TIDES

GRAVITY

Remember gravity is an invisible force of attraction between two objects.

What celestial object is attracted to the Earth?

The moon

What celestial object is the Earth attracted to?

The Sun

EFFECT OF THE MOON ON THE EARTH

The moon’s gravity also pulls on the Earth, but because the Earth is solid, it doesn’t seem to respond.

However, the Earth is mostly covered with water. Water is a liquid and does respond to the pull of the moon. This response creates tides.

TIDES

The how and the why of tide formation is very complex.

This presentation will try to simplify how and why tides form.

To do this….

IMAGINE THAT THERE IS NOTHING BUT WATER COVERING THE EARTH.

NOW IMAGINE LOOKING AT THE EARTH FROM ABOVE THE NORTH POLE.

N

LET’S ADD THE MOON TO THIS SYSTEM.

N

THE MOON PULLS ON THE WATER.

This creates a bulge.

N

BUT BECAUSE THE EARTH IS SPINNING THERE ARE ACTUALLY 2 BULGES CREATED.

You’ll learn more about this in a course called Physics.

N

THIS BULGE OF WATER FOLLOWS THE MOON AS IT

ORBITS THE EARTH.

N

IT TAKES THE MOON ABOUT 28 DAYS TO REVOLVE AROUND THE

EARTH.

N

THIS MAKES THE TIMES THAT THE TIDES OCCUR CHANGE IN A PREDICTABLE

PATTERN.

BUT THERE IS MORE GOING ON THAN JUST THE REVOLUTION OF THE MOON AROUND

THE EARTH .

The Earth rotates every day.

N

LET’S PUT A PERSON ON THE EQUATOR OF THE EARTH.

Remember you are looking at the Earth from the North Pole, so this person appears to be on

the edge of the Earth.

AS THE EARTH ROTATES, THE PERSON WILL MOVE THROUGH TWO VERY HIGH BULGES OF WATER

AND TWO AREAS OF LOW WATER EACH DAY .

WE CALL THESE AREAS HIGH TIDE AND LOW TIDE .

Remember in a 24 hour period of rotation, there are 2 high tides and

2 low tides.

This is what it looks like with both motions.

SPRING TIDES

When the Sun, Earth, and Moon are in a straight line- the variation between high tide and low tide is greatest. This occurs during the full moon and new moon.

High

Tide

LowTide

NEAP TIDES

When the Sun, Earth, and Moon are in a perpendicular arrangement- the variation between high tide and low tide is not great. This occurs during the first and third quarter moon.

High

Tide

LowTide