Bad AU

Yesterday, the Open University's Stardate programme attempted to involve the British public in a large experiment to calculate the distance of the Sun by using times for third contact. Unfortunately, their final value was about 161 million km, or about 10 million kilometres too much - note that although the Sun's average distance is 149.6 million km, the Earth moves in an ellipse so is sometimes closer or further away.

So what caused the discrepancy? As the only distribution of times I saw was a very brief, poor resolution plot on the TV, it is difficult to say. The biggest problem may be that nobody had synchronised their clocks before taking measurements so people may have measured to the nearest second but that second could have been a minute or so out! Or as Brian Campbell points out in the StarDate forum, it might be because the TV viewers might have used the time for the Egyptian camera, although I think that is probably taken into account when you enter your numbers as it checks how you did the observation.

Another explanation could be due to different filters being used. We noticed yesterday that third and fourth contacts happened later in our H alpha telescope images than in images taken using a white light filter. This could also be seen in the TV images as third contact happened later on the Egyptian images (which looked like they were in H alpha) than Adam Hart-Davis (the presenter) was saying from the Royal Greenwich Observatory images.

The H alpha filter only lets through a very particular part of the spectrum - red light at a wavelength of 656.3 nanometres. At this colour, you actually see out into the chromosphere of the Sun which is about 2500km thick. As Venus is about 1/3 the distance to the Sun, this is equivalent to about 800km at Venus. The planet is about the same size as the Earth at 12400 km so this is about about 15.5 times less than the diameter. The time between third and fourth contacts was about 19 minutes so a 15.5th of that is about 1.2 minutes i.e. because you can see the chromosphere with the H alpha filter, you get an extra 1.2 minutes of transit! This agrees quite well with what we saw yesterday. Putting in a later time for third contact will cause the distance of the Sun to be artifically increased.

I wonder what filter the OU were using to measure the second contact with? If they used a H alpha filter they would start the transit an extra 1.2 minutes as well giving a possible increase of 2.4 minutes overall!

Posted in astro blog by Stuart on Wednesday 09th Jun 2004 (14:06 UTC) | Permalink
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