The Moon and Tides - What causes tides?

1. Tides

5. 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

6. Location of HighTides
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

7. High tides are aligned under the moon.
Describe the location of tides
Low Tides are aligned at a right angle to the high tides.

9. 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.
9

10. 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

11. 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.

12. 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.

14. 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.

15. 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.