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Entry 423, on 2006-11-09 at 13:35:55 (Rating 1, Activities)

Today I observed a transit of Mercury. A transit occurs when an inferior planet (one closer to the Sun than Earth, ie Venus or Mercury) passes between the Sun and Earth. The planet is seen as a black dot crossing the surface of the Sun. These occur quite rarely because the tilt of the planets' orbits usually means they cross above or below the Sun's disk. I travelled to the center of Australia in 2004 to see a transit of Venus (see my blog entry for 08-06-2004) but this event was visible from New Zealand.

Yesterday we had some unusually bad weather for the beginning of summer (southern hemisphere) and the forecast was for more bad weather today which would clear during the afternoon. This would be after the transit had finished, of course. I wasn't confident of clear conditions so I was really pleased when my worst expectations turned out to be wrong and we had sunny, clear conditions.

In the photo I have included with this entry you can see Mercury (the small black dot above center near the right) crossing the Sun. The larger black object below it is a sunspot. There are also some smaller spots above center on the left. I took this photo with a 400mm lens on my Canon EOS350D digital SLR.

This photo really emphasises the scale of the Solar System. Even though Mercury is 58 million km closer to Earth than the Sun it is still tiny (its diameter is 4800 km) compared to the Sun (its diameter is 1.4 million km). Even a medium size sunspot, like the one in the photo, makes most planets look small.


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