The force that drives the tides is not gravity itself but the variation in it across the Earth: the side nearer the Moon is pulled more strongly than the centre, and the far side less, producing two bulges. Because it depends on the cube of distance, the Moon's tidal effect is about 2.2 times the Sun's despite the Sun's far greater mass. The horizontal component of this force does the real work of moving water.
Tide-generating force goes as distance cubed, which is why perigee matters more than people expect.
Also heard as
- tidal force
- tractive force
Related terms
Equilibrium tide
The theoretical tide an ocean covering the whole Earth would show if it responded instantly to the tidal forces.
Spring tide
A tide of larger than average range, occurring around new and full moon when Sun and Moon pull in line.
Perigean spring tide
An especially large spring tide that occurs when new or full moon falls close to the Moon's nearest approach to Earth.
Declination
The angle of the Moon or Sun north or south of the celestial equator, which makes the two daily tides unequal.