June 3, 2012

A Little Black Dot

I don't actually have a picture for this post. I am instead going to write a bit about the events of tomorrow, June 5th. As I wrote yesterday, there happens to be a transit of Venus occurring this tuesday. This is one of the rarer astronomical events that we can easily predict and observe.

A transit is very simple. As seen from earth, the planet Venus passes directly in front of the sun. We see a small black dot which slowly moves across the face of the sun. It is very much like a very tiny solar eclipse. During the 2004 transit, Venus was in front of the sun for around 6 hours, and the same will be true this time around.

The transits happen on a 243 year cycle with pairs of transits 8 years apart, separated by first 121, then 105 years. The first transit to be predicted was the 1631 transit. Kepler predicted it in 1927, but his calculations were not sufficiently accurate to determine that it would not be visible from Europe. As such, there were no observations made during that transit. He had predicted that in 1639 there would be a near miss and not an actual transit.

Horrocks fiddled with the calculations and deduced that there would in fact be a 1639 transit. He and his friend Crabtree both observed the transit, but due to clouds and sunset, they were only able to observe a very short part of the transit.

The next pair of transits was 1761 and 1769. For these transits, observers were sent all across the world. The reason for this was that Halley had predicted that if you observed the transits from widely spaced points on the earth, one could use parallax to determine the distance from the earth to the sun. The earth is large enough that Venus takes a different path across the sun depending on where you observe from.

At the time, they had calculated relative distances to the planets, but they did not have a baseline to determine actual hard distances. The distance they came up with was 153,000,000km plus or minus 1,000,000. That is very close to the currently accepted 149,600,000km. During the pair of transits in 1874 and 1882, the distance to the sun was refined to 149,590,000km plus or minus 310,000km.

Anyhow, the transit on tuesday kicks off at around 3pm and goes on until sunset here in Victoria. If anyone is in Victoria there are many options if you would like to take a look at the transit yourself. The Royal Astronomical Society of Canada is putting on several observing sites around the city, the DAO out on Observatory Hill is having an open house and finally UVic is putting on a big open house on the 5th floor of the Bob Wright Building.

We will have welders glass and solar sunglasses to observe unaided, as well as several projection telescopes to look at projected image of the sun. There will also be several solar telescopes to look directly at the sun. We will also have our 32 inch telescope running looking at stars and other planets which we can see during the daytime. If the weather fails we will also have our projector set up to watch feeds of the transit from somewhere sunny...

Cross your fingers for clear skies!

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