
Related to tides: neap tides, spring tides
Tidesclick for a larger image
Tides: (a)(i) gravitational acceleration on Earth due to Moon, (ii) differential (tidal) acceleration found by subtracting a from other vectors on Fig. (1); (b) spring (top) and neap tides


Distortions of a planet, star, etc., produced by the differential gravitational attraction of other astronomical bodies on parts of that planet, star, etc. The Sun and Moon combine to generate two tidal bulges in the Earth's oceans, one directed toward the Moon and the other diametrically opposite. If the Earth is assumed to be spherical and to be covered with water, then the gravitational pull of the Moon will have a different force at different points on the Earth's surface (see illustration (a)(i)). The gravitational acceleration at the Earth's center is equal to the centripetal acceleration of the Earth–Moon system. If this acceleration (which is a vector quantity) is subtracted from the surface accelerations, the differential (tidal) acceleration is found (illustration (a)(ii)), showing the two tidal bulges on the Earth–Moon line.

The daily rotation of the Earth and the slower eastward revolution of the Moon in its orbit produce (usually) two high tides and two low tides every 24 hours 50 minutes. The rotation and lunar revolution are also responsible for a tidal lag between the time when the Moon crosses the local meridian and the time of high tide.

The tidal pull of the Sun is less than half that of the Moon but reinforces it at full Moon and at new Moon to produce very high, or spring tides; these tides are exceptionally high when the Earth is close to perihelion and the Moon is close to perigee (see illustration (b)). The lowest high tides, or neap tides, occur when the Moon is at quadrature. The tidal height depends on the topography of the coastline and on the area of the adjacent continental shelf. Tides are also generated in the Earth's atmosphere and in the solid Earth.

Lunar tides are raised by the Earth as a consequence of the Moon's eccentric orbit. They trigger moonquakes and transient lunar phenomena. See also tidal friction.

References in classic literature ?
A thousand times no; and I wept tears of sweet sadness over my glorious youth going out with the tide.
Trusting to the girl's skill and making no use of the rudder, he eyed the coming tide with an absorbed attention.
Give me a hand with this rope, and we'll drag her up as far as we can; and then when the tide goes out we'll try another scheme.
The tide ran strong, I took care to lose none of it, and our steady stroke carried us on thoroughly well.
Yes, monsieur," replied one of them, "we are only waiting for the tide.
San Rafael Creek, up which we had to go to reach the town and turn over our prisoners to the authorities, ran through wide-stretching marshes, and was difficult to navigate on a falling tide, while at low tide it was impossible to navigate at all.
While I was doing this, I found the tide begin to flow, though very calm; and I had the mortification to see my coat, shirt, and waistcoat, which I had left on the shore, upon the sand, swim away.
Jo could not speak, and for several minutes there was no sound but the sigh of the wind and the lapping of the tide.
Then he turned to Yolland, and asked, "Is there any chance of finding her, when the tide ebbs again?
For this purpose, return all to your posts; within an hour, we shall have the ebb of the tide.
On this mighty tide the black ships--laden with the fresh-scented fir-planks, with rounded sacks of oil-bearing seed, or with the dark glitter of coal--are borne along to the town of St.