On 2012 August 31, the IAU resolved to define the AU as exactly 149,597,870,700 meters. Of course, they didn’t just pull that value out of a hat. I take it as a statement that measurements have converged enough that any variation is in the noise.
When you see such a precise value being entered into reference books, it’s easy to forget that 250 years ago, determining the AU was goal that triggered a massive international effort. Andrea Wulf, in her wonderful book Chasing Venus, tells some of the incredibly complex history of the 1761/1769 Transit of Venus expeditions.
There was some scientific interest in the 2012 Transit of Venus, but it certainly wasn’t in determining the value of the AU. Despite that, there are still good reasons to calculate the astronomical unit from measurements from June’s Transit. There’s tremendous education value in reducing the data. The math is straightforward, and crunching the numbers doesn’t require the head-wrecking tedium endured by human computers in the 18th and 19th centuries. Working through the process is the entire point.
This image, from an article in Universe Today, gives you an idea just how fiddly the AU calculation would be. This composite shows the effects of a large baseline, and yet the parallax is still small. Thanks to modern GPS receivers, the station positions can be measured precisely. With exquisite optics so easily available, none of us had to suffer through the black drop nightmares that plagued explorers like Captain James Cook. Even so, there’s still some uncertainty in calling the exact moments of Second and Third Contacts.
A recently published article describes the efforts of Team Hetu’u to use the 2012 Transit of Venus results to work out the Earth’s distance from the Sun. (“Hetu’u” is the rapanui word for “star,” a rather appropriate name for an effort centered on Easter Island.) A world-wide group of students sifted through the data, crunched the numbers and came up with the value of 152 million kilometers, ±30 million kilometers. Compared to the high-precision value adopted by the IAU, this might not seem very impressive. But, I’ll point out that the now-accepted value is well within the calculation’s error margin. These students did a remarkable job!
So — Bravo to Team Hetu’u!