Tides ebb and flow with the moon.
Though the sun's mass, and its gravitational pull, is greater than the moon's, the moon is closer and so wields twice the gravitational pull on Earth's oceans. The moon revolves around Earth every 23 hours and 53 minutes, or the time of a tidal day, but because the moon and earth have elliptical orbits, a full tidal cycle actually takes 19 years. Variations in the distance and alignment of the sun and moon to the Earth result in several different types of tides.
Tidal Variations
The moon's gravity affects each object on and in the Earth. When gravitational forces act on the Earth, objects distort, including water and the Earth itself. Oceans distort in the form of two tidal bulges. Oceans on the side of the Earth closest to the moon exhibit a large tidal bulge. Oceans on the opposite side of the planet also exhibit a smaller tidal bulge, as the moon's gravitational pull moves the Earth toward the moon and away from the water. As the Earth rotates under these two bulges, any coastline on the planet's surface generally experiences two high tides and two low tides during each full rotation. Of course, there are exceptions to this simplistic model.
Spring Tides
One complicating factor that creates different types of tides is the sun's gravitational pull. When the moon is in its new and full phases, the Earth, moon and sun are aligned in a formation that adds the sun's gravitational pull to the moon's gravitational pull. This phenomenon, known as spring tide, results in higher-than-normal high tides and lower-than-normal low tides.
Neap Tides
When the moon is in its first and third quarters, the sun and moon are aligned at right angles to the Earth. Because the sun's gravitational pull works against the moon's pull in this alignment, both high tides and low tides are weaker. This phenomenon is known as neap tide. Spring tides and neap tides alternate at 1-week intervals.
Perigean Tides
Like other heavenly bodies, the moon's orbit around the Earth is elliptical. Once each month, the moon passes by the Earth at perigee, which is about 30,000 miles closer than at apogee, or the furthest point in orbit. When spring tides and perigee align, the moon's extra-strong pull results in unusually high tides, known as perigean tides. Perigean tides take place at intervals that are slightly longer than six months, according to the Woods Hole Oceanographic Institution.
Diurnal Tides
If one high tide and one low tide take place in a lunar day, it's known as a diurnal tide. Diurnal tides create 24-hour, 50-minute tidal periods. Oceans in partially enclosed basins, such as the Caribbean and the Gulf Coast of the U.S. often experience diurnal tides.
Semi-Diurnal Tides
When two high and two low tides of the same height take place in a lunar day, it's called a semi-diurnal tide. Semi-diurnal tidal periods are 12 hour and 25 minutes long. Most coastlines experience semi-diurnal tides.
Mixed Tides
Mixed tides consist of two high tides and two low tides of varied heights during a lunar day. Most open coastlines, such as the west coast of the U.S., experience mixed tides.
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