Spend a day by the shore and you can watch the water level climb and fall. Follow it for a month and the range between high and low water grows and shrinks in a regular pattern. The Moon provides most of the forcing, with the Sun adding its share. Their relative positions decide whether those effects reinforce or partly cancel; Earth's rotation and the shape of each ocean basin then turn that forcing into local heights and times. The diagram below begins with the three bodies, then connects spring and neap tides to the tide table used at a harbour.
What are tides?
Tides are the regular rise and fall of sea level driven by the gravitational pull of the Moon and the Sun. Sea level usually rises and falls twice a day, and over a month the tidal range grows and shrinks with the phase of the Moon. The Moon is the main cause; the Sun's tide-raising effect is less than half the Moon's.
What causes tides?
Tides are caused by the tide-raising force: the Moon pulls on different parts of Earth with slightly different strength, harder on the near side and more weakly on the far side, so the whole Earth system is stretched along the Earth–Moon line. That difference falls off with the cube of distance, which is why the tide-raising force is noticeably stronger when the Moon is at perigee. Earth's rotation and the shape of each ocean basin then turn that force into the different tide times and heights of individual ports.
Why are there two high tides a day?
Because the tide-raising force raises a bulge on both sides of Earth: on the side facing the Moon the water is pulled a little more, and on the far side the solid Earth is pulled away more than the water, so both sides bulge relative to the middle. As Earth rotates, a place passes through two bulges and two troughs; because the Moon also moves along its orbit, one "lunar day" is about 24 h 50 min and successive high tides are about 12 h 25 min apart.
What causes spring tides and neap tides?
Spring tides occur around new and full moon, when the Sun and Moon pull along the same line and reinforce each other, giving the largest range between high and low water. Neap tides occur around first and last quarter, when the two pulls are nearly at right angles and partly cancel, giving the smallest range. "Spring" describes the range, not an especially high sea level; the actual height at any port still comes from its tide table.
The Moon sets the beat, but not each harbor's high-tide time
The Moon sets the broad tidal rhythm; coastlines, seafloor shape, and weather decide how that rhythm appears at each harbor.
Why simplified diagrams show two tidal bulges
The Moon pulls on the whole Earth, but slightly more strongly on the near side and less strongly on the far side. Tides arise from that difference, so an equilibrium-tide diagram stretches the system in both directions.
Real oceans never hold a tidy ellipse that follows the Moon. The model only shows the tidal-force pattern; continents, seafloor topography, and basin resonance make actual tidal waves far more complicated.
Spring tides describe the range, not necessarily the highest water
Around new and full Moon, the Sun's and Moon's tide-raising forces roughly line up, and the difference between high and low tide usually grows: a spring tide. At first and last quarter the two directions are close to a right angle, the range often shrinks, and neap tides result.
A spring tide can fall by day or in the middle of the night, and it does not mean every place reaches its highest water ever. It only says the range is larger on those days; the actual water level also depends on the harbor's location, wind, air pressure and sea state.
A real trip to the shore still needs a local forecast
Near new and full Moon the tidal range is often larger, producing spring tides; near quarter phases it is often smaller, producing neap tides. A spring tide describes range, not a guaranteed record water level.
Tidal waves change as they enter straits and harbors or turn around headlands. For tide pools, fishing, diving, or boating, use the nearest station prediction together with wave conditions and official warnings.
Tidal forcing measures the change in gravity across Earth
The Moon pulls on ocean, rock, and Earth's centre together. The near side is closer to the Moon and accelerates a little more than the centre; the far side is farther away and accelerates a little less. Subtract Earth's shared acceleration and the near side stretches Moonward while the far side falls behind, leaving one bulge on each side. The quantity that matters is how much gravity changes across one Earth diameter—a gravity gradient.
The Sun's total pull on Earth is much stronger than the Moon's, yet the Sun is about 150 million kilometres away. Its pull therefore changes less from the near side of Earth to the far side. The nearby Moon supplies the larger tidal gradient. At mean distances, solar tidal forcing is about 46% of the lunar value, enough to change the range appreciably.
Tidal forcing grows with the mass of the distant body and falls very quickly with distance. Distance enters as a cube, so the move from lunar apogee to perigee produces a noticeable increase even though the distance changes by only a fraction of the total.
The same difference in pull acts on the Moon, and it is what slowly braked the Moon's spin until it matched its orbit. Why the Moon always keeps the same face toward Earth is where the tidal-locking lesson picks up.
Two bulges and a lunar day of about 24 hours 50 minutes
Imagine Earth covered by a uniform ocean. The two ends of the Earth–Moon line carry the bulges; the troughs lie 90 degrees away. As Earth rotates, a location passes through two bulges and two troughs during one lunar day, giving two ideal highs and two lows. This equilibrium picture is a guide to the period. Real ocean water does not keep a neat elliptical shape while circling the planet.
While Earth turns, the Moon moves eastward by about 13 degrees per day. A meridian must rotate for roughly 50 extra minutes before it faces the Moon again, making the mean lunar day about 24 h 50 min. The principal lunar semidiurnal constituent, M2, has a period near 12 h 25 min. That rhythm underlies the day-to-day delay seen at many ports.
A coast need not receive two equal highs and lows. Stations may be semidiurnal, diurnal, or mixed, and consecutive highs can differ greatly. Continents, seabed shape, the Coriolis effect, friction, and basin resonance steer the tidal wave; in many seas it rotates around amphidromic points.
Spring and neap describe the size of the tidal range
At new Moon, the Moon lies between Earth and the Sun; at full Moon, Earth lies between the other two bodies. Both arrangements are nearly straight lines, so the long axes of the lunar and solar tides align. Their sum tends to raise high water, lower low water, and enlarge the range: this is a spring tide. It need not contain the highest level of the year. Here spring means to rise or well up and has nothing to do with the season.
Near first and third quarter, the Sun and Moon appear about 90 degrees apart. A solar bulge then lies close to a lunar trough, so the components partly cancel. High water tends to be lower, low water higher, and the range smaller: a neap tide. Lunar forcing remains, and the water continues to rise and fall.
The mean synodic month lasts 29.5306 days. New to full Moon, and full Moon to the next new Moon, each take about 14.765 days, setting the interval between spring configurations. Neap configurations sit midway, about 7.383 days away. Ocean basins take time to respond, so the largest or smallest range at a station often follows the lunar phase by one to several days. That delay is the age of the tide.
One spring tide can differ from the next
The Moon's orbit has an eccentricity of about 0.055, producing a difference of tens of thousands of kilometres between perigee and apogee. Distance affects tidal forcing by its cube, so a new or full Moon near perigee gives stronger-than-average lunar forcing: a perigean spring tide. News reports often use king tide for an unusually high astronomical-tide period. It is not a precise astronomical category, and the label alone does not predict flooding.
Earth's orbit around the Sun is also slightly eccentric. Solar tidal forcing is a little stronger near perihelion in early January and a little weaker near aphelion in early July. The change is smaller than the lunar-distance effect, but it still combines with phase, lunar distance, and the local station constituents.
Longer orbital cycles also matter. In a fixed ecliptic frame, the direction of lunar perigee precesses in about 8.85 years and the nodes regress in about 18.6 years. JPL's 5.997-year value is the cycle of the argument of periapsis relative to the moving node; the engine advances that argument and the node separately, then combines them to obtain the fixed-frame perigee direction. Changes in lunar declination produce diurnal inequality, while those long cycles slowly modulate station ranges. The diagram has neither a complete lunar perturbation theory nor station harmonic constants.
Read model dates and harbour tide times separately
Lunar and solar forcing set the basic periods. A harbour level appears only after the tidal wave has travelled to that coast. The wave goes around continents and through straits, changes speed with depth, and may resonate in a basin. A lunar transit, syzygy, or model bulge aimed at a longitude does not mark high water there.
Station forecasts begin with long water-level records. Analysts separate the record into M2, S2, K1, O1, and other constituents, determine the local amplitude and phase of each, and add them to predict water level. A subordinate station may also need time and height corrections. Nearby ports can therefore reach high water at different times.
A tide table mainly predicts the astronomical tide. Onshore wind can pile water against the coast; low pressure, river discharge, swell, and storm surge add further departures. Follow official warnings and closures whenever a typhoon or deteriorating local conditions are present.
Spring tide says that range tends to be large during that part of the cycle; neap says it tends to be small. Neither term gives the water level, current speed, or wave height at a particular moment. Check level, current, waves, weather, and shore geometry separately before entering a tidal flat, diving, fishing, or navigating.
How to read the pages: use this lesson for the trend in tidal range around a date. Use the tide page for predicted high and low times and heights at a selected station. Plan timing from the station forecast and make safety decisions from current conditions and official warnings.
Spring and neap describe the size of the tidal range
Geometry
Near phase
Ideal range
Mean repeat
Nearly aligned (0° or 180°)
New, full
Lunar and solar tide patterns reinforce; larger ideal range
About every 14.765 days
Nearly square (unsigned separation near 90°)
First, third quarter
Partial cancellation; smaller range
About 7.383 days after spring
FAQ
Why does the Moon create a bulge on both sides of Earth?
The Moon pulls on ocean, rock, and Earth's centre together. The near side is closer to the Moon and accelerates a little more than the centre; the far side is farther away and accelerates a little less. Subtract Earth's shared acceleration and the near side stretches Moonward while the far side falls behind, leaving one bulge on each side. The quantity that matters is how much gravity changes across one Earth diameter—a gravity gradient. The Sun's total pull on Earth is much stronger than the Moon's, yet the Sun is about 150 million kilometres away. Its pull therefore changes less from the near side of Earth to the far side. The nearby Moon supplies the larger tidal gradient. At mean distances, solar tidal forcing is about 46% of the lunar value, enough to change the range appreciably.
Which Moon phases produce spring and neap tides?
At new Moon, the Moon lies between Earth and the Sun; at full Moon, Earth lies between the other two bodies. Both arrangements are nearly straight lines, so the long axes of the lunar and solar tides align. Their sum tends to raise high water, lower low water, and enlarge the range: this is a spring tide. It need not contain the highest level of the year. Here spring means to rise or well up and has nothing to do with the season. Near first and third quarter, the Sun and Moon appear about 90 degrees apart. A solar bulge then lies close to a lunar trough, so the components partly cancel. High water tends to be lower, low water higher, and the range smaller: a neap tide. Lunar forcing remains, and the water continues to rise and fall.
Can this interactive diagram tell me today's high-tide time?
Lunar and solar forcing set the basic periods. A harbour level appears only after the tidal wave has travelled to that coast. The wave goes around continents and through straits, changes speed with depth, and may resonate in a basin. A lunar transit, syzygy, or model bulge aimed at a longitude does not mark high water there. How to read the pages: use this lesson for the trend in tidal range around a date. Use the tide page for predicted high and low times and heights at a selected station. Plan timing from the station forecast and make safety decisions from current conditions and official warnings.
Orbital numbers and tide references come from public NASA/JPL and NOAA technical pages; the equations describe scientific facts. StarWindow wrote the program and lesson and drew the Canvas graphic. No source code, image, texture, logo, or article text from those sites is reproduced. NASA material is generally not copyrighted in the United States, although its identity and media-use rules still apply. This page uses only the required numerical data and links to each original source.
The Earth element table covers 1800–2050. Lunar position uses a mean Keplerian orbit with long-term apsidal and nodal precession. The model omits the full periodic perturbations, light time, irregular Earth rotation, basin dynamics, and station harmonic constants. Do not use it for navigation, surveying, eclipse work, telescope pointing, or coastal tide prediction.