Why High Tide Comes 50 Minutes Later Every Day

If you visit the same beach at the same time two days running, the tide will not be in the same place. Coastal areas get two high tides and two low tides every lunar day — and a lunar day is not 24 hours long, but 24 hours and 50 minutes.

The two tidal bulges, caused by gravity and inertia, rotate around the Earth as the moon's position changes. The bulges are high tides; the flat sides are low tides. Credit: NOAA.

The lunar day

A lunar day is the time it takes for a point on the Earth to turn all the way around and end up in the same position relative to the moon. It is longer than a normal day because the moon travels around the Earth in the same direction the Earth spins. The Earth needs an extra 50 minutes to "catch up" to the moon.

Tides as waves

Tides are very long waves that move across the ocean, driven by the gravity of the moon and, to a lesser extent, the sun. When the crest of the wave reaches a coast, that coast has high tide; when the trough arrives, it has low tide.

Why there are two high tides

Picture the ocean shaped like a football pointing at the moon. Its two pointed ends are high tide; its flat sides are low tide.

• The near bulge. The moon's gravity is strongest on the side of the Earth facing it, pulling the ocean toward the moon.
• The far bulge. On the opposite side the moon's pull is weaker, and inertia wins: the water there resists being pulled back, and the ocean bulges outward.

As the Earth spins, different places turn to face the moon, and the tides cycle around the planet.

Keeping watch

National Ocean Service scientists use advanced tide recording systems and satellite imagery to monitor tides and water levels, and use the data to predict the ocean's behavior and protect coasts and coastal communities.

Sources

Based on "How frequent are tides?" from NOAA's National Ocean Service, a work of the United States government in the public domain.