Ex Die in Diem



So there are many questions to which I don’t know the answer. That’s a good thing, even if my job as a teacher somewhat revolves around being able to answer other people’s questions. If I had all the answers, not only would I be an even more frustrating person to talk to, I’d be incredibly bored all the time. Unanswered questions being a good thing, in many ways it was a blessing to find myself asking the same question for almost fifteen years without a satisfactory answer, even if those ways were not obvious to me as I asked it.

Why are there two tides a day?

It seems like an innocuous enough question. The tide comes in and out, then in and out again every day. This behaviour is actually not that common: the sun only does its trip across the sky once a day, ditto the moon. I sleep once a day, shower once a day, shave once a day, plants have pores which open and close once a day, lizards bask once a day: why do the tides feel they have to do their thing twice? Did they notice our teeth-brushing regimen and grow jealous? Have they somehow become confused by the twelve-hour clock?

Much as I’d love to, I can’t really satisfy myself with an answer that relies an the anthropomorphising of a natural phenomenon: there is presumably a better explanation, relying on physical principles and understood by some learned individual who might be convinced to share. So began my search. I asked all kinds of people, from teachers to sailors to those dwelling on islands separated by tidal causeways, each questionee with their own avenue for interest in this riddle. Each conversation followed a similar path:

Do you have any idea why there are two tides each day?

Well, the tides are caused by the moon pulling the water up, aren’t they? And then the earth spins around, and so from where we are, it looks like the tide comes in and out. You get spring tides when the moon and the sun are lined up, and neap tides when they work against each other.

Okay, but why two a day? You just described one in, one out each day. That’s not what happens.

You’re absolutely right. I guess I have no idea, in that case.

The thing that really gets me looking back is the way that people would describe spring and neap tides. Almost everybody I asked in that decade and a half had something to say about the tides getting bigger and smaller each month. I genuinely love that people were and are so keen to share what they know that they’ll bring in extraneous detail like that. Still, I didn’t get an answer.

It turns out that this is one of those problems that only just fits into your intuitive sense of how the world works. We’ve established so far that the tides are basically bulges in the seas and oceans that don’t turn with the earth, which explains the daily cycle in and out. All that’s missing is some sort of reason for there to be two bulges instead of one.

Rather than leaving you hanging, I’m going to invoke one of the triumvirate: Isaac Newton. Newton’s theory of universal gravitation is startlingly simple: everything (that has mass) attracts everything else (that has mass), with the attraction getting weaker the farther apart they are. To explain the tides, I want you to picture three things being attracted to the moon: the water on the moon side of the earth, the earth itself, and the water on the far side of the earth.

It seems obvious that the water on the moon side is closer to the moon than the earth is, and so is attracted more strongly: bulge number one. Hopefully you can also see the other half: the earth is closer to the moon than the water on the far side, so it is attracted more strongly, effectively being pulled out from under the oceans, and producing bulge number two. Add in some spinning, and we have successfully explained the two-tides-a-day situation. Congratulations.

As an interesting addendum, the tidal bulges drag behind the rotation of the earth a little, which produces a torque on the moon, pushing it further away from the earth at the expense of the earth’s rotational speed. That means that today was longer than yesterday but shorter than tomorrow. Pretty cool, no?

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