Tuesday, June 8, 2010

"Time and Tide Wait for No Man"

Continuing the topic of tides, we now know why there are two high tides and two low tides at most places on the earth daily (see “Swimming with the Tide in Hawaii”). How high or low the tides are depends on the relative location of the earth, moon and sun.

Spring Tides

When the moon is full or new, the gravitational pull of the moon and sun are combined. The combined gravitational attraction of the two bodies produces a very strong tide that “springs forth” onto the coast, and thus is called a spring tide. (Spring tides have nothing to do with the season of the spring.) They occur when the Earth, the Sun, and the Moon are in a line. The gravitational forces of the Moon and the Sun both contribute to the tides. Spring tides occur during the full moon and the new moon.


Neap Tides

During the moon's quarter phases the sun and moon work at right angles, causing the bulges to cancel each other. The result is a smaller difference between high and low tides and is known as a neap tide. Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons. http://home.hiwaay.net/~krcool/Astro/moon/moontides/

Tide Charts


Tide charts show a tremendous amount of information (please follow link to see larger size). They show the times of sunrise, sunset, moonrise and moonset. They also show the phase of the moon and the times of the two high and two low tides each day. And the vertical axis shows the height or depth of the tide in feet as a variance to the Mean Lower Low Water (MLLW) level – defined in the last post.

You can see for example that in Hilo the high tide this week will be on Saturday with the new moon at about three feet above MLLW and occurring at 15:55 (3:55pm), and lowest tide that day will be about 0.7 feet below MLLW at 8:30am. And these are spring tides because the sun, moon and earth are in a straight line with each other.

Four feet is about the maximum variance of the tides at Hilo which isn’t much as you can see by the two photos below taken from our home here.

Low Tide

High Tide
The highest tides in the world at over 50 feet (!) are at the Bay of Fundy in Nova Scotia, Canada. The Bay of Fundy even has an interactive site where you can see the impact of such tides at http://www.bayoffundytourism.com/.

1 comment:

  1. Great topic Wes.
    I never looked into why, but our part of the Texas Coastal Bend had only one tide cycle per day. Also, there is a Navy development outfit in Bethesda MD that has a large model towing tank so large in volume, it is rumored that it has tides.


Mahalo for leaving a comment!~WesIsland