T IDES

G RAVITY  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

E FFECT OF THE MOON ON THE E ARTH  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.

T IDES  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….

I MAGINE THAT THERE IS NOTHING BUT WATER COVERING THE E ARTH.

N OW IMAGINE LOOKING AT THE E ARTH FROM ABOVE THE N ORTH P OLE. N

L ET ’ S ADD THE MOON TO THIS SYSTEM. N

T HE MOON PULLS ON THE WATER. This creates a bulge. N

B UT BECAUSE THE E ARTH IS SPINNING THERE ARE ACTUALLY 2 BULGES CREATED. You’ll learn more about this in a course called Physics. N

T HIS BULGE OF WATER FOLLOWS THE MOON AS IT ORBITS THE E ARTH. N

I T TAKES THE MOON ABOUT 28 DAYS TO REVOLVE AROUND THE E ARTH. N

T HIS MAKES THE TIMES THAT THE TIDES OCCUR CHANGE IN A PREDICTABLE PATTERN.

B UT THERE IS MORE GOING ON THAN JUST THE REVOLUTION OF THE MOON AROUND THE E ARTH. The Earth rotates every day. N

L ET ’ S PUT A PERSON ON THE EQUATOR OF THE E ARTH. Remember you are looking at the Earth from the North Pole, so this person appears to be on the edge of the Earth.

A S THE E ARTH ROTATES, THE PERSON WILL MOVE THROUGH TWO VERY HIGH BULGES OF WATER AND TWO AREAS OF LOW WATER EACH DAY.

W E 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.

S PRING T IDES  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

N EAP T IDES  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