The Gravitational Connection
Tides are primarily caused by the Moon's gravitational pull on Earth's oceans. Since gravity decreases with distance, the ocean water on the side of Earth facing the Moon experiences a slightly stronger pull than the water on the far side. This difference creates two tidal bulges — one facing the Moon and one on the opposite side.
Spring Tides: When Moon and Sun Align
During new and full moons, the Sun, Earth, and Moon are aligned (a configuration called syzygy). During these times, the gravitational forces of the Sun and Moon combine, creating especially high "spring tides" (the name comes from the German "springen," meaning to leap — not the season). Spring tides have both higher high tides and lower low tides than average.
Neap Tides: When Forces Oppose
During first and last quarter moons, the Moon and Sun are at 90° angles relative to Earth. Their gravitational forces partially cancel each other, resulting in "neap tides" — tides with a smaller range between high and low water.
Why There Are Two High Tides Per Day
Earth completes one rotation in 24 hours, but during that time the Moon has moved slightly in its orbit. This means a complete tidal cycle takes about 24 hours and 50 minutes — explaining why high tide comes about 50 minutes later each day, and why most coastal locations experience two high tides and two low tides approximately every 24 hours.
Sources
- NOAA Ocean Service: Tides and Water Levels
- NASA: What Are Tides? (science.nasa.gov)
- Pugh, D.T. (1987). Tides, Surges and Mean Sea-Level. John Wiley & Sons.