Tides

A tide is the periodic rise and fall of a body of water due to

gravitational interactions between the sun, moon and Earth.

Tides - Definition

Location of High Tides

Tide-generating forces produce 2 Tidal bulges:

1. Toward Moon on side of Earth facing Moon

2. Away from Moon on side of Earth opposite Moon

Earth rotates into and out of tidal bulges, creating high and low tides

High tides are aligned under the moon.

Describe the location of tides

Low Tides are aligned at a right angle to the high tides.

What causes tides?

Tides are created by the imbalance between two forces squeezing water to either side of the Earth:

1. Gravitational force of the Moon and Sun on Earth

• If distance increases (), then gravitational force greatly decreases()

• The moon then pulls harder on the water under it

2. Centripetal (center-seeking) force required to keep bodies in nearly circular orbits

• Earth’s movement around the Earth-Moon Barycenter pushes water out opposite the moon.

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Gravitational forces on Earth’s tides are mainly due to the Moon

• Force decreases with increasing distance

• Force is directed toward the Moon’s center of mass

Centrifugal Force opposing Earth’s inward motion around the barycenter.

• Force pushes water opposite the moon.

• Force is directed opposite the location of the barycenter.

Barycenter

• The Earth and the Moon together make a system; The Earth – Moon System.

• Although it appears that the moon orbits the Earth, the moon actually orbits a common center of gravity between the Earth and the moon.

• This common center of gravity is referred to as the barycenter.

• Both the Moon and the Earth move about this center of gravity.

• It is this point that revolves around the sun.

• This point is located 900 miles from Earth’s surface.

1st High Tide on near side of the moon.

• Gravity grows weaker with distance and pulls harder on

the near side of the moon.

Tidal Bulge – Why 2 tidal bulges

2nd High Tide Opposite the moon.

• Earth’s movement around the Barycenter creates a force that helps push water out opposite the Earth from the moon.

Tidal Bulge – Why 2 tidal bulges

With these forces present, tides are generated because the

large volume of water in the ocean.

The tides are thus squeezed under their own weight pushing

water towards the moon and along the Earth-Moon line.

Smaller bodies of water like a lake, water in a glass, or a cup

of coffee do not have enough volume to squeeze the liquid

towards the Earth-Moon line.

Therefore, smaller bodies of liquid do not exhibit tides.