Tides and Water Levels

First published: 2008

Brief summary

NOAA tutorial on tides as gravitationally forced periodic motion.

Article

Tides are periodic changes in sea level produced mainly by the gravitational attraction of the Moon and Sun, together with the motion of the Earth–Moon system. The Moon generally has the larger tide-generating effect because it is much closer to Earth.

The gravitational force is not identical across Earth. Differences between the side facing the Moon, Earth’s centre and the far side produce two broad tidal bulges, while Earth’s rotation carries coastlines through these regions.

The relative positions of the Sun, Moon and Earth alter tidal range. Spring tides occur near new and full moon when the solar and lunar effects reinforce one another, while neap tides occur near the quarter moons when they partly oppose one another.

Observed tides differ greatly between locations because ocean-basin shape, coastline geometry, water depth, weather and local resonance modify the astronomical forcing. NOAA combines observations and harmonic analysis to predict the timing and height of tides.

Source details and credits

• Source / publisher: NOAA Ocean Service
• Source type: Tutorial
• URL type: WWW
• Credits: NOAA Ocean Service
• URL: https://oceanservice.noaa.gov/education/tutorial_tides/