What Causes the Tides?
The tides in the Bay of Fundy (the funnel-shaped bay near the top of this photo) are the largest in the world.
CREDIT: Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC
Tides may seem simple on the surface, but the ins and outs of tides confounded great scientific thinkers for centuries – they even led Galileo astray into a bunk theory.
Today people know that the gravitational pulls between the earth, moon and sun dictate the tides. The moon, however, influences tides the most.
The moon's gravitational pull on the earth is strong enough to tug the oceans into bulge. If no other forces were at play, shores would experience one high tide a day as the earth rotated on its axis and coasts ran into the oceans' bulge facing the moon .
However, inertia -- the tendency of a moving object to keep moving -- affects the earth's oceans too. As the moon circles the earth, the earth moves in a very slight circle too, and this movement is enough to cause a centrifugal force on the oceans. (It's centrifugal force that holds water in a bucket when you swing the bucket in an overhead arc.)
This inertia, or centrifugal force, causes the oceans to bulge on the opposite side facing the moon. While the moon's gravitational pull is strong enough to attract oceans into a bulge on the side of the earth facing the moon, it is not strong enough to overcome the inertia on the opposite side of the earth. As a result, the world's oceans bulge twice – once when they are on the side of Earth closest to the moon , and once when they are on the side farthest from the moon, according to the Wood's Hole Oceanographic Institution in Wood's Hole, Mass.
Geography complicates the tides, but many places on Earth experience just two high and two low tides every 24 hours and 50 minutes. (The extra 50 minutes is caused by the distance the moon moves each day as it orbits Earth).
The sun and the tides
"Solar tides" are caused by the sun's gravitational pull and are weaker than lunar tides.
The sun is 27 million times more massive than the moon, but it is also 390 times farther away. As a result, the sun has 46 percent of the tide-generating forces (TGFs) that the moon has, according to the National Oceanic and Atmospheric Administration (NOAA).
Solar tides are therefore often considered just variations on lunar tides.
Local geography can vary tide strength as well.
Just north of the coast of Maine in Canada, the Bay of Fundy has a unique “funnel shape” at just the right position to creates the largest tides in the world. Water in the bay can rise more than 49 feet (15 meters) or about as high as a 4-story house.
FORCE, the Fundy Ocean Research Center for Energy, estimates the Bay of Fundy pushes110 billion tons (100 billion metric tons) of water with every tide.
Recently, local leaders have moved to take advantage of the tides.
In July, Maine's Governor John Baldacci and Nova Scotia's Premier Darrell Dexter signed a Memorandum of Understanding to share research and ideas in tidal and offshore wind sources of renewable energy, according to Business Weekly.
Understanding tides: then and now
When Galileo Galilei attempted to explain tides in 1595, he left the moon out of this theory and focused on the inertia of the oceans and his correct idea that the earth orbited the sun, according to a NOVA documentary.
It wasn't until 1687, that Sir Isaac Newton explained that ocean tides result from the gravitational attraction, according to NOAA.
Got a question? Email it to Life's Little Mysteries and we'll try to answer it. Due to the volume of questions, we unfortunately can’t reply individually, but we will publish answers to the most intriguing questions, so check back soon.
Life's Little Mysteries: Gift Edition Hardcover Book
Uncover the truth behind more than 100 mysteries that surround us every day with our new hardcover book! Perfect for gifts and classrooms, and suitable for all ages. Some of the included mysteries are:
- Why Do Cats Land on Their Feet?
- How Long Does it Take to Make Petrified Wood?
- What Everyday Things Around Us Are Radioactive?
Find out all of this and much, much more in our NEW hardcover book.
It makes a great gift idea for all ages. more info>>