What is the primary cause of the variation in high and low tides?

The primary cause of the variation in high and low tides is gravitational interactions, specifically the gravitational pull of the moon and the sun on Earth's oceans. The moon's gravity has the most significant effect because it is closer to Earth, creating a bulge of water that leads to high tide on the side of the Earth facing the moon. At the same time, a corresponding bulge occurs on the opposite side due to the centrifugal force caused by the Earth-moon system's rotation. As the Earth rotates, different areas experience these bulges, resulting in the rise and fall of water levels we observe as high and low tides.

The sun also plays a key role, albeit to a lesser extent compared to the moon, as its gravitational force also affects ocean water levels. When the sun, moon, and Earth align (during full and new moons), the tidal effects are amplified, leading to what are called spring tides, where high tides are even higher than usual and low tides are lower. Conversely, when the sun and moon are at right angles relative to Earth (during the first and third quarter moons), this results in neap tides, where the difference between high and low tides is less pronounced.

This gravitational dynamic is the fundamental principle that underpins tide variability