The word “tides" describes the regular rising and falling of the ocean's surface. Large lakes, such as the Great Lakes, also have tides, but the change in water level is only inches. Ocean tides are measured in feet.
Tides are caused by gravitational forces of the moon and the sun. The sun is huge, but it is 360 times farther from the Earth than the moon.
Even though the moon is much smaller than the sun, it has two times more influence on tides than the sun, simply because it is much closer to the Earth.
The moon's gravitational force pulls on water in the oceans and causes bulges that create “high tide." The moon's gravitational pull is strongest on the side that faces the Earth. You might think that the opposite side of the Earth would experience a low tide, but that would be incorrect!
Meanwhile, on the other side of the Earth, there is another high tide created by the moon slightly pulling the Earth away from the still water on that side.
There are a few different types of special tides that occur depending on the phase of the moon. Spring tides are strong tides that occur when the Earth, the moon and the sun are in a line.
"Neap tides" are weak tides that occur when the gravitational forces of the moon and the sun are perpendicular to one another, pulling on the Earth from different directions. Neap tides occur during the quarter moon phase.
- Isaac Newton first explained tides scientifically in 1686.
- The highest tides in the world occur in the Bay of Fundy in Nova Scotia, Canada.
- Because the moon moves around the Earth, it's not always in the same place at the same time each day. So, the times for high and low tides change by 50 minutes each day.