Ocean Tides and the Moon
Moon's Gravity and Ocean Tides
The Moon's gravitational pull is the main driver of Earth's ocean tides. Because gravity weakens with distance, the Moon pulls more strongly on the ocean water nearest to it than on Earth's center, and more strongly on Earth's center than on the water on the far side. This difference in gravitational pull across Earth's diameter is what actually creates the tides.
- Near-side bulge: Water on the side of Earth facing the Moon gets pulled toward the Moon more than the solid Earth beneath it, creating a tidal bulge (high tide).
- Far-side bulge: On the opposite side, Earth itself is pulled toward the Moon more than the distant water is, so the water "lags behind" and forms a second bulge. This is why there are two high tides, not one.
- Low tides occur in the regions between the two bulges, where water has been drawn away toward the bulge areas.
As Earth rotates beneath these two bulges, most coastal locations experience two high tides and two low tides roughly every 24 hours and 50 minutes. This is called a semidiurnal tidal cycle. Some locations experience only one high and one low tide per day (diurnal cycle), depending on geography and the shape of the coastline.
Celestial Alignment and Tidal Patterns
The Sun also exerts a tidal force on Earth's oceans. When the Sun's pull lines up with the Moon's, tides become more extreme. When they work against each other, tides become more moderate.
- Spring tides occur during new moon and full moon phases, when the Sun, Moon, and Earth are roughly aligned. The gravitational forces of the Sun and Moon reinforce each other, producing higher high tides and lower low tides than average. Places like the Bay of Fundy in Canada see dramatic tidal ranges during spring tides.
- Neap tides occur during first quarter and third quarter moon phases, when the Sun and Moon are at right angles relative to Earth. Their gravitational forces partially cancel out, producing smaller tidal ranges with lower high tides and higher low tides than usual.
Spring and neap tides alternate roughly every two weeks, following the Moon's phases.
Tidal Forces of Moon vs. Sun
You might expect the Sun to dominate tides since it's far more massive than the Moon. But tidal force depends on how much gravity changes across an object, not just how strong it is overall. That change depends on distance much more steeply than regular gravitational force does.
Tidal force is inversely proportional to the cube of the distance:
The Moon is roughly 400 times closer to Earth than the Sun. Even though the Sun is about 27 million times more massive than the Moon, that distance factor (cubed) wins out. The result: the Sun's tidal force is only about 46% of the Moon's. The Moon is the dominant tide-raiser.
The Sun's contribution becomes most noticeable during spring tides (when it adds to the Moon's effect) and neap tides (when it partially offsets it). Especially large spring tides can occur when the Moon is also near perigee (its closest approach to Earth), sometimes called perigean spring tides.
Tidal Variations and Measurements
- Tidal range is the vertical difference in water height between high tide and low tide. It varies enormously by location. The Bay of Fundy has tidal ranges exceeding 15 meters, while some areas in the open ocean see less than a meter.
- Amphidromic points are locations in the ocean where the tidal range is essentially zero. Tides rotate around these points. You can see them on tidal maps as the "hubs" that tidal patterns swirl around.
- Local geography, including the shape of coastlines, the depth of the ocean floor, and the width of bays, strongly affects how large or small tides are at any given location. That's why tidal patterns vary so much around the world.