ArticlesIf you have ever left a towel at the water's edge and returned to find it soaked, you have met the tide. Tides are among the most regular events in nature, yet the explanation is often told in a way that leaves people puzzled: if the Moon pulls the ocean toward it, why is there a high tide on the opposite side of the planet too? And why does the Sun, which is vastly more massive, matter less than the Moon? This article walks through why tides happen, using explanations published by the U.S. National Oceanic and Atmospheric Administration (NOAA) and NASA, and clears up a few common misunderstandings along the way.
The short answer
Tides are long, slow waves of water created mainly by the gravitational pull of the Moon and, to a lesser degree, the Sun. NOAA describes tides as long-period waves that roll around the planet as the ocean is pulled back and forth by the gravity of the Moon and Sun as they interact with the Earth in their monthly and yearly orbits [2]. NASA puts the everyday experience this way: when you watch the tide roll in or out, you are seeing a cycle of small changes in how the planet's oceans are distributed [3]. The rest of this article explains why that redistribution happens twice a day on most coasts and why its size changes over a month.
Gravity, mass, and distance
NOAA traces the explanation to 1687, when Sir Isaac Newton showed that ocean tides result from the gravitational attraction of the Sun and Moon on Earth's oceans. Newton's law of universal gravitation says the attraction between two bodies grows with their masses and shrinks with the square of the distance between them [1].
That is where the Sun puzzle begins. NOAA notes that the Sun is about 27 million times more massive than the Moon, and its gravitational attraction on Earth is more than 177 times greater than the Moon's. Yet the Moon controls the tides. The reason, according to NOAA, is that tidal forces are not the same thing as the simple attractive force. Tide-generating force varies inversely with the cube of distance, not the square. The Sun is about 390 times farther from Earth than the Moon is, so its tide-generating force is reduced by roughly 390 cubed, or about 59 million times. Taking its enormous mass into account as well, the result is that the Sun's tide-generating force is about half the Moon's [1].
It helps to separate two ideas. Gravitational attraction is how hard something pulls overall. Tidal force is about how much that pull differs from one side of Earth to the other. A nearby object changes its pull sharply across the width of a planet, while a faraway object pulls almost equally on every part of it. Distance matters more for tides than for gravity itself, which is why a nearby Moon beats a distant but massive Sun. This difference-in-pull framing is a standard way to read the cube relationship that NOAA describes; the cube rule itself is the cited fact.
Why there are two bulges
NASA's explanation of the opposite-side puzzle is worth reading closely. The Moon's gravity tugs on all of Earth, not only on the water facing it. The strongest pull falls on the points closest to the Moon and the weakest on the points farthest from it, but every bit of water is affected [3]. As the Moon's gravity tugs, it shifts Earth's mass and distorts the planet very slightly, much like a football that is elongated toward and away from the Moon. NASA adds that this effect on the solid Earth can be detected with scientific instruments, while the movement of oceans is simply visible from a beach [3].
NASA offers an analogy: pouring a bucket of water on a table. The water does not float upward; it is easier to slide it around the surface. In the same way, the ocean is not lifted away from Earth. It is pushed and squeezed around the globe by gravitational pull and other forces until it ends up bulging on the side nearest the Moon and on the side farthest away [3].
Because Earth rotates inside that layer of water, coastlines pass through both bulges every day. Those bulges are the high tides. NASA notes that most shorelines see two high tides and two low tides per day, and that the time from one high tide to the next is a little over 12 hours [3].
Why the schedule shifts each day
If the Moon stayed still, the tide would repeat every 12 hours exactly. It does not, because the Moon is orbiting Earth in the same direction that Earth rotates. NASA explains that it takes extra time, roughly 50 minutes, for a spot on Earth to rotate around and sit beneath the Moon again. As a result, tide times drift later each day [3]. If you plan a beach walk or a boating trip, this is why a tide chart from last week will not match this week.
NASA also warns that tides do not line up perfectly with the Moon overhead. The bulges sit slightly ahead of the Moon, and the tides you see locally are affected by the shapes of continents, ocean basins, wind, and storms [3]. That is why local forecasts are the reliable way to predict the water level at a particular harbor. NOAA's tide and tidal current predictions account for the positions of the Moon and Sun as well as local conditions [2]. A simple physical model explains the general pattern, but it cannot replace a published forecast for your coastline.
Spring tides and neap tides
The Sun's smaller but real influence shows up in a monthly rhythm. Twice each lunar month, around the new Moon and full Moon, the Sun, Moon, and Earth are nearly in a line. NOAA explains that in both cases, the Sun's pull is added to the Moon's, so the oceans bulge a bit more than usual. High tides are slightly higher and low tides slightly lower than average. These are called spring tides [2].
The name causes confusion. NOAA states that a spring tide is a historical term unrelated to the season of spring; it comes from the idea of the tide springing forth. Spring tides occur twice each lunar month all year long, regardless of the season [2]. If you hear someone say spring tides happen in spring, this is the correction.
About a week after a spring tide, the Sun and Moon sit at right angles as seen from Earth. NOAA says the Sun's bulge then partly cancels the Moon's bulge, producing moderate tides called neap tides. High tides are a little lower and low tides a little higher than average, and neaps occur around the first and third quarter Moon, when the Moon looks half full [2]. NASA offers a handy trick based on the same facts: you can tell a spring or neap tide is coming without going near the water, simply by looking at the Moon. Full or new Moons mean spring tides; half Moons mean neap tides [3].
A practical example
Imagine you want to explore tide pools. The best chance to see more of the shore is a low tide that is lower than average. According to the sources above, those occur around new and full Moons, when spring tides stretch the range between high and low water. During a neap tide, the tide range is smaller, so less of the shore is exposed [2][3]. For safety, the flip side applies: the same spring tides bring higher high tides, so a rocky path that is dry at midday can be covered later. Use a local tide table, and let the Moon phase only tell you roughly what to expect.
This example is an interpretation of the cited explanations, not advice from NOAA or NASA. Local geography can change tide ranges dramatically from one coast to another, and weather can raise or lower water levels beyond what a tide table predicts [3].
The Moon feels tides too
The pull goes both ways. NASA points out that Earth has about 80 times the Moon's mass and its gravity also deforms the Moon. The effect is smaller than ocean tides, but it can be measured with lasers. Earth's gravitational tide can change the lunar surface by about 4 to 6 inches (10 to 15 centimeters), which astronomers must account for when bouncing lasers off reflectors on the Moon [3]. Tides, in other words, are not just a coastal curiosity. They are a general feature of how massive bodies interact.
Common misunderstandings
The tide is not a river current. NOAA's description of tides as long-period waves reminds us that the ocean is being redistributed globally, and the water level rises and falls over hours at a shore [2].
The Moon does not lift the ocean off the planet. NASA's bucket analogy shows that water slides around Earth's surface, ending in bulges [3].
The Sun is not irrelevant. It supplies about half the Moon's tide-generating force [1], and its alignment with the Moon explains why tides swing larger and smaller through the month [2].
The Moon overhead does not give the exact high tide. Local timing differs because the bulges lead the Moon and because geography and weather matter [3].
Takeaway
Tides come from a gradient in gravity: the Moon pulls a little harder on the near side of Earth than on the far side, and the ocean flows into two bulges as Earth turns through them. The Sun adds roughly half as much tide-generating force, so when it lines up with the Moon the tides grow, and when it is at right angles they shrink. For planning a day by the water, the Moon phase gives a helpful hint, but a local tide forecast tells you what the sea will actually do.
Sources
[1] NOAA National Ocean Service, Tides and Water Levels: What Causes Tides? - https://oceanservice.noaa.gov/education/tutorial_tides/tides02_cause.html
[2] NOAA National Ocean Service, What is a spring tide? - https://oceanservice.noaa.gov/facts/springtide.html
[3] NASA Science, Tides - https://science.nasa.gov/moon/tides/