Antarctica

Week 23 Going... Going... Gone. Back Again! - Sunset

19th March - 26th March 2000

Monday : There is a lot of debate about when the sun is going to set. To me the sun sets when you can't see it anymore. But we have times for when the middle of the sun is on the horizon - for when it will go below the horizon, but without correction for atmospheric affects - and guesses. The rotation of the earth is of course 360 degrees a day. Another way of looking at it is that we see things not on the earth move 360 degrees a day - which is 15 degrees an hours - which is 1 degree every 4 minutes. So if you watch the sun set closer to the equator it doesn't matter much if you have the suns position off by a degree or two. Sunset to within 10 minutes is fine. But here where the suns motion is almost all horizontal to the horizon, it makes a big difference. But since it's very overcast today, with blowing snow, it's looking like we won't see the sun set anyway.

Tuesday : The sky has cleared up!!! There is still quite a bit a blowing snow which obscures the exact horizon. But we can still see the sun through it quite clearly. I go over to Mapo to work, and the sun's bottom limb has just gone bellow the horizon.

In most places around the world if you measure the temperature of the air as you move away from the ground (well, lets call it surface) the temperature decreases. This is because the earth re-radiates the energy absorbed by the sun, and the higher you get the further you are away from this heat source. But under some condition there can be a cooling processes at the surface, such as over water and ice, and the temperature increases away from the surface. This is called a temperature inversion, and is quite common here. Combined with the effects of gravity, the air is denser at the surface. This seems fairly obvious, but it is slightly more so here, especially just above the ice.

Another way of thinking about this is that if you are looking at the horizon, just above the horizon, you are looking through more air, and the higher above the horizon you look the less air you are looking through. It's like having a prism on the horizon, fat at the bottom, the at the top. Light going through a prism get bent. The result of our air prism is that we can see below the horizon. This makes it difficult to estimate when the sun will set, as the amount we can see below the horizon changes, and depends on things we can't measure. Another effect of a prism is that that white light gets separated into it's rainbow components. And it just happens from where we sit, if we think of the sun setting, and having it's colours slightly separated, the last colour we should see is green (the blue light gets scattered in the air before it reaches us). This is called the green flash. But the atmosphere isn't nice and even. Air tends to form layers, which don't mix well in the vertical direction (unless you get heat driven convection). So when we look at the horizon we are likely to be looking through slabs of different density air. This distorts the image of the sun into all sort of weird shapes, and is called the lantern effect. The air is also always moving, so the image of the sun is always changing right on the horizon.

At around mid-day the sun was getting very low on the horizon. We could only see the upper limb, and this was all distorted into a flat disk shape with jagged edges. Every so often bubbles would come off the top before popping into nothing. As the sun got very low, so that it nothing more than a line on the horizon, small dots of green appeared above the sun blinking like a traffic light, until finally the sun disappeared. Zach and I had a beer to celebrate the start of winter, only to the see the sun rise again slightly as it went past the very slight rise in the horizon. Now we know - the horizon isn't quite as flat as it looks!!

That night there is a sunset party in sky lab. But the wind has picked up considerably. Even though the sky is quite clear, the horizon is completely obscured by blowing snow. Even if the sun was up, we weren't going to see it. So that is it for the sun. We sit around and have a few quite drinks.

Wednesday : I have to go out to Mapo early in the morning. The front doors are still shut because of a bit of wind. When I open the small door I'm surprised how light it is. When I walk to the top of the hill I'm even more surprised to find the sun shining quite brightly - well it's there!!! I work at Mapo most of this day. Every 10 minutes I have to get up and look at what the sun is doing. Because the horizon is clear of blowing snow I could see the shapes the sun was being distorted into quite clearly. With binoculars I could see it changing very quickly - growing taller - being sliced by dark lines - the edges all jagged - and every so often a line or bubble of green on top.

While the sun was being spectacular in one direction, the very full moon was sitting just above the earths shadow, above the horizon, directly opposite the sun. Having had nothing but a bright blue sky all summer (except when it's cloudy!), it was great to have a sky with colour - pastel colours all around the horizon, and deep greyish colours high in the sky. The light from the sky also makes the snow ice surface look completely different. It becomes a deep steel grey, but the sastrugi look white against the snow. Looking out at the moon, I could easily imagine the surface of the moon looking like the plateau at this moment.

That night there are more people in Sky Lab watching the sun. Having disappeared once, I think everyone realised - it ain't coming back, and wanted to watch it one more time. Having had to the sun all summer, we suddenly become sun worshipers, sitting in chairs staring at it. The green on the very upper edge is the brightest I've seen it so far - bars of it appearing every so often. Even blues are visible. With the telescope it's an amazing sight. While the air looks fairly clear the presence of ice crystal is obvious from the faint sun pillar that extends a fair distance into the sky. I want to stay and watch the sun set once and for all, but by 10 o'clock I can't keep my eyes open any longer.

Friday : A distraction from the sunset. We play radio dart with MCM and Scott base. This is done by getting a team together, somewhere that an HF radio can be used, and radioing scores back and forth. We have to play a simple game, like 301, as it would take forever otherwise. But 301 is one of my least favorite darts games. To start you have to hit a double, and to finish it must be a double. And I'm terrible at hitting doubles. I also don't like playing doubles because it so long between throws, and radio darts is much worse. At pole we have enough people to have 2 teams. The first game is terrible, with SP1, and SP2, taking forever to double in. SP2 (I'm on SP1) finally wins the third game. But I've been throwing so badly I don't want to hang around for another. At the last moment I decide to stay. We start okay, but so does everyone else. No one is throwing that good, with a high score of 81 so far. Then I hit a high point and throw 85 twice in a row. On my next go we need 10 to win. With my first dart I hit double 5. Worth staying for!!

Saturday : It's a wonderful evening outside today. The wedge of the earths shadow is a couple of degrees above the horizon, and above the deep dark grey is a band of orange. The moon is getting quite high in the sky, and the position of the sun is marked by a light coloured patch. The rest of the sky is a deep purple.

Sunday : Is it cold today! I have to go out and fix a problem at SPASE, and it's -71degC!! My goggles fog up on the way back, so have I to lift them. This means I have to walk backward most of the way to the dome, or have the wind into my face. As it is my eyes keep freezing shut. But what is that on the horizon? The sun again!!! But even with the sun making a faint appearance on the horizon, the sky is looking dark enough to see stars. I keep looking, expecting to see one any minute.

To be continued .......

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