Why are there two tides a day? - Elise Cutts

In 326 BCE, the mighty army of Alexander the Great was exhausted.

Tired of monsoon rains and fruitless fighting in India, Alexander’s forces mutinied and demanded a retreat.

But their bad luck followed them home. While marching along the Indus River, the water’s current suddenly reversed and a massive wave crashed down on the weary soldiers.

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 shock for Alexander, who was accustomed to the Mediterranean’s mild tides.

But tidal bores are just one of many ways tides can surprise.

2,000 years after Alexander, Isaac Newton deciphered the laws of gravity and offered the first gravitational explanation of tides.