Tuesday, 27 June 2017

Tropic of Cancer

I might write about Henry Miller's shockin' and widely banned book Tropic of Cancer at some time, but not now. I am today writing about the line on the globe at 23°26′13.3″ N and its companion the Tropic of Capricorn down South. Why? Because I read recently that the names are a stonkin' great fib. Because the tropics are a movable feast both as to latitude and with respect to the 'fixed' stars. The first people to look up in the sky and pay attention noted that the sun doesn't rise to the same height through the year: it was higher and hotter in the Summer. During Winter in Norway and Alaska, the sun appears to have trouble with take-off and skims the horizon if it appears at all at all. About 2,000 years ago, this phenomenon was observed, tallied and quantified and named. When the sun was at its highest in and around the Mediterranean, the Sun was [just] in the constellation Cancer. And that was the label that appeared on maps and globes

That was a clever piece to deduction, if you think about it, because when you can see the sun you defo can't see the stars in any constellation. I know my star sign is Gemini, because I used to make Birth Charts. That's a convenient sign to be born under because it means that I can regularly view Castor, Pollux and the other Geminian stars in December when star-gazing doesn't require being out after my bed-time. Just like some of my best friends are Jewish, many of them are woowah, tree-hugging alternative types. They are not above saying "Oh, you're Libra, that's why you are so balanced" OR "Aha, Virgo, no wonder you don't have children". Labelling which is somehow acceptable while "Black people are natural runners" or "Fat people are [un]happy" are unspeakable. I really have no idea when the other Zodiacal signs fall during the year and I never read horoscopes in the newspaper - I avert my eyes lest my thinking mind is soiled. Racism is waaaay better than astrology at binning people into groups. [aside Dara O'Briain et al. on Private Browsing]

Well it turns out that when the Sun tips the Summer Solstice nowadays it is in the constellation Taurus having passed through the whole of Gemini since Hipparcus of Rhodes first deduced the precession of the equinoxes. It is hard enough to reconcile the calendar with the tropical year because the day is not an exact multiple of the year and we have to fix the discrepancy with leap days every 4th February . . . except when we don't. And I've attempted to explain perihelion before. The Tropical Year is the length of time between Summer [or Winter] equinoxes. The Tropic of Cancer's shift to Taurus is caused by changes in the Sidereal Year - the position of the sun w.r.t. the fixed stars. Ptolemy, building on the earlier calculations of Hipparcus, worked out that every 100 years things slip about 1° but his time-scale baseline was too short and his estimation of how long the cycle of precession lasts was too long. Consensus now is that it will take ~25800 years to get back to the state things were when Hipparcus was still walking and disputing.

You should also note that the super-accurate 23°26′13.3″ N is slipping towards the equator at a rate of 15m every year so that 13.3" will be 12.8" next year.

The 1970s hit hippie musical Hair sang ensemble "This is the dawning of the Age of Aquarius" but nobody can agree when our solar system will move into that constellation. Indeed it may not have arrived yet, which is s good thing because then we'd be all wet.

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