Winter is coming…

A yearly sunset in a sight to be behold. Watching the sun gradually deflate in its gently spiralling arc towards the horizon, turning the sky from a light radiant blue, through the palette of pastel colours, back to a monochromatic diffuse deeper shade of indigo. This primed the conflict in our minds of what the excitement of what a true Antarctic experience would bring, versus what the six long months without sunlight would do to our mind and spirits.

During life at more temperate latitudes, I always rued the onset of winter, where the sun would be yet to rise on the way to work, and down before I could home to enjoy it. The melancholy that that time of the year ushered in was slightly offset by the winter and snow sports, but the return of long days, warm weather and all the activities that returned with it, provided the solace of dealing with a standard winter.

Here, life is on a much more profound scale. During the weeks of February and March, we watched the temperature drop by up to a degree a day as the suns angle of inclination dropped through the single digits, meaning a cross section of its rays were spread across an ever increasing area of ground, and with that dissipation, the warmth of the sun disappeared. At its highest point on the summer solstice in late December, the sun reaches 23.5°, which correlates to the angle of the earth varies in, compared to the ecliptic plane of orbit around the sun.

As we watched our shadows get longer and longer, the sky transitioned through shades of yellow, orange and red. Maybe one day, down here will become a lovers retreat, where you can amorously watch the sunset for weeks on end, rather than the minutes you get elsewhere.

The exact time of the sun setting is a variable entity and dependent on a range of different variables, as the resident weather experts informed us. Being on the very bottom of the planet, you would assume the sun sets on the equinox. But that assumes a perfectly spherical earth, no atmospheric impact, that you are right on the surface rather than any elevation, and the measure is of when half of the disc of the sun is below the horizon.

But what we consider to be the ‘sunset’ is when the last of the visible rays of light disappear, rather than the technical reference being just when half is visible. The sun depresses at just under one arc-minute (a measure of angular distance in the sky) per hour, and the sun is around 30 arc minutes in diameter, so it takes 15-16 further hours for the top half of the sun to disappear.

Then there is the issue of atmospheric distortion. Light hitting a more dense substance is bent towards the line perpendicular to the surface of the two different substances. It is for the same reason that were you to try and spear a fish in the water, unless directly below you, you are likely to hit a spot above it. As a result, light is bent around the curve of the earth by the atmosphere. Whilst the atmosphere here is a lot thinner than at sea level, the cold increases its density, meaning it has roughly the same density as at sea level.
When we watch the sun setting, whilst sitting on a beach back home, by the time our view of the sun first reaching the horizon, it in fact has already completely set, but we are just seeing light bent from over the horizon. The refractive effect is approximately 34 degrees, meaning that here, we get roughly another 36 hours of visible sun, after it has passed below the horizon.

Then there is the issue of elevation. The higher your vantage point, the further the visible horizon stretches. From our vantage point in the main elevated station, we can see approximately 7 miles further compared to at ground level. For each mile equates to around a further arc-minute of visibility, and therefore an extra hour of sun.

With all of these calculations and permutations streaming through our consciousness, everyone was keenly (with a dose of trepidation) awaiting the final glimpses of sunlight, and the twilight that would shepherd in winter. For those that had been there since the end of October, it was to be the first sunset in 5 months. Low and behold, the wind shifted to Grid North (heading from the warmer moister Weddell Sea region, rather than the drier colder East Antarctic plateau). Misty overcast skies mixed with windblown ice crystals to reduce the visibility to a mile or so, and ruin the views we were anticipating of the sky transitioning through its pastel spectrum.

A despondent group of Polies moped around, disappointed the chance to see this sentinel waypoint in the year looked likely to pass without our observation. But late on the 23rd, the skies cleared, and the slim sliver of the sun beamed its radiant beams around the horizon to us to everyones delight.

One of the idiosyncrasies of the setting sun is the transition in its colours. Just as the sky is blue because the blue light is scattered by the atmosphere, allowing other wavelengths to transit, as the sun sets, the sunlight passes through more and more atmosphere given its low angle of incidence. This refracts out more and more wavelengths of light, giving it a more red appearance. It is for this same reason, that the setting sun in dusty dry environments such as the subsaharan African dry season, or a smog-ridden city has a deep red appearance, due to particulates in the atmosphere affecting shorter wavelengths.

As the sun departs our line of sight and heads over the horizon, the light is bent by the atmosphere, with the red light being bent less than green and blue light. But at a certain point, the green light at the top of the sun is bent just the right about to pass to the observers position, whilst the green light of the body of the sun is bent and falls short, but red light from the body of the sun still makes it to the observer. Therefore, the elusive green flash can be seen as a green sparkle on the top of the setting sun. Typically at temperate latitudes, this happens in the passing of a few seconds, so is difficult to see, and even harder to photograph. Luckily for us, the equivalent of a few seconds of the suns path equates to minutes here, so we all revelled in peering through telescopes, cameras and binoculars to see what looked like a stream of green ants marching back and forward on the top of the sun. At times we even saw a blue flash, as the air here is some of the cleanest in the world, so even less particulates to refract short wavelength light away. This was a pretty magical evening for the crew, with a papparazzi line up of cameras busily snapping away well into the night.

This evening doubled as also being the dying minutes of my 20s. Come midnight, the last remainders of the sunset watchers sang me the traditional happy birthday ballad, as well as a more disturbing song about getting old, dying and being forgotten. At least I won’t forget what was a special way to bring in my 30s. Supposed to be the best decade right?

The clear weather also luckily were coupled with low winds, so a couple of us took up the offer to climb the Atmospherical Research Observatory’s (ARO) 50m tall meteorological tower to gain a new perspective on our barren world. From the top of the tower, on one side was the red/yellow glow of the final remnants of the setting sun. On the other, in a shroud in a pink hue, fell the shadow of the earth, from which we watched the full moon rise, reminding us that night was here, and the long winter was coming…

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2 Responses to Winter is coming…

  1. Peter Hicks says:

    Loving all the science and the excellent commentary. You have a real gift for writing. However you are wrong in one respect. Surprisingly your 50’s can be pretty fantastic.

  2. Tiff says:

    Winter is coming…….. and yes your 30’s are great because you are old enough but not old enough to know better😉

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