Moon & Sun Gravity
The astronomical tide — the rhythmic twice-daily rise and fall predicted on every tide chart — is driven by the gravitational pull of the moon and, to a lesser extent, the sun.
The basic mechanism
The moon’s gravity tugs on Earth’s oceans, raising a bulge of water on the side of the planet facing the moon — and, less obviously, a second bulge on the opposite side caused by centrifugal effects in the Earth-moon system. As the Earth rotates beneath these two bulges, any given coastline experiences two high tides and two low tides each day. This is called a semi-diurnal tide, and it’s the pattern you see all along the Outer Banks oceanfront.
Spring tides and neap tides
Twice a month, around the full moon and new moon, the sun and moon line up with the Earth and their gravitational pulls add together. The result is a spring tide (the name has nothing to do with the season) — higher highs and lower lows than average. The OBX oceanfront might see a 4.5 ft range on a spring tide versus a typical 3.5 ft.
A week later, at first and third quarter moons, the sun and moon pull at right angles to each other. Their effects partially cancel, producing neap tides with smaller ranges — sometimes only 2.5 ft on the OBX oceanfront. Surf fishermen often notice that the action slows during neap tides because there’s less water movement to push bait into the sloughs.
Perigean and apogean tides
The moon’s orbit isn’t circular. Roughly once a month it reaches perigee (closest to Earth) and tidal forces are about 20% stronger than at apogee (farthest). When a new or full moon coincides with perigee — sometimes called a “king tide” or “perigean spring tide” — the OBX can see ranges of 5+ ft. These are the events most likely to flood low-lying spots like the Mirlo Beach section of NC-12, even without a storm.
Why the sound side barely feels the moon
The Albemarle and Pamlico Sounds are connected to the Atlantic only through narrow inlets — Oregon Inlet, Hatteras Inlet, Ocracoke Inlet, and a few others. By the time the astronomical tide squeezes through these chokepoints, most of its energy is lost. The result: lunar tides in Pamlico Sound are typically only a few inches, and wind becomes the dominant driver of water levels there.