Tides
• We experience
tides on Earth
when the ocean
level rises and
falls.
• We experience 2
high tides and 2
low tides in a
little over one
day.
• But what causes
tides?
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Tides and Gravity
• The gravitational pull of the
Moon is the most important
reason we experience tides
• The Sun is also important.
• Tides occur when the Earth
gets “stretched” by the
Moon’s gravity.
• The parts of the Earth
closest to the Moon get
pulled more than parts in the
opposite direction.
• It is the difference in the
strength of gravity with
distance that causes tides.
2
The Tidal Bulge
• Since water can flow more
easily than rock it “piles
up” into a bulge pointing
towards the Moon and a
bulge pointing away from
the Moon.
• The bulge on the side of
Earth farthest from the
Moon is due to the Earth
being pulled “out from
under” the water on the far
side.
• This varying amount of
gravity is the “stretching
force” that causes the tidal
bulges to form along the
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Earth-Moon line.
Two Tides a Day
• When we are beneath one of the
tidal bulges we experience “high
tide”
• In the space between the bulges
we experience “low tide”
• As the Earth spins beneath these
tidal bulges we get carried into
and then away from two high and
low tides each day.
• Because the Moon moves in its
orbit the tidal bulges move as well
so it takes a little more than 12
hours for two high tides to occur.
4
The Sun’s Role
• Certainly the Sun contributes
to tides as well but to a lesser
degree.
• When the Moon and the Sun
are nearly aligned (that is at
New Moon and Full Moon)
the tidal forces are the
strongest and we get more
extreme high and low tides.
These are called Spring
Tides.
• When the Moon is at 1st and
3rd Quarter the Earth
experiences less extreme
tides. These are called Neap
Tides.
5
Tidal Friction & Tidal Braking
The Moon is moving away
from the Earth at 1 inch/year.
The Earth is slowing down by
0.002 seconds/century
• As the Earth spins it is
constantly running up against
the tidal bulges.
• The Earth forces the bulges to
move slightly off of alignment
with the Moon. This force is
called Tidal Friction.
• The tidal bulges themselves
exert a gravitational influence
on the Moon causing it to speed
up and move to a slightly higher
orbit.
• The pull of the Moon on the
bulges slows the Earth’s
rotation slightly. This is called
Tidal Braking.
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Tidal Braking & Spin Orbit
Resonance
• The Moon has one face
always towards the Earth due
to tidal braking.
• Tides on the Moon have
caused its spin to perfectly
match its orbital period.
• Pluto and Charon also
experienced tidal braking
and they are locked into
keeping one side always
facing each other.
• Most of the moons in the
solar system are tidally
braked as well.
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Other Examples
of Tides
Tides are responsible
for:
• ripping interacting
galaxies apart
• pulling gas from
one star to another
in binary systems
• and melting the
interior of Io as it
interacts with
Jupiter and the
other Galilean
satellites.
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