Thursday, 19 June 2014

The Great or Platonic Year

Recently I gave a list of things I might write about here, with an invitation to readers to pick a subject, or indeed suggest another. A day or two later I set a ten-question quiz.

Number of people sending in answers to quiz: 
Number of people suggesting blog subjects: 

That's not even two people, but one, in the dual capacities of subject-suggester and quiz-answerer. So I shall write about what she asks about, and one of the things she asks about is the ‘Great’ or ‘Platonic’ year.

As with most things called ‘Platonic’ the connection with Plato is remote or non-existent. Plato did, like many philosophers, entertain the idea of cycles of time: everything happening all over again hundreds or thousands of years later exactly as it happened before, or perhaps, as Marx said, first as tragedy and then as farce. But Plato’s astronomical knowledge or speculation has little to do with how the term ‘Platonic Year’ is used now, because this depends on that impressive-sounding thing the


Gosh. Don’t be frightened. The Earth goes round on its own axis once every twenty-four hours, and round the sun in a great big oval once every year. But as most people know, the axis from North to South Poles on which the Earth does its daily spin is slightly tilted, so that at different times of year we get more or less sun; more or fewer hours of daylight. We get seasons in fact. And on two days of the year — one in Spring, one in Autumn — it happens that the hours of daylight and the hours of night-time are the same. (Twelve hours each. (duh.)) These are the equinoxes.

The equinoxes ‘Precess’ — that is to say, they happen a little later each year. (Or is it a little earlier? I’ll come back to that, as they say.) So it really is true that Spring (and every other season) comes later and later year by year, but by an amount so small only an astronomer could measure it. (Astronomy might almost be defined as the science of the very very big or long and the very very small or short). But why do the equinoxes precess? Well, think of a child’s spinning-top: when you first set it going it looks quite stationary and stands up straight, spinning fast. But as it slows down a touch, it starts to wobble: looking from above, you see that the top of the top — the handle, if it’s one of those clever ones that work on a sort of barley-sugar pushy rod through the centre — is describing slow circles, which get bigger and bigger until the whole thing falls over.

The Earth is doing much the same. The axis is already tilted, and that axis is, ever so slowly, going round and round. The picture below might make this clearer. It was kindly supplied by NASA. (Kindly my arse: they want people to be interested in astronomy because then they’re less likely to complain when NASA spends a fortune sending a couple of golf-playing overgrown schoolboys to the moon.)


Oops! I’m not sure, but the picture seems to suggest that the precession is in the opposite direction to the Earth’s spin. Yes, I rather think it must be: watch what happens when a top finally falls over. So then the equinoxes occur that teensy bit earlier, not later, each year? I dunno. (Ignorance, madam; sheer ignorance.) But anyway I hope you get the idea.

Oh, yes: the time taken for the axis to complete a full circle is, then, the Great or Platonic year, and it’s about 26,000 ‘ordinary’ years long.

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