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Posts from the ‘Weather/Tides’ category

The Tide is High . . .

The tide is high and I’m holding on . . . in attempting to explain the following.

Truth is, even thought I taught science for years and live by the tides, I get a little dizzy when I strive to explain how our winter tides can be particularly high and low.

Why this is topical is because, this Friday, November 25th, there will be an extremely high tide and a very large tidal exchange. Tide tables for Alert Bay indicate that the ocean will bulge to a 17.44′ (5.3m) high at around 12:30. Then it will drop by 17.16′ (5.2m) to a 0.28′ (8.5cm) low tide around 7:30 PM. It is relevant that this is happening in the winter.

To challenge my teacher skills, I will try to be as succinct as possible in explaining what is causing this.

  • Tides are caused by the gravitational pull of the moon and the sun.
  • The strength of this gravitational pull is constantly changing because the earth is rotating on its axis (cycle of a day; 23 hours and 56 min) while orbiting around the sun (cycle of a year; 365.24 days) and the moon is orbiting around the earth (cycle of a month; 27.3 days).
  • I will not get into the detail of how this happens, but because of the movement of the moon relative to the earth, a “tidal day” is about 24 hours and 50 minutes long so the cycle of high and low tides is about 50 minutes later each day. In our position on earth, we get “mixed semi-diurnal tides”  which means that over 24 hours and 50 minutes we get a high tide, then a low tide, another high tide (of a different height that the first high tide), and then another low tide (of a different height that the first low tide). 
  • When the moon and sun are in line with the earth, their gravitational forces combine and the high tides are higher and the low tides are lower. These strong tides are known as Spring tides (but they have NOTHING to do with the season “Spring”!) There are two scenarios when the moon and sun are in a line. (1) Full Moon and (2) New Moon. Please see diagram below. 

Two scenarios when the combined gravitational force of the moon and sun is the greatest. (1) Full moon shown on left (lit side of the moon faces earth); (2) New moon shown on the right (dark side of the moon faces earth). Source

  • This Friday, we have a New Moon. The moon will be in a line between the sun and the earth. The sun and the moon will therefore combine their forces on the ocean making for a really high tide and a really low tide.
  • HOWEVER, it is also significant that it is winter for us now. The gravitational pull is even greater because, the way the earth travels in its yearly elliptical orbit around the sun, it is closest to the sun during the northern hemisphere’s winter. The earth is closer to the sun so there is an even greater gravitational pull.
  • Wait, if the earth is closest to the sun in the winter WHY is it colder for us right now?! This is because of the way the earth tilts. Our half of the world is tilted away from the sun right now. The southern hemisphere is basking in the rays.

How was that explanation?

How about this animation? Does it offer any greater clarity?


Okay, how about this . . . Blondie, from 1980!  

Your reward for “hanging on” in wanting to understand why  . . . the tide is high! 

Update – November 26, 2011

As a very valued by-product of his reading this blog, Rod Gray dared go where I did not and shared an explanation of why we have 2 high tides and 2 low tides per day in our area (i.e mixed semidiurnal tides):

“The “main” high tide, the one caused by the earth rotating under the bulge in the ocean caused by the moon’s gravitational pull is easy enough to understand, but what about the other tide, caused by a bulge on the other side of the earth? The (very simplified) explanation goes like this:

To say that the moon orbits the earth is not precicely correct. Actually, the moon and the earth both orbit around a common point which is their center of combined mass. This point happens to be within the earth, but not at its center. Thus, the position of the earth (within the earth/moon system) is not stationary, but travelling in a relatively small circle (whose period is that of the moon’s orbit around the earth). Therefore, just as a person on a merry-go-round feels an apparent centrifugal force (seemingly) pulling them away from the center, the ocean (on the opposite side from the bulge caused by the moon’s gravity) also bulges in response to the apparent centrifugal force caused by the earth “orbiting” around the earth/moon center of gravity. In other words, one of the high tides is caused by the pull of the moon, and the other high tide is caused by centrifugal force.”

Yeah Rod!  And thank you!