Monday, September 12, 2011


Although tides are best appreciated from a comfortable beach chair, truly understanding the motion in the ocean requires a trip to the moon. As you may have noticed, the moon is Earth’s only natural satellite, locked in orbit by the Earth’s gravitational pull. But the moon has a gravitational pull of it’s own, which tugs on the planet’s oceans; the water on Earth’s moon-facing side – or sublunar point – bulges out slightly, while the water on the opposite side – or antipodal point – is pulled slightly “in,” towards the Earth’s core.

But lunar gravity alone isn’t enough to create tides. Although we generally say, “the moon orbits the earth,” this isn’t quite true. When two celestial bodies are locked in orbit, they revolve around their common center of gravity; because the Earth is a bigger and exerts more gravity, this central point is much closer to it than the moon – somewhere between the Earth’s core and the equator. 

The earth’s off-kilter orbit creates an inertial force that pushes matter on its surface outward. This creates a bulge of water on the Earth’s antipodal point, while making water on its sublunar point dip slightly “in.” With combined inertial and lunar gravitational forces, we get water bulges on both sides of the earth at sublunar and antipodal points.

So what does this mean? Let’s say, for example, Baltimore is at the sublunar point – its high tide, and the ocean’s height is at its max. As the day goes on and the earth rotates on its axis, the ocean recedes during ebb tide, hitting low tide in about six hours – Baltimore is halfway between the sublunar and antipodal points. Later still, the ocean rises during flood tide, reaching a second high tide in another six hours – Baltimore is at the antipodal point. This cycle continues, with another low tide around six hours later.

Like Baltimore, most places in the world experience semidiurnal tides – two high tides and two low tides a day. But because the moon gradually rotates, it shifts slightly relative to the earth each day, starting the cycle of tides about an hour earlier than the day before.

But there’s one little big thing we’ve yet to factor in: the sun. About twice a month, during the full and new moons, the moon, sun and Earth align in a syzygy. During syzygy, the sun’s added gravitational force creates a spring tide, making the high tides higher and the low tides lower. Similarly, when the sun and moon form a ninety-degree angle with the Earth, their competing gravitational forces disrupt each other, causing a neap tide with more moderate tidal fluctuations.

With your brain freshly flooded with tides, the next time you find yourself soaking up the sun in a comfortable beach chair, remember to thank the moon.  

Source: 1, 2, 3