lördag 6 augusti 2011

Sun up/down at Davis and Mawson stations

In the "911! 8.5.11" video Ursu says that  the sun rises and sets at the wrong time at Davis and Mawson stations.

Now this is indeed easy to get wrong. I put up an explanation for Mawson yesterday that made sense for sunrise at Mawson but when I started doing the same for Davis and, for completeness sake, looked at the sunset it didn't fit.

The reason it's easy to get wrong is that we are dealing with both local time vs UTC and the problem of knowing exacly when sunrise and sunset occur. They are defined as "the times when the upper edge of the disk of the Sun is on the horizon" and in Antarctica in the winter the sun is just below the horizon for a very long time before it actually rises above, and in that time it still gives off enough indirect light to make the precise timing of the "horizon-breach" difficult to see (especially when the webcam doesn't particularly track the sunrise/set).

But what we can do, however, is measure the time between when it just about becomes bright and when it just about becomes dark and use that to find out at what time noon is.

First thing to know is that the stills at the respective site are timestamped in that stations local time (this also means that the easiest way to match sun up/down to the times given at timeanddate is to just look at the stills). However if you download that image you can see that the filename is the UTC time, so for Mawson the file might be called M1108061327s.jpg and show 6:27 pm (which is 18:27 mil time). So from that we can deduce that Mawson time is UTC+5h. Doing the same for Davis reveals a 7h difference.
The video time is also presented in UTC.

In these images from the Mawson video we see that the light starts to show at about 03:00UTC and is gone by about 13:00UTC (1:00pm) :

So the time span is 10h, indicating that noon should be at 10/2 + 03:00 = 08:00UTC
Since we know Mawson time is UTC+5, Mawson noon should occur at 8+5 = 13:00 local time and if we look at the timeanddate site for solar noon at Mawson it's 12:55 local time. Perfect fit.

The current video images from Davis makes it a bit harder to discern exact time, since it's clody right now, but there seems to be light between about 02:30UTC and 11:30UTC

The time span here is 9h, so noon should occur at 9/2 + 02:30 = 07:00UTC.
Add the seven hours UTC-local difference and you'll end up with 14:00 local time. Timeanddate says 13:55 .

So we see that there's nothing mysterious in the sunrise and sunset times on neither Davis nor Mawson.