tidal force

Tidal forces arise because gravity is not uniform across an extended body. The near side of a body is pulled more strongly than the centre, and the centre more strongly than the far side. The net effect is a stretching along the line connecting the two bodies and a compression perpendicular to it. The tidal acceleration scales as Δa ≈ 2GMd/r³, where d is the size of the body and r is the distance to the attractor.

The Moon's tidal force raises two bulges in Earth's oceans — one on the side facing the Moon (pulled more than Earth's centre) and one on the far side (pulled less, effectively left behind). The Sun produces a tidal force about 44% as strong as the Moon's. When Sun and Moon align (new and full moon), their tidal forces add to produce spring tides; when perpendicular (quarter moons), they partially cancel to produce neap tides.