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The September Equinox and Look at what I just found!

Sine and Cosine waveforms.

Sine and Cosine waveforms.

Today is an equinox, a day of  the year when the length of of time between sunrise to sunset is equal to the time between sunset to sunrise and the sun is directly overhead of the equator.  Here in the northern hemisphere it is the autumn  equinox and nights will be getting longer while the amount of daylight becomes shorter.   It is the exact opposite in the southern hemisphere.  The rest of this discussion all applies to the southern hemisphere also, but in the interests of not always repeating myself I will limit it to just the northern hemisphere.  Just remember the southern hemisphere is just the exact opposite of the northern.   (i.e when days get longer in the northern hemisphere  they get shorter in the southern hemisphere and vice-versa.)

Another interesting thing about this day is this is the day when the length of the daylight changes the most each day.   Although it is not 100% accurate to say the length of daylight plotted over a year forms a sine wave, it is very close to true on most of the earth.  (Wimpy words alert… I said most, and I said close to true.)   If you are on the equator the length of daylight does not change at all over the year.  If you happen to live on one of the poles, the length of daylight goes from 24 hours of daylight to 24 hours of darkness.  Anywhere from north of the equator to south of the Arctic circle the length of daylight is a sine-wave with an average value of 12 hours and an amplitude that depends upon how far north.  The longest day is usually about June 22 and the shortest day is about December 22.  Another way of saying all this is the length of the day is a sine wave with a period of 365 day and it starrs on March 22, the first day of spring.   (It actually has a period closer to 365 1/4 days… see why I used the wimpy words!)

While the length of the day is related to a sine function, the rate of change is a cosine function with the maximum change on the day light when the length of day light is at near the average value of 12.  Although it is a little more complicated the main reason the length of daylight changes is because the earth resolves around the sun just like the unit circle we discussed in Trianges, The Unit Circle and Radians. I discuss this in a little more depth in this video I created:

Now for those of you that really want to go into depth about the wimpy words I used above I suggest two Wikipedia articles:  The first is Equation of Time.   The second is some very interesting photographs of an interesting solar function called Analemma.

Now, back to actually making things.  When I was looking at the last video I created about using barrels for flotation devices I ran into a video of some really interesting commercial pontoon boats or barges using exactly that.   The thing that they did was to capture the barrels within truss sections to give the platform a lot of stiffness.  And we learned all the principles of doing that back when we talked about the popsicle construction of bridges and cranes.  They just added some more construction and engineering with full sized aluminum truss members.  But in general, we are actually learning some really good stuff!!   The video is embedded here for your viewing enjoyment.

As always I hope you enjoyed this post.


I had no part in the creation of the last video and it is simply embedded by the permission of the owner by their YouTube settings.

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