Supermoons and king tides: how much higher the tide really gets

What a supermoon is

The moon doesn’t circle Earth in a perfect circle. Its orbit is an ellipse, so every month it passes a closest point, perigee, and a farthest point, apogee. Its average distance is about 238,900 miles (384,400 km), but that changes quite a bit: in 2027, perigees range from about 222,000 to 230,000 miles (357,300 to 370,000 km), and apogees from about 251,000 to 252,600 miles (404,000 to 406,600 km).

A supermoon is a full moon that happens with the moon near perigee. Astronomers prefer the term “perigean full moon,” because supermoon isn’t an official astronomical term. It was coined in 1979 and took off decades later in the media and on social networks. The original definition also included new moons near perigee, which you can’t see but which count just as much for the tides.

Why there’s no single definition

With no official cutoff, everyone draws the line in a different place:

Definition	What counts as a supermoon
The 90% rule (most common)	A full moon at least 90% of the way to perigee in that orbit. NASA uses it in its explainers
By distance	A full moon closer than a round number, such as 360,000 or 362,000 km
By timing	A full moon within a few hours, or at most a day and a half, of perigee
The strictest	Only the closest full moon of the year

That’s why two reputable sources can say the same year has one supermoon or four. Under the 90% rule, NASA counts three or four supermoons a year, always in a row, because perigee lines up with the full moon for a few months and then drifts away. Under the strict definitions, you get one or two.

How much bigger and brighter it looks

Less than the headlines suggest. Here are the figures for a full moon at a very close perigee (the first row is NASA’s; the other two come from the orbit’s distances):

Compared with…	Apparent size (diameter)	Brightness
The year’s most distant full moon (a “micromoon”)	up to 14% bigger	up to 30% brighter
A full moon at average distance	less than 8% bigger	a little brighter
An ordinary perigean full moon	less than 2% bigger	about the same

The difference with a micromoon is real. In 2027, the closest full moon will be about 222,200 miles (357,600 km) away and the farthest, on July 18, about 252,300 miles (406,000 km), about 30,000 miles (48,000 km) farther. But you never see both at once. NASA itself notes that a supermoon can be hard to spot by eye, and the difference only becomes obvious in two photos side by side.

It’s still worth a look. A clear full moon night is always beautiful, and catching it as it rises over the horizon, with the coast or a lighthouse for scale, makes for a much better photo. You’ll find the moonrise time for your spot on the Moon tab of every TidePlanet place.

How much higher the tide really gets

This is where a supermoon does make a difference, though not as much as people think. The moon’s tidal pull grows quickly as it gets closer, because it depends on the cube of the distance. At the January 2027 perigee, about 222,000 miles (357,300 km) away, the moon will stretch the ocean about 25% more than at its average distance and about 46% more than at an average apogee.

That doesn’t mean 25% more water, for three reasons:

• The sun doesn’t change. At new and full moon, its pull adds to the moon’s, and that share is the same with or without perigee.
• The ocean doesn’t respond in proportion. The shape of each coast and bay amplifies or dampens the tide in its own way, as we explain in what causes tides.
• The peak comes later. Spring tides usually peak a day or two after the full or new moon, not on the night itself.

Still, the effect is real. NOAA calls these perigean spring tides and gives these figures:

• they happen 6 to 8 times a year;
• it’s not uncommon for high tides to be more than a foot higher than during an apogean spring tide, when the moon is farthest away;
• in Anchorage, Alaska, where the tidal range tops 30 feet, the difference can be 3 feet or more;
• during a perigean spring tide, high water can run half a foot or more above the average daily high tide, which US charts call mean higher high water (MHHW).

Compared with an ordinary spring tide, the gain is much smaller. NOAA says it’s only a couple of inches in most cases.

Type of coast	Example	What perigee adds
Huge tides	Anchorage, Cook Inlet	3 feet or more over an apogean spring tide
Moderate tides	Charleston, San Francisco, Puget Sound	A couple of inches over an ordinary spring tide, and around a foot over an apogean one
Small tides	Most of the Gulf of Mexico	Barely noticeable: wind and air pressure matter more

If you’re curious about how Europeans rate tide size, the tide coefficient calculator turns any day’s tide into a number from 20 to 120. A perigean spring tide shows up as a higher coefficient than an ordinary spring tide. Our guide to spring and neap tides explains the scale.

King tides: sea level rise you can see

In the US and Australia, the highest tides of the year are called king tides. NOAA is clear that it’s a popular, non-scientific term. King tides are usually perigean spring tides, sometimes helped along by the season.

What makes them interesting is how they’re used. As sea level rises, today’s exceptional high tides become tomorrow’s ordinary ones. So several agencies ask people to photograph the coast on those days:

• In California, the state’s Coastal Commission runs the California King Tides Project. It collects photos of the shoreline during the year’s highest tides and adds them to an interactive map, with a simple question in mind: what will the coast look like when today’s king tides become everyday high tides? For the 2026-27 season it lists November 24-26 and December 23-25, 2026, and January 21-22, 2027, right with the supermoon. The Commission picks the dates from the highest predicted astronomical tides and warns that predictions leave out storm surge and El Niño, so the real water level can run higher. NOAA’s own king tide page shows one rolling in at Pier 96 in San Francisco.
• On the East Coast, king tides already flood streets under clear skies, sometimes called sunny day flooding. NOAA notes that the supermoon of December 14, 2016, caused moderate flooding on the streets of Charleston, South Carolina.
• In Australia, Maritime Safety Queensland publishes the year’s highest summer tide and highest winter tide for each port, and citizen photo projects have run there too.

NOAA expects minor high tide flooding during perigean spring tides to become more common as sea level rises.

The supermoons of 2027

Supermoons come in runs, and the January and February 2027 full moons close out one that begins with the full moons of November and December 2026. These are the 2027 full and new moons that fall closest to perigee. Dates and times come from the U.S. Naval Observatory in Universal Time (UTC: US Eastern time is 5 hours behind in winter and 4 in summer), and distances from NASA’s ephemerides.

Date and time (UTC)	Phase	Distance	What it is
January 22, 12:17	Full moon	222,200 mi (357,600 km)	A supermoon by any definition: the closest full moon of the year, about 14½ hours after the closest perigee of 2027
February 20, 23:23	Full moon	225,700 mi (363,300 km)	A supermoon under the 90% rule, but not the strict ones, since it comes more than a day and a half after perigee
July 4, 03:02	New moon	223,000 mi (358,800 km)	Perigean new moon: invisible, but it brings perigean spring tides
August 2, 10:05	New moon	222,100 mi (357,400 km)	Perigean new moon, on the day of the total solar eclipse
August 31, 17:41	New moon	223,900 mi (360,300 km)	Perigean new moon

Two notes. The December 13 full moon sits right at the 90% line, so some lists include it and others don’t. And on TidePlanet’s 2027 moon calendar we use a strict definition: a supermoon is a full moon within a day and a half of a perigee closer than 362,000 km (225,000 miles). That’s why it marks only January as a supermoon, with a note adding February under the 90% rule, and lists the July and August new moons as perigean spring tides.

For the tides, what matters is the date of the spring tide, not the moon phase. Spring tides usually peak a day or two after the full or new moon, but every coast has its own lag: in California, for example, the Coastal Commission expects the highest tides around the January 2027 supermoon on January 21 and 22. Your local tide table is the only safe bet.

When it really matters: perigee plus a storm

A perigean spring tide on its own rarely causes trouble. According to NOAA, high tide flooding along US coasts generally starts when water levels run about 1.5 feet above the average daily high tide, while a perigean tide alone can reach about half a foot or more above it. To get the rest of the way, you need more:

• Low pressure. Under a storm, the sea rises roughly 1 centimeter per millibar, a bit over an inch for every 0.1 inch the barometer drops.
• Onshore wind, which piles water against the coast.
• Big waves, which run far above the still-water level.
• High river flow in estuaries and tidal rivers.

When all that lines up with a king tide, water can top seawalls and flood coastal roads. In Charleston, Miami or Norfolk, a few extra inches can be the difference between a dry street and a flooded one. On the Gulf Coast, where tides are small, the supermoon barely matters and the wind is in charge.

Perigean spring tides also bring the lowest low tides of the year, the minus tides that expose tide pools and clam flats. But with a bigger range, the water comes back faster. Before you head out, read our guide to tide safety.

How to make the most of a supermoon on the coast

1. Check the date of the full or new moon and of perigee on the moon calendar.
2. Find the spring tide: in your local tide table, the highest tide usually comes a day or two later.
3. Watch the forecast: pressure, wind and surf for those days. With a storm and onshore wind, stay off jetties and seawalls with waves breaking over them.
4. For king tide photos, pick a safe spot with something for scale. The Coastal Commission reminds people to watch for slippery rocks and big waves.
5. To see the moon, check the moonrise time: just after it clears the horizon is when it compares best with the landscape.

A supermoon isn’t a rare event, but it’s a great excuse to see how the moon moves the sea.