Lunar tides, illustration. Tidal phenomena arise from the changing gravitational field strength of one celestial body, felt across another celestial body. In this case, the Moon (top right) stretches the Earth's oceans (lower left) into an ovoid (egg-like) shape. The Moon's gravity weakens with distance (red circles). The gravitational force of the water on itself maintains the symmetry and ensures a tidal bulge on both sides of the Earth (with a larger tide on the side facing the Moon). The Earth is not locked by tidal forces and rotates its solid surface past its tidal bulges, resulting in high and low tides. The tidal bulge in the Earth's oceans is pulled ahead of the Moon by the Earth's rotation (red arrows).