How the Tides are Created

April 02, 2018 Posted by: Nicole Ornelas
At Cabrillo National Monument, a trip to the tidepools is based specifically around the tide. Within park hours and during low tide, 0.7 feet below sea level or lower, a preserved ecosystem of marine creatures can be explored. On the other hand, during high tide, the entire tidepool area at Cabrillo National Monument is hidden below the surface of San Diego’s beautiful coastline. 

Cabrillo National Monument Tidepools at sunsetNPS Photo/Nicole Ornelas: Cabrillo National Monument Tidepools at sunset.

Tides are the alternating rise and fall of the sea level within a 24-hour period. In San Diego, this fluctuation in sea level is moderate and can range between two to ten feet within a day. However, in some areas like the Bay on Fundy in Canada, the tidal fluctuation can be up to 53 feet!   

So what causes this shift in sea level within any given day? Across the globe, the gravitational forces of the Moon and the Sun pulling on the Earth produce what we know as the tides. As the Earth rotates around the Sun and the Moon rotates around the Earth, the focus of these gravitational forces change position and inertia follows.

Graph showing what causes tides
Source of Image: 

When the Moon and Sun are aligned with the Earth, here in San Diego, we have the greatest high and low tides. When in alignment, both gravitational forces amplify one another (notice the bulge of water depicted in green or black in the images above and below). These are called Spring Tides. 

When the Moon and Sun are perpendicular to each other, the gravitational forces are diffusing one another and dispersing the overall force. In San Diego, this is when we have little to no change in our tides. These are called Neap Tides.

Figure showing the difference between spring and neap tides
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The gravitational force between two objects is dependent on the mass of the objects and the proximity of the objects. 

Gravitational Force Equation
Source of Image:
Equation for Gravitational Force. 

Using the Gravitational Force Equation, you may notice that proximity trumps mass to a varying degree. Thus, although the Sun has a much greater mass than the Moon, the Moon still has more of an effect on the San Diego tides due to its relative closeness to the Earth. 

Next time you are exploring the tidepools anywhere around the globe, think about the great force of the Sun and the Moon. 

You can find information about the low tides at Cabrillo National Monument from the chart below, or from the following links from the Scripps Pier Webcam (courtesy of Ed Parnell at Scripps), which show the tide charts for 2018-2019.

Please note that the following charts are predictions for the Scripps Pier in La Jolla. They should be used as guidelines. Actual times for the Cabrillo tidepools could vary by approximately 10 minutes earlier than what is shown.

2018 Tide Charts
2019 Tide Charts

Last updated: April 2, 2018

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