Tonga,  VaVa'U

How we Choose to Waste our Time

You may be wondering what we do with our time? We post these blogs, and in them we tell tales of gallant exploration of far-flung paradisaical islands, exciting hitherto-unknown life forms, life-threatening nautical exploits and of course haughty camaraderie with fellow adventurers. But what about the rest of the time?

Well, the rest is fairly uninspiring to be honest. We spend our time much as I hope retirement will pan out if we ever get around to making any money, ‘providing’ for our futures and having the luxury of retiring properly, at the appropriate age, as tradition dictates.

We get up every day at about 8:30am, sit around for a bit, have some breakfast, sit around some more, check the weather forecast if one is available, decide what we’d like to do that day and then come up with a plan for doing those things. We sleep for about 9 or 10 hours every night, watch films, TV shows or play cards in the evenings and talk a lot of gumpf about politics (well, Sarah talks and I say ‘ungh’ every now and then), the merits of various anchor designs (now our roles are reversed), rant about the injustices of the world, or we talk about science. Science is a popular topic on Bob, since we’re both verified geeks. Last night’s topic of conversation is one that we both found particularly stimulating, and which I thought I might share with you all here for the simple reason that I think it’s pretty cool.

As we were wandering back from town yesterday afternoon we were contemplating the laws of physics, when one of us mentioned that by the time we arrived back at the beach we could expect the tide to have risen. Then a question popped into my head. We know that the tides are caused (mainly) by the gravitational pull of the moon pulling water one way or another across the surface of the Earth. And, we have learned from Isaac Newton that energy cannot be created or destroyed (yes yes, I know it really can be, but for all practical purposes let’s stick with classical Newtonian Physics). We also know that in order for stuff to be accelerated, and moved from one place to another (work done) requires an input of energy. Where, therefore, does the energy necessary to move all those millions of tons of water every day come from? Sarah got the answer pretty quickly, but I wasn’t convinced. She thought it must come from the Earth itself. Or rather, the momentum of the Earth spinning on it’s own axis (to simplify things). But surely, if the energy is coming from the rotation of the Earth then the Earth must be slowing down? Rapidly. After all, there’s a lot of ocean that needs to be moved about twice every day.

According to our (very) rough calculations: Let’s assume that 70% of the surface of the Earth is covered by oceans, and that the average oceanic tidal range is 0.6m. That corresponds to a total of about 440 trillion tons of water moved every day. That’s a really big number! But is it? Well, a quick Google search (we can do Google on board now, for as long as we’re in Tonga) told us that the mass of the Earth is a whopping 5.927 x 1024Kg (that is, 5,927 with 21 zeros after it!). So the total tidal movement of water, daily, is only 0.00000742 percent of the mass of the Earth. Not so much after all.* In fact, another Google search told us that the Earth is slowing down, as Sarah’s hypothesis would demand. When the Earth and moon were relatively new 4.5 billion years ago each day (each rotation of the earth) lasted only 5 hours. Our days are getting longer. 2 milliseconds every 100 years to be precise. Furthermore, we are losing the moon, which is being pushed out from it’s orbit at the rate of a few centimetres per year. Sooner or later it will break free from the clutches of it’s possessor and spin off all by itself into space.

So, that’s how we sometimes spend our time. Not much use to anyone, but occasionally kind of cool nonetheless 🙂

Finally, since this post might be a touch tedious without the addition of some aesthetically-pleasing visual stimulus, here’s a lovely picture of the moon that Sarah snapped a few days ago, complete with well-timed bat flying across it:

* Most of the energy is in fact conserved. The water doesn’t get accelerated and displaced in one direction and then accelerated and displaced in another. If it did, we’d slow down by about 24 hours per year according to our calculations. I’m guessing the much smaller figure of 2ms every 100 years might be because the water actually moves as a continuous tidal wave around the planet. The kinetic energy is conserved. Hence, the only energy losses are friction losses in the form of heat. Alternative views, musings and rebuttals are welcome!


  • Brooksie

    Well! I wish I was there to add to this conversation, which although stimulating, is, in my ‘umble opinion, flawed in that it does not take into account life, the universe and everything, whence the answer is simple – 42! That said, I do think that you are restricting your thinking by not taking into account the greater panoply – more than just the solar system itself – dark matter? Hmmm – wadderyerthink? Although I cannot fault Sarah’s thinking, I opine that the cause of the earth slowing is due to a plethora of factors, not least of which is gravity – enter Einstein. He would have a thing or two to say over a glass of schnapps in Bob!

    Love to you both


  • Stephen brooks

    An interesting read, but i dont understand a word of it! 😊
    My understanding was, as the moon passes over a large body of water, the gravitational pull causes the water to rise slightly (upwards)! And the tide goes out, as the moon moves away, the water goes down to its original level, and the tide comes back in!

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