Why are there two tides a day?
đˇď¸ TĂ o lao
In 326 BCE, the mighty army of Alexander the Great was exhausted. Tired of monsoon rains and fruitless fighting in India, Alexanderâs forces mutinied and demanded a retreat. But their bad luck followed them home. While marching along the Indus River, the waterâs current suddenly reversed, and a massive wave crashed down on the weary soilders. This unexpected event was a tidal bore, a phenomenon that occurs when extremely high tides push seawater up a river. The wave was likely quite the sock for Alexander, who was accustomed to the Mediterraneanâs mild tides. But tidal bores are just one of many ways tides can surprise. Two thousand years after Alexander, Isaac Newton deciphered the laws of gravity, and offered the first gravitational explanation of tides.
As Newton correctly identified, tides are choreographed by the motions of celestial objects, and Earthâs tides in particular are mostly driven by the Moon. Many coastal comm communities connected lunar and tidal activity long before Newton, but the precise nature of this relationship is actually quite nuanced. The attractive froce of gravity gets weaker with distance, so the Moonâs gravity tugs strongest on the side of the Earth that faces it. There, gravity pulls the oeans up into whatâs called a tidal bulge. Yet at the same time, another tidal bulge forms on the planetâs opposite side. This might seem like gravity defying behavior. But thatâs because we often think of the Moon as orbiting the Earth. When in reality, the Earth and Moon orbit each other around a shared center of mass roughly 1700km below the planetâs surface.
In this context, the Earth is like a child holding on to a carousel. And just like a riderâs hair flies out behind them, Earthâs water stretches away to create that second tidal bulge. Within that orbit, the Earth rotates once a day, moving points on its surface in and out of these bulges. This results in two daily high tides, when areas are inside each bulge, and two daily low tides, when places are between them. But as Newton recognized, itâs not just the Moonâs gravity that pulls on Earth, our Sun tugs the tides too. In fact, the Sun is why tidal strength varies with the phases of the Moon. Lunar phases coincide with different gravitational lineups of the Moon, Sun, and Earth.
For example, high tides are highest when the Moon is full, creating extreme spring tides. And low tides are lowest when the Moon is half-full, making tiny neap tides. Subtleties in the orbits of these celestial bodies introduce even more complexity and tidal varieties. And the strength of all these tides depends on the local landscape. Flat, enclosed lakes and seas generate the weakest tides, while bays and narrow inlets produce the strongest. Well, at least the strongest tides on Earth, there are even more dramatic tidal forces on our solar systemâs other celestial bodies. Millennia of Jupiter and Saturnâs gravitational kneading has generated enough heat on their respective moons of Enceladus and Europa to create oceans beneath their icy crusts. Jupiterâs moon Io endures the strongest tidal forces in the solar system, fualing intense volcanic activity. And in other planetary systems, some planets orbit so close to their stars, that extreme tidal forces lock them in place.
This tidal locking can leave the sun-facing hemisphere boiling while the other freezes in internal night. You wonât find a half-meting, tidally-locked planet in our solar system, but giving enough time, tidal forces would lock the Earth to the Moon. As Earthâs oceans churn to keep pace with the Moon, the water creates friction that slows our planetâs rotation. And after roughly 50 billion years, this process will have slowed Earth down enough for it to become tidally locked to the Moon. But before you start to sweat, you can take the solace in the knowledge that the Sun will have already died and taken us with it billions of years earlier.