Spring tides, neap tides, and king tides

Watch one tide station for a month and the daily rhythm turns out to have a rhythm of its own: the range swells, shrinks, and swells again, twice per lunar month. The moon's phase sets that beat, and the moon's distance decides how hard it hits.

Spring tides: the sun and moon pull together

At new moon the sun and moon line up on the same side of Earth; at full moon they pull from opposite sides. Either way their tidal bulges stack, and for a few days the highs run higher and the lows lower than average. These are spring tides— from the water “springing up,” nothing to do with the season — and they arrive every two weeks, usually a day or so after the phase itself.

Neap tides: the pulls fight each other

At first and last quarter moon, the sun sits at right angles to the moon and partly cancels it. The result is neap tides: the flattest curve of the month, with modest highs and lows and gentler currents. If you need the calmest window for work near the waterline, neaps are it.

King tides: add perigee

The moon's orbit isn't a circle. Its distance from Earth swings from about 405,000 km at apogee down to about 363,000 km at perigee— and because tidal force falls off with the cube of distance, that ~10% swing in distance is worth roughly 30% in tide-raising force. (Yes, that's where this site gets its name.)

A few times a year, perigee lands within a day or two of a new or full moon. The spring tide that follows is a perigean spring tide— the “king tides” of news coverage. They're the highest astronomical tides of the year, and on low-lying coasts they can push water into streets on a clear, windless day. That's why king tides have become a preview of sea-level rise: they show, on a schedule, what routine flooding will look like.

What it means in practice

• Springs— the month's lowest lows for clamming and tidepooling, its strongest currents in inlets, and the days to double-check dock lines and dinghies.
• Neaps — small ranges, weak currents, the forgiving days for a first paddle or shoreline work.
• King tides — watch marinas, causeways, and low roads around the predicted high, especially if weather piles on: a storm riding a perigean spring tide is the flooding worst case.

You can see all of this on Perigee. The astronomy pagetracks the moon's phase and a 30-day calendar for any location, and every station page plots the predicted curve — watch the envelope breathe from springs to neaps over a month. For how those curves are computed in the first place, read how tide predictions work.