Tides

Tides are the daily rise and fall of sea level at any given place. The pull of the Moon’s gravity on Earth is the primary cause of tides and the pull of the Sun’s gravity on Earth is the secondary cause.  The Moon has a greater effect because, although it is much smaller than the Sun, it is much closer. The Moon’s pull is about twice that of the Sun’s.

Tides are caused by the gravitational attraction of the moon to ocean water. Image courtesy of CK-12. 

To understand the tides it is easiest to start with the effect of the Moon on Earth. As the Moon revolves around our planet, its gravity pulls Earth toward it. The lithosphere is unable to move much, but the water is pulled by the gravity and a bulge is created. This bulge is the high tide beneath the Moon. On the other side of the Earth, a high tide is produced where the Moon’s pull is weakest. These two water bulges on opposite sides of the Earth aligned with the Moon are the high tides. The places directly in between the high tides are low tides. As the Earth rotates beneath the Moon, a single spot will experience two high tides and two low tides approximately every day.

High tides occur about every 12 hours and 25 minutes. The reason is that the Moon takes 24 hours and 50 minutes to rotate once around the Earth, so the Moon is over the same location every 24 hours and 50 minutes. Since high tides occur twice a day, one arrives each 12 hours and 25 minutes.

The tidal range is the difference between the ocean level at high tide and the ocean level at low tide. Waves are additive, so when the gravitational pull of both the Sun and the moon is in the same direction, the high tides are higher and the low tides lower than at other times through the month. These more extreme tides, with a greater tidal range, are called spring tides. Spring tides occur whenever the Moon is in a new-moon or full-moon phase, about every 14 days.

Spring tides happen when the gravitational pull of both the moon and the sun is in the same direction. Image courtesy of CK-12. 

Neap tides are tides that have the smallest tidal range, and they occur when the Earth, the Moon, and the Sun form a 90o angle. They occur exactly halfway between the spring tides, when the Moon is at first or last quarter.  At neap tides, the tidal range is relatively small.

Neap tides ocur when the graviatational pull of the moon and sun are at a 90ºangle. Image courtesy of CK-12. 

Source: Tides. Retrieved from http://www.ck12.org/earth-science/Tides/lesson/Tides/ on September 1, 2013. 

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Last modified: Monday, 12 February 2018, 12:37 PM