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Journal

No 58 - December 2008

Total eclipse of the sun on 1st August 2008

Earlier this year I travelled to China to see the total eclipse of the sun from a site in the Gobi desert. It was my first total eclipse. I travelled all that way because, apart from visiting China for the first time, the site had the best chance of being cloud free over the whole path of the eclipse. The path stretched from the Canadian Arctic Islands, over Greenland, the Arctic Ocean, through Siberia and into China.

The point of longest eclipse was a remote spot in northern Siberia. There totality could be experienced for 2 minutes and 27 seconds. At our spot in the Gobi desert totality was estimated to last 1 minute and 54 seconds.

On 31st July we arrived in Jiuquan - our base for the eclipse. This city is situated on the edge of the Gobi and is famous as a tourist spot because it is at the western end of the Great Wall. That evening we enjoyed an open air 'Total Eclipse' concert laid on by the local Council which, I reckon, five thousand people attended.

outer corona

Totality could be experienced at the city but as it was on the edge of the track of the umbra this would only be for a few seconds. On the morning of the 1st the assembled UK groups - totalling three hundred people - were given a presentation on the eclipse including tips on photography. After the presentation and lunch we were driven fifty kilometres, in a convoy of coaches, across the desert to the exact point where the middle of the totality track crossed the road. There we were joined by parties from Holland and people from all over China.

Our viewing spot was at 40°18' N and 99°30' E. First contact would be at 18:16 local time, second contact at 19:13, third at 19:15 and fourth contact (when the moon finally moved off the sun's disk) at 20:08. We were going to be looking at an evening eclipse.

The viewing place was very scenic. It was stony desert, with a few low hills in the middle distance, at an altitude of 4,000 feet. This was unusual enough for someone from Scotland - but, in the further distance was the Qilian Shan, a mighty rampart of snow covered mountains with glaciers. The highest peaks reach 19,000 feet (5,800 metres).

We arrived about an hour before first contact, which gave us enough time to get our equipment set up. This was achieved without any hitches probably because of the many rehearsals in my back garden in Edinburgh. I had the specific objective of imaging the outer corona of the sun and had assembled the gear to do that. I would be using a Canon DSLR, a 400 mm Canon lens and a Mylar filter. The Mylar filter was for imaging the sun outside totality. It would not be required during totality. The trickiest part of the whole exercise was to remove the filter at the onset of totality and putting it back at the end of totality. This had to be done quickly but without disturbing the system so that the sun remained in focus. There would be no time to adjust the focus during totality. In any case I just wanted to actually watch the eclipse while doing the images with a remote switch. This was possible because in the short time the image of the eclipsed sun would remain within the field of view of the camera. I found that my settings did capture structure in the outer camera but the inner corona was overexposed. I could see why people used two cameras!

author and camera
The author and equipment.

I took the advice of veterans and spent some time during the hour of preparation to make sure that the camera and lens were exactly focussed on infinity. You cannot assume that the infinity mark on the lens is precisely right because the setting is affected by temperature (expansion or contraction of the lens and lens mounting). Otherwise conditions for observing at our site were very good. No obstructions, a gentle breeze and little turbulence.

Up to totality the sun was successfully imaged with settings of f 5.4, ISO 200 and an exposure of 1/1200 second. During this phase of the eclipse we were considerably worried by the presence of some cumulus clouds. Fortunately the clouds, while still around, never interfered with observations. During totality the exposure was switched to 1 second. This was not satisfactory for imaging the outer corona. A 0.25 second exposure was better. In all 16 quality images were taken during totality, which was a good outcome. These will end up in the database of the BAA's Solar Section.

It was an exciting experience just watching the eclipse. In the half hour before totality the temperature noticeably cooled. The sun could not be observed with the naked eye until the onset of totality itself. The cover image gives an idea of what the scene looked like to the naked eye. In addition to the eclipse sun you could see Venus and Mercury and the edge of the umbra in the distance.

The total eclipse was over all too quickly. We retired to a picnic in the desert while the coaches started taking people back to Jiuquan. We thought that this pleasant moment had gone on too long when we found ourselves on the last bus that would only get back at midnight. Yet, when it got dark, we were still driving across the desert under a clear sky. We persuaded the driver to stop and had a wonderful look at the constellations from a dark sky site at an unfamiliar latitude. It was nice to see Scorpius and the centre of the Milky Way in Sagittarius.

Des Loughney


Contents

Cover page

IYA astro-happening at Science Festival

Johann von Lamont - Bavaria's Scottish Astronomer Royal

Never the TWAN shall meet?

Total eclipse of the sun on 1st August 2008

Forthcoming events

Exoplanets come into view

Recent observations

Variable star observing workshop

Society news

About the ASE Journal


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