plan to watch one.

Which upcoming Full Moons will bring an eclipse?

NASA's table lists every lunar eclipse from 2021 through 2030, and none falls on the Full Moons of November 24 or December 24, 2026. The next lunar eclipse in the table is the penumbral one on February 20, 2027, followed by penumbral eclipses on July 18 and August 17, 2027.

Does the size of the shadow change?

A little. NASA calls the 2.7 ratio typical. The Canon says Earth's umbra is largest when Earth is at aphelion, and the band of penumbra around the umbra measures 1,887.7 arcseconds at aphelion and 1,951.9 at perihelion. The Moon itself ranges from 1,763.0 to 2,011.8 arcseconds. So how far from the plane a Full Moon can sit and still touch the shadow shifts slightly from one eclipse to the next.

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.
• Why Do Eclipses Happen? - NASA Science, NASA Science. https://science.nasa.gov/eclipses/geometry/. Opened November 9, 2026.
• Lunar Eclipses and Solar Eclipses | NASA Space Place – NASA Science for Kids, NASA Space Place. https://spaceplace.nasa.gov/eclipses/en/. 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 Eclipse Figures, NASA Eclipse Web Site (Goddard Space Flight Center). https://eclipse.gsfc.nasa.gov/LEcat5/figure.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.
• 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.
• 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.
• 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.
• Eclipses of the Sun and Moon, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/data/UpcomingEclipses. Opened November 9, 2026.
• Dates of Primary Phases of the Moon, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/data/MoonPhases. Opened November 10, 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

• Why Most New Moons Bring No Solar EclipseThe Moon's orbit is tilted about five degrees, so at most New Moons its shadow passes above or below Earth.
• Total Lunar EclipseA total lunar eclipse happens when the entire Full Moon passes into the darkest part of Earth's shadow.