If you don’t live on the coast – or if you do, but don’t actually spend a great deal of time on the beach or in the salt marsh – you probably don’t give much thought to tides. I certainly didn’t before I moved here. I knew they were caused by the gravitational pull of the moon, but I didn’t know the specifics of how it works.
Now, of course, tides play a role in my daily life. If I’m teaching a salt marsh ecology class, what the tide is doing affects where I can take my students (and how wet we’ll get getting there). Last Friday, right around the new moon, I forgot to check the tide schedule ahead of time and arrived in the marsh with a class to discover that pretty much the entire thing was under water. Jekyll Island really looked like an island for once! (Normally the marsh separating us from the mainland looks mostly like a grassy plain.)
I was, of course, witnessing a spring tide. Turns out the moon isn’t the only thing that affects how the water comes and goes: while the moon is the major player, the sun has a role as well.
Twice a month, at the new moon and the full moon, we experience what are called spring tides, times when the high tides are extremely high and the low tides unusually low. This happens because the sun and the moon are lined up and working together, with the sun augmenting the normal tug of the much closer moon.
The pull of the moon (combined, here, with the sun) causes a bulge of water on the side of the Earth facing the moon, and because the Earth is spinning we also get a corresponding bulge on the opposite side – don’t ask me to explain why exactly, it involves more complicated physics than what little I can recall from my junior year of high school. In any case, you can see clearly why we have two high tides and two low tides every day.
About a week later, we’re now at the first quarter of the lunar cycle, a.k.a. a half moon. This means that instead of spring tides we’re getting on toward neap tides.
Now the sun and the moon are working against each other. The moon wins, because it’s much, much closer, but the sun still has enough effect to smooth out those bulges quite a bit. At this time of the month the difference between low and high tide is not nearly as great. Between this and the fact that some of my favorite night walk activities to do with kids don’t work so well in the glare of the full moon, the lunar calendar has been on my mind a lot lately! Hope someone out there finds this interesting.