Equinox

Last Friday (for most parts of the world – Saturday for us ‘advanced’ people in New Zealand) was the equinox. Loosely described, that’s when the day and night are of equal length – from this moment on and for the next six months all us southern hemispherers will be experiencing longer periods of darkness than daylight as we head into winter, while the northern hemisphere types get to laugh at us while they cruise into summer. Come 22 September (or 23 for us awkward people at the leading extremity of the time zones), we’ll swap round again.

Did you spot the deliberate mistake? I said it is six months between the two equinoxes. That’s approximately true, but not exactly. Count up the days, you’ll get 186 from the March date  to the September date, but just 179 from the September equinox to the March one.  Put it another way, us unfortunate southern hemispherers have to endure an entire week more ‘winter’ than do our friends in the north. (Northerners would say it’s our punishment for hogging all the best features of the night sky, but that’s another subject.)

 

So why is this? It’s the same reason as a talked about earlier regarding the analemma – the earth does not orbit the sun in a circle. It’s not far off a circle (about 3% difference between its closest and furthest distances from the sun) but the discrepancy is big enough to make a noticeable difference, and the length of the seasons is one of them. The earth reaches perihelion (its closest  to the sun) early January (4th this year). At this point, it is moving its fastest (Kepler’s 2nd law). Contrast that to early July, when it is moving slowest.

This means our planet skips round from the September to March equinoxes that bit quicker than the March to September case, and, as such, us kiwis get a raw deal. But so do the Australians, so at least we can be happy about that.

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