Guam Day 116 - Saturday Tide Tables

5/18/2022 - I am doing double posting to catch up from Mom and Dad visiting. Please look back in posts if you are interested in reading more about their trip here. We will continue back in a more daily cadence starting with this post. 

Mom and Dad visiting gave Joan and I a chance to see so many new things in new ways. Tides were one of these items that came up over and over as we spent days at the beach. At this point in time low tide occurs around 4pm in the afternoon, and with low tide came this unusual site of seeing Tumon bay have little islands of coral that were visible. The first image may be a little difficult to see, but the dark brown color near the horizon line is the edge of the protective reef, all of which is above water. Strange because we have never seen the tide this low on Guam leading to the question why are we seeing this occur now?

Tides are not something I ever worked to understand or even question as my time living next to a large body of water has been relatively short. Also, having these shallow reef protected bays really brings this changing of water levels to light that may not be as visible in other places. Ok, so physics class in High school and college taught me that two large bodies that are close together create an attraction force due to their mass. And, I remember my college professor saying you can see this in the changing of the waves, but that's were the conversation stopped and really the heavy math started to come out and I never looked back.

The second image is of a first stop site I found that does predictions of tidal conditions based on where you live in the world. Its quite impressive actually, if you like math and science, it looks like lots of time, theory and data collection has gone into this field of study. Ultimately the work has produced an app that shows this graph. Link below:

https://www.tide-forecast.com/locations/Guam-Marianas/tides/latest

Warning, this next part is where the technical bits com in. You have been warned!!

This shape of this wild graph has to do with only three things, the position of the sun, the position of the moon, and the position of the earth. Bear with me as I try to simplify complex astrophysics into something readable in a few paragraphs. The large changes in the graph have to do with the moon orbiting around the earth, the gravitation force from the moon is pulling the large mass of the oceans towards the direction of the moon at its closest position to the earth. So if the moon is right over head the tide will be the highest you will see, and if the moon is at the horizon (90 degrees from overhead) the tide will be at the lowest. The physics is quite crazy from here, if the moon is on the other side of the earth (180 degree from your viewpoint) the ocean is restricting pull from the moon and you will have a high tide again. Very simply though every 90 degrees of rotation of the moon, the tide changes either from low to high or high to low, a sine wave for you math lovers. The moon rotates around the earth on a 28 day cycle so you see a small shift day to day of when the low and high tides occur.

Let's throw the moon out now and just think about the sun and the earth. The same gravitational pull exists  but is much smaller due to the distance. So on this graph these small changes in the peaks of the sine wave are due to the suns gravitational pull on the earth and the oceans. The complicated factor now has to do with both the orbit of the earth around the sun, and the spinning of the earth on its axis. Guam is in the northern hemisphere so lets use that for the example. Summer months the tides will be higher due to the closest point between the earth and sun being on the line of the Tropic of Cancer, also the spinning of the earth will only produce a high tide during the day. Winter months (haha winter months on Guam) will result in the lowest tide (caused by the sun) and will occur either at sunrise or sunset. Lots to wrap a mind around. 

Putting these two things now together is where my brain struggles to capture all these moving pieces. Simply, I think about each of the moon and the sun as two sine waves that add and subtract on each other. Remember in math class where a trough of the wave can element the peak of another? That is what is happening here, the moon will generally pull more strongly than the sun, except in certain times where they cancel each other out. The highest tide is when these forces each are at their maximum, and lowest the opposite. 

I do greatly enjoy the theory and thinking this through, if you don't I would recommend just checking the link the next time you want to know if you will be grounding your paddle board on the reef or not. That's all I have, more interesting pieces of Guam!