Solar Spotters 
The last transit of Venus across the Sun for more than a century has come and gone, but it’s not forgotten. A team of astronomers will be studying yesterday’s transit for a while in an effort to get a more precise measurement of the size of the Sun.
The Sun is about 865,000 miles in diameter, give or take; different techniques and instruments give measurements that vary by hundreds of miles.
Part of the problem is that the Sun is a ball of gas without a solid surface, so its “edge” is a bit fuzzy. And part is just that different instruments can see things a little differently from each other.
So in the last decade, astronomers from the University of Hawaii and Stanford tried something new. They used a Sun-watching satellite to plot two transits of the planet Mercury across the face of the Sun. Watching the transits from space eliminated the blurring effects of Earth’s atmosphere, making it possible to precisely time when Mercury first and last appeared to touch the Sun.
The astronomers knew how fast Mercury orbits the Sun, so timing its passage across the Sun yielded a new measurement of the Sun’s diameter, which was released earlier this year: 865,374 miles, with, the astronomers say, an error of no more than 40 miles.
Yet even that isn’t good enough, so the astronomers hope to refine it with observations of yesterday’s transit of Venus — a dark spot shedding a little light on the Sun.
Script by Damond Benningfield, Copyright 2012