By Emile Heskey
Updated Mar 24, 2022
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Low and high tides are among the most dramatic natural events along coastlines and tidal rivers. Their magnitude—the tidal range—varies with geography and the relative positions of Earth, moon, and sun. In some places, the difference between the lowest and highest water levels is a modest few feet; in others, it can exceed 50 feet.
High and low tides occur roughly twice a day, separated by about six hours. The tidal range is the vertical difference between these two extremes. Coasts with steep seafloor gradients, such as those around the Bay of Fundy in southeastern Canada, can experience tidal ranges of 5 to 10 feet (1.5–3 meters) or more. The Bay of Fundy, in particular, boasts the world’s largest tidal range—over 50 feet (15 meters) during spring tides, according to NOAA.
The moon’s gravitational pull is the primary driver of tides. When the moon aligns with a specific location on Earth—once every 24 hours and 50 minutes—it pulls water toward it, creating a tidal bulge on that side of the planet. A second bulge forms on the opposite side because the Earth’s core is pulled more strongly toward the moon than the water on that side. These aligned bulges generate high tides on both sides of Earth, while the areas midway between the bulges experience low tides.
The sun also exerts a gravitational influence, albeit weaker due to its greater distance. When the moon, sun, and Earth align—at new and full moons—their combined forces amplify each other, producing the highest tides, known as spring tides. Conversely, when the moon is at first or third quarter, the sun’s pull partially offsets the moon’s, resulting in lower tidal ranges called neap tides.
The moon’s orbit is elliptical, so its distance from Earth varies. At apogee, the moon is farthest away, reducing tidal ranges; at perigee, it is closest, amplifying them. Consequently, the difference between low and high tides can fluctuate noticeably over a lunar month.
