Tides

Tides are the regular rise and fall of sea level produced by the gravitational attraction of the Moon and, to a lesser extent, the Sun on the Earth's oceans. The Moon's gravity pulls most strongly on the water nearest to it, drawing it into a bulge, while a second bulge forms on the opposite side of the Earth. As the planet rotates beneath these bulges, most coasts experience the sea rising to high tide and falling to low tide in a repeating cycle through the day.

The Sun also raises tides, but because it is far more distant its tidal effect is weaker than the Moon's despite its enormous mass. The combined pull of the two bodies varies as their positions change. When the Sun, Earth, and Moon are aligned, at new and full Moon, their gravitational forces reinforce one another to produce spring tides, with the greatest difference between high and low water. When the Sun and Moon are at right angles relative to the Earth, their effects partly cancel, giving the smaller neap tides.

The strength of tides also changes with the Moon's varying distance from the Earth along its orbit. When the Moon is nearest, its gravitational pull is stronger and the resulting tides are higher, so that especially large spring tides occur when alignment coincides with the Moon's closest approach. Because tides follow the predictable motions of the Moon and Sun, their timing and height can be forecast far in advance, which is important for navigation, fishing, and coastal activities.

Frequently asked questions

What causes tides?

Tides are caused mainly by the Moon's gravitational pull on the oceans, which raises bulges of water, with a smaller contribution from the Sun.

What is the difference between spring and neap tides?

Spring tides, with the largest range, occur when the Sun and Moon align and reinforce each other. Neap tides, with the smallest range, occur when they are at right angles.

Why is the Sun's tidal effect weaker than the Moon's?

Although the Sun is far more massive, it is much more distant, and tidal force falls off sharply with distance, so the nearer Moon dominates the tides.