Spring and neap tides explained

The sun and moon together, then apart

Tides are caused mainly by the moon's gravity, which draws the ocean into a slight bulge on the side of the Earth facing the moon and another on the far side. The sun does the same thing, but because it is so much farther away, its tide-raising effect is a little under half that of the moon.

At new moon and full moon, the sun, Earth and moon lie roughly in a line. The two sets of bulges coincide and reinforce each other, producing the large range of spring tides. At the first and last quarter, when the moon appears half lit, the sun and moon pull at right angles to each other as seen from Earth. The sun's effect partly cancels the moon's, and the result is the small range of neap tides.

This is a simplified picture. Real oceans are broken up by continents and respond to these forces as large, slow waves moving around ocean basins, which is why the tide at any one place has its own timing and size. The alignment of sun and moon still sets the fortnightly rhythm on most coasts.

The two-week cycle

The moon takes about 29.5 days to run through its phases. Since both new moon and full moon produce spring tides, springs come round every 14 to 15 days, with neaps falling roughly a week after each set of springs. The change is gradual. The range grows a little with each tide for about a week, peaks, then shrinks for about a week.

Not all spring tides are equal. The moon's orbit is not a perfect circle, and when a new or full moon falls close to the moon's nearest approach to Earth, called perigee, the springs are larger than usual. On many coasts the springs near the March and September equinoxes also tend to be among the largest of the year.

The pattern is clearest where there are two similar tides a day. On coasts with one tide a day, or with strongly mixed tides, how far north or south of the equator the moon stands can matter as much as its phase, and the largest tides do not always line up neatly with new and full moon.

Why they are called spring tides

The word comes from an old sense of spring meaning to rise or well up, the same sense found in a spring of water or in springing to one's feet. The term describes a tide that rises strongly. Neap is a very old English word of uncertain origin that has been applied to the weaker tides for centuries.

The lag after new and full moon

The largest tides do not usually arrive on the exact day of the new or full moon. The ocean takes time to respond, and at many places the greatest range comes a day or two later. This delay is known as the age of the tide. It differs from place to place, so the dependable way to find the biggest tides of the fortnight is to look at the heights in the local tide table, not to count days from the moon phase.

What it means on the water

A larger range means more water has to move in roughly the same amount of time. Where the range is big enough to notice, that has several practical consequences.

Stronger currents: tidal currents generally run faster at springs than at neaps, sometimes much faster in narrow channels, and the quiet period around slack water tends to be shorter.

Higher highs: spring high waters reach farther up beaches, slipways and sea walls. Combined with an onshore wind and low pressure, they are the tides most associated with coastal flooding.

Lower lows: more shoreline, rock and sandbank is exposed. That is good for exploring the shore and for seeing ground that is normally covered, and bad for boats that need depth over a bar or at a ramp.

Faster change: the water rises and falls more quickly, so a sandbank or rocky ledge can be cut off sooner than expected on a rising spring tide.

Neap tides are gentler on all counts. Currents are weaker, slack periods tend to last longer, and the water neither rises as high nor drops as low. That often makes neaps the easier time for paddling through tidal channels or diving where there is strong flow, while springs are the time to see the lowest shoreline. Anglers have their own views on which they prefer, discussed in the article on fishing the tide.

Planning around the cycle

Before choosing a date, check where the coming days sit in the cycle, not only the times of high and low water. Compare the day's range with the ranges a week either side. That shows at once whether it is a spring, a neap or something in between. CatchWindow location pages show a tidal-range figure and a week-ahead view where a tide station applies, which makes the trend easy to see. For passage planning and safety decisions, rely on official tide and current tables and the marine forecast.

Remember too that predictions describe the astronomical tide only. A spring tide with a strong onshore wind behind it can rise higher than the table says, as explained in the guide to reading a tide table.