"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.

Redfishpro.org

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/

Redfishpro.org

Tide Charts

http://freetidetables.com/?tides_data%5bttiq%5d=-16,2

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/.

Swimming with the Tide in Hawaii

For those of you who do not remember their 9^th grade General Science Class, like me, the ocean’s tides have always been sort of a mystery – e.g., why are there two of them each day?

A search of the literature reveals a complicated, and even somewhat controversial, explanation of why and how tides occur. And some parts of the world do not even have two tides per day! So I will keep it general about why tides occur, but be more specific about what happens.

http://www.soest.hawaii.edu

First, a key definition: Mean Lower Low Water (MLLW): the average of the lowest tide recorded at a tide station each day during the recording period, usually nineteen years. It is the “0” in tide charts, but more about them later.

Simplifying, and ignoring inertia, tides are created because the Earth and the moon attract each other, like magnets. The moon tries to pull at anything on the Earth to bring it closer. However, the Earth is able to hold onto everything except the water.www.hiwaay.net

The gravitational attraction is strongest on the side of Earth that happens to be facing the Moon, simply because it is closer. This attraction causes the water on this “near side” of Earth to be pulled toward the moon (see below).

www.HowStuffWorks.com

On the opposite of Earth (the “far side”), the gravitational attraction of the Moon is less because it is farther away. Thus, the moon’s gravity creates two bulges of water. One forms where Earth and Moon are closest, and the other forms where they are farthest apart. That then means in most of the country, each day there are two high tides and two low tides. The ocean is constantly moving from high tide to low tide, and then back to high tide.

www.TidesfromSpace

A high tide is as high as the water will reach before it starts to fall again. It is highest when the Earth and Moon are closest, and the other daily high tide is somewhat less than the highest tide (shouldn’t these tides have different names (?)). A low tide is as low as the water goes before it starts to rise again. And the same with the two daily low tides; one is lower than the other.

http://freetidetables.com/?tides_data[ttiq]=-16,2

A common misconception is the thought that since there are four tides daily they must be on a six hour schedule. It takes the Earth about 24 hours to rotate once, relative to the Sun. But, because the Moon is moving with respect to Earth and the Earth is spinning, it takes the Earth a little longer to complete a rotation relative to the Moon—24 hours and 50 minutes. Thus, two daily tides occur separated by 12 hours and 25 minutes. www.Woods_Hole.edu

The amount of rise of fall in the tide is directly related to the relative location of the earth, moon and sun, but we’ll address that next time.