Why does the sea have a high tide and a low tide every day?
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Why does the sea have a high tide and a low tide every day?
The short answer
We get tides because the Moon's gravity pulls the near side of Earth harder than the far side, stretching the ocean into two bulges — one facing the Moon and one on the opposite side. As Earth spins once a day, every coast is carried through both bulges, so most places get two high tides and two low tides each day.
How it works
Gravity gets weaker with distance, so the Moon tugs the ocean nearest it harder than the water on the far side of Earth. That difference in pull stretches the planet's water into two bulges: a near bulge lifted toward the Moon, and a far bulge made of water that is pulled the least and so gets left behind. The bulges roughly stay lined up with the Moon while Earth rotates underneath them, so any given beach sweeps into a bulge (high tide), out of it (low tide), into the second bulge (high tide again), and back out — about two highs and two lows per day.
What people get wrong
Many people think there is only one high tide a day, on whichever side the Moon is over, or that the Moon 'sucks' the water up like a magnet. In fact there are usually two bulges and two high tides a day, and the tricky far-side bulge is not pushed away by anything — it forms because that water feels the weakest pull and lags behind the rest of Earth.
The catch
The near-side bulge is easy to picture because the Moon is right there tugging the water up, but the far-side bulge is the mind-bender: it exists because that water is pulled the least, not the most. And real tides are not perfectly even or exactly twice a day everywhere — the Sun adds a smaller tide, and the shapes of ocean basins and coastlines shift the timing and size, so some places get lopsided or unusual tides.
Questions kids ask
Why are there two high tides a day instead of one?
Because the Moon pulls the near side of Earth harder than the far side, it stretches the ocean into two bulges — one toward the Moon and one on the opposite side. Earth spins through both bulges each day, so most coasts get two high tides.
How can there be a bulge on the side facing away from the Moon?
The water on the far side feels the weakest pull from the Moon, so as the rest of Earth is tugged toward the Moon, the far water gets 'left behind.' That lag piles it up into a second bulge.
Does the Sun cause tides too?
Yes, but the Sun's tides are smaller than the Moon's even though the Sun is far more massive, because the Sun is so much farther away. When the Sun and Moon line up you get bigger 'spring' tides; when they pull at right angles you get gentler 'neap' tides.
Why doesn't every place get exactly two equal tides at the same time?
The shapes of ocean basins, coastlines, and the angle of the Moon all change how the water actually moves. Some places get two roughly equal tides, some get two unequal ones, and a few get only one a day.
For grown-ups
Tides come from the difference in the Moon's gravity across Earth (the tidal force), which falls off as 1/r³, not from the Moon's pull itself. In Earth's free-falling reference frame the near side is accelerated toward the Moon and the far side is left behind, producing two bulges; Earth rotating beneath them yields roughly two highs and two lows per lunar day (about 24h 50m). The Sun contributes a smaller tide that creates spring and neap tides, and real ocean-basin geometry and resonances set the actual local timing and range.