How the Moon moves the oceans, twice a day
The Moon's pull raises two bulges of water on Earth. Here is why most coasts get two high tides a day and why parts of the Gulf of Mexico can differ.
The Moon’s gravity pulls on the whole Earth, and water moves more easily than land. The result is a tidal force that makes Earth’s water bulge out on the side closest to the Moon. A second bulge forms on the opposite side, and those bulges of water are the high tides.
The far-side bulge looks odd at first. It comes from the difference between the Moon’s pull at a given spot and its average pull across the whole planet, which stretches the water on both sides.
As Earth turns through both bulges, most coastlines get two high tides and two low tides in roughly 24 hours and 50 minutes. The extra 50 minutes appear because the Moon keeps moving along its orbit while Earth rotates.
The Sun adds a smaller tide of its own. At new and full moon the lunar and solar tides reinforce each other and produce spring tides. At the quarter moons they work against each other and leave unusually small neap tides.
Why the Gulf is different
Local geography changes the pattern. The Gulf of Mexico is listed among places with a diurnal tide, which means only one high tide and one low tide a day, and many areas of the Gulf follow that pattern. That is why a local tide table is a better guide than a general rule.
Sources
- What causes tides?, NOAA
- Tidal cycles, NOAA Ocean Service