What Causes Tides?
QUICK ANSWER
Tides are caused mainly by the Moon's gravity pulling on Earth's oceans, with the Sun contributing about half as much. The pull creates two tidal bulges on opposite sides of Earth. As the planet rotates through these bulges, we get the rising and falling tides at the coast.
Tides are caused by the Moon's gravity, with help from the Sun. The Moon pulls on Earth's oceans hard enough to create bulges in the water on opposite sides of the planet. As Earth rotates through these bulges, we get the predictable rising and falling tides at the coast. The whole system is one of the most reliable astronomical phenomena visible from Earth.
What causes the tides?
Mostly the Moon's gravity. According to NOAA, tides result from the gravitational attraction of the Moon and Sun on Earth's oceans. The Moon's pull is the dominant force, about twice as strong as the Sun's despite the Sun being vastly more massive. The reason is distance: the Moon is much closer to Earth than the Sun, and tidal force depends on distance more strongly than gravity in general. The closer body wins, even at the Moon's relatively small mass.
Why are there two high tides each day?
Because the tidal bulges form on opposite sides of Earth. The Moon's gravity pulls Earth's oceans toward the Moon on the side facing it, creating one bulge. On the opposite side of Earth, the Moon's pull is weakest, and Earth itself is pulled slightly toward the Moon more than the water there. The effect leaves a second bulge on the far side. As Earth rotates through these two bulges every 24 hours and 50 minutes, coastal areas experience two high tides and two low tides daily.
How does the Sun affect tides?
Significantly, but less than the Moon. The Sun's gravity also creates tidal bulges, though only about half the size of the Moon's. When the Sun and Moon are aligned (during new moon or full moon), their effects combine to create larger spring tides. When they're at right angles (during first or third quarter), they partially cancel each other out, producing smaller neap tides. The cycle from spring tides to neap tides repeats every two weeks, providing predictable patterns that mariners have used for thousands of years.
Why is the tidal cycle 24 hours 50 minutes?
Because the Moon is also moving. As Earth rotates on its axis, the Moon is simultaneously orbiting Earth in the same direction. Earth has to rotate a little more than once (about 50 extra minutes' worth) to catch up to where the Moon has moved. Each high tide occurs about 50 minutes later than the previous day's, which is why tide schedules at coastal locations shift gradually over time.
Tides are caused by the Moon's gravity pulling on Earth's oceans, with the Sun contributing about half as much. The pull creates two bulges of water on opposite sides of Earth, which we experience as high tides as our planet rotates through them. The tidal cycle takes 24 hours and 50 minutes because the Moon is also moving in its orbit. Tides have shaped coastlines, ecosystems, and human history for as long as both Earth and Moon have existed.
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