Twice each lunar month, around the first and third quarter moons, the tidal range reaches a minimum. These are neap tides. They occur when the gravitational pulls of the Moon and Sun are at right angles relative to Earth, so the tidal bulges partially cancel rather than reinforce each other.

High tides are lower than average and low tides are higher than average, producing the smallest difference between high and low water. Neap tides alternate with spring tides, which occur at new and full moon when the range is greatest. The exact range also depends on local coastal geometry and bathymetry.

Mariners, coastal engineers, and intertidal biologists track the spring-neap cycle because it affects navigation depths, the duration of intertidal exposure, and the timing of certain ecological processes. Tide tables predict the cycle years in advance with high accuracy.

• Minimum tidal range in the lunar cycle
• Occurs at quarter moons
• Sun and Moon pull at right angles
• Alternates with spring tides