A supermoon happens when a full moon coincides with the moon’s closest approach to Earth in its orbit. Because the moon doesn’t travel in a perfect circle, its distance from Earth changes constantly, and when a full moon lines up with the near point of that oval-shaped path, the result is a moon that appears up to 14 percent larger and 30 percent brighter than the most distant full moon of the year.
Why the Moon’s Distance Changes
The moon orbits Earth in an ellipse, not a circle. At its nearest point (called perigee), it sits about 356,400 kilometers from Earth. At its farthest point (apogee), it swings out to roughly 406,700 kilometers. That’s a difference of about 50,000 kilometers, or roughly 12 percent of the total distance.
The orbit is elliptical because of gravitational tugging from multiple sources. The sun’s gravitational pull on the moon is actually more than twice as strong as Earth’s pull, and it constantly warps the moon’s path. The changing positions of the sun, moon, and Earth, the slight bulge around Earth’s equator, and even the gravity of other planets all combine to shift the shape and orientation of the moon’s orbit over time. The orbit’s elongation isn’t fixed; it stretches and compresses in a slow, ongoing cycle driven primarily by the sun’s influence.
How Perigee Lines Up With a Full Moon
A full moon occurs when the sun, Earth, and moon are roughly aligned, with Earth in the middle. Astronomers call this arrangement syzygy. Meanwhile, the moon reaches perigee once every 27.5 days or so, while the cycle from full moon to full moon takes about 29.5 days. Because these two cycles run on slightly different clocks, perigee drifts through the lunar phases over time. The full cycle, where perigee shifts from one full moon phase all the way around and back again, takes about 14 months.
When perigee and a full moon happen to land close together in time, you get a supermoon. The closer the timing overlap, the more dramatic the effect. A commonly used threshold defines a supermoon as a full (or new) moon occurring when the moon is within 360,000 kilometers of Earth. The term itself was coined in 1979 by astrologer Richard Nolle, who described it as a new or full moon happening at or near 90 percent of its closest possible approach. It’s not a formal astronomical classification, but the concept has been widely adopted.
How Big and Bright It Actually Looks
The numbers are real but modest. Compared to the smallest, most distant full moon of the year (sometimes called a micromoon), a supermoon can appear up to 14 percent wider and 30 percent brighter. In practice, your eye has trouble detecting a 14 percent size difference without a side-by-side comparison. The brightness boost is more noticeable, particularly on clear nights, casting stronger shadows and lighting up landscapes more than a typical full moon.
Much of what people experience as a “giant moon” on supermoon nights is actually a separate phenomenon: the moon illusion. When the moon sits near the horizon, framed by trees, buildings, or mountains, your brain perceives it as enormous. Photographs prove the moon is the same width near the horizon as it is high in the sky. The illusion is a quirk of how the brain processes visual information, not an atmospheric or gravitational effect. A supermoon near the horizon triggers both the real size increase and the illusion at the same time, which is why those rising-moon photos can look so spectacular.
Effects on Tides
A supermoon does have a measurable physical effect: higher tides. Every full and new moon already produces spring tides, the highest and lowest tides of the month, because the sun and moon pull in alignment. When the moon is also at perigee, those spring tides get an extra boost, called perigean spring tides. High tides during a perigean spring tide are typically more than a foot higher than spring tides when the moon is at its farthest. In places with extreme tidal ranges, like Anchorage, Alaska (where the normal tidal range exceeds 30 feet), the difference can reach 3 feet or more.
For most coastal areas, this means slightly more flooding risk during already high tides, especially if a storm surge coincides with the event. The effect is predictable and well understood by oceanographers, but it’s worth knowing if you live near the coast.
When the Next Supermoons Occur
Supermoons aren’t rare. They typically happen a few times per year, sometimes in clusters of consecutive months. The next super full moon falls on December 23, 2026. Supermoons can also occur during a new moon, when the moon sits between Earth and the sun and isn’t visible at all. The next super new moon arrives on May 16, 2026. These dates shift slightly depending on your time zone, since the precise moment of perigee and full moon can fall on different calendar days in different parts of the world.