Sep 23, 2017

All hail the sun, whether yours is coming or going

Some idle thoughts about the southern spring equinox

Ben Sandilands — Editor of Plane Talking

Ben Sandilands

Editor of Plane Talking

South Pole, ‘spring equinox’, this morning

And now for something completely different

At 6.02 am this September 23 in eastern Australia, it was the southern spring equinox. The calculated moment when the days become longer than the nights for the next three months, while at the same instant, 10.02 pm Greenwich mean time on September 22 in say London, it marked the autumn equinox, and the similarly long march downslope toward longer nights than days.

The science class texts made it all seem so simple back at school, as did a number or morning radio and TV programs a few hours ago.  The March and September equinoxs are the moment when the sun will rise for the only time in a year at one pole, and simultaneously set, for the only time in 2017 at the opposite pole.

But the detail doesn’t quite match up with the broad brush of ‘simplified’ science.  The days in the southern world have been a bit longer than the nights for a day or more because the moment of sunrise is officially calculated as being when the mid point of the solar disc lines up with the horizon, in air that has an assumed pressure, is at sea level, and at a temperature which is too cool to be common place in many equatorial or high tropical climes, or impossibly hot in say, deepest and highest Antarctica.  Air pressure, temperature and altitude all distort the stability of an image of an object near or on an horizon. At the south pole this morning it was -70C, at an altitude of 2835 metres, not +15C at zero metres as assumed by the sunrise and sunset gurus in their presumably comfortably climate controlled laboratories.

The uppermost part of the solar disc actually rose over the South Pole at 4.53 am on September 21, New Zealand time, because the good people at the Amundsen-Scott base keep NZ time, as does the science community a long way to their north at McMurdo Sound.

In fact the South Pole scarcely ever gets truly dark because that pedantically defined centre point of the solar disc will never tilt lower than 23.5 degrees below the horizon from that location and it has to have sunk to 18 degrees below that line before ‘astronomical twilight’ in which the light of the sun can still be detected as a faint sky glow, is said to no longer have any affect.

That doesn’t leave much time for utterly truly, pitch dark skies at the poles considering that for the six months during which the sun isn’t above the horizon the moon will be, and for about two weeks out of every four.

Which is why those huge telescopes being built in Chile, and other telescope colonies on places like Mauna Kea in Hawaii, or Siding Spring Mountain in NSW, can enjoy in theory longer periods of much darker skies than the poles.

In fact somewhere on the Equator would be perfect for deep sky astronomy, if not occupied by millions of fun loving neon drenched citizens as in Singapore, since every night the sun would be somewhere like 89 degrees below the horizon. For a fleeting moment of course.

Anyhow, there are more pressing issues than the almost limitless opportunities for pedantry that attend such seemingly straightforward matters as the march of the seasons (which never keeps in step in Australia anyhow). One is that a grass fire will incinerate the incredibly dry pastures of the highlands that surround this reporter and his community at this hour. Plans have been made, escape routes are constantly updated, and just about everyone knows what each person can or will do.

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5 thoughts on “All hail the sun, whether yours is coming or going

  1. Dan Dair

    “escape routes are constantly updated, and just about everyone knows what each person can or will do”

    Asbestos Korf-Balls, maybe.?

  2. comet

    Fires: Be careful. Grassfires usually travel faster than bushfires, and can travel up a slope faster than most people can run.

    The seasons: It’s strange that in Australia the seasons are said to change on the calendar month. The calendar was devised by Pope Gregory – who wasn’t very good at maths – and has nothing to do with the seasons.

    1. Dan Dair

      Which was my point about asbestos Korf-balls.!
      Only good for downhill, fast.
      & asbestos, so fire-resistant.!
      Roll through it to safety instead of trying to run away from it uphill.?

      Hmm…. I might patent it.?
      Or at least e-mail Mr & Mrs Korf to see if they patented it before me.?

    2. Dan Dair

      “Fires: Be careful”

      Can’t fault you there mate.! Always a good bit of advice.!!!

  3. Tango

    Well it hard foze on top of the glass tbel out back yesterday and it was 27 this AM

    They call it fall, we call it early winter. Fall lasts about two weeks.

    Kind of arificiail me thinks.

    My brother got a test qeusion wrong about Seond grade, what season is it (this was November) .

    He looks out the window, 2.5 feet of snow on the ground, think on what it was temp wise (-40) going to school, yep, its dark and its 10 am, Winter!

    Quite a battle ensued between the parents (come on lets be real) teacher, no, per all agrreemtns its is fall. My folks were not ones to knock down teachers but they sure went to bat.

    Finally having lost, they had a talk with my brother. Technically she is right, realistically she is totally wrong and as you go through life you are going to run into those situations. You are right, but we can’t win this one but we agree with you.

    Don’t ever be afraid to stand up for what you think is right, its those who don’t who are wrong.

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