You check the calendar and see tonight is a full moon. What kind of tides should you expect over the next day or two?

Spring tides — a large range, because the Sun and Moon are aligned and their pulls add

Neap tides — a small range, because the full moon is farthest from the Sun's influence

Spring tides — a large range, because the Sun and Moon are aligned and their pulls add

🌊 Why the Sea Breathes: What Makes Tides

Tides are the slow rise and fall of the sea caused mainly by the Moon's gravity. The Moon tugs hardest on the water nearest it, pulling it into a bulge — a high tide. Curiously, a second bulge forms on the opposite side of Earth, because that water is pulled least and effectively 'left behind'. As Earth spins beneath these two bulges, most coasts pass through two high tides and two low tides each day, roughly every 12 hours and 25 minutes (this is called a semi-diurnal pattern).

The Sun pulls too, but from much farther away, so its tidal effect is only about 46% as strong as the Moon's. What matters for a skipper is how these two pulls line up over the month.

At new moon and full moon, the Sun and Moon line up, so their pulls add together — producing spring tides, with the highest highs and lowest lows (nothing to do with the season). At the half moons (first and last quarter), the Sun and Moon pull at right angles and partly cancel — giving neap tides, with the smallest range. This cycle repeats roughly every two weeks, so a beach that shows wide sand at spring low may barely uncover at neaps.

For a skipper this is practical, not academic: tidal range decides whether you clear a shallow bar, dry out against a wall, or have enough water to enter a harbour. Spring tides also mean stronger tidal currents — the next thing to reckon with.