The Transit of Venus


(This 1106th Buffalo Sunday News column was first published on June 3, 2012.)


Photo by Alan Friedman of the 2004 Transit


There are many numbers that are important to scientists. Here is one of them: 92,955,807.2730. Known as AU, that is the astronomical unit: the mean distance from the earth to the sun, expressed here in miles.


Units are central not only to the scientific enterprise but also to every one of us. They allow us to measure and to compare. You are familiar with many of them: inch, mile, cent, dollar, liter, bushel, minute, hour, the list goes on and on. Those units let us know which is farthest away, who has more money, whether we can get there on time. They are a defining characteristic of our civilization: they separate us from the other animals.


It is important then for units to be well defined. We don't want an inch to mean different things to different people. And since that AU is the basic unit of measure for astronomical distances, that remarkable 12-digit accuracy is useful for such things as space flight.


Now the question arises: how was that AU, the distance from the earth to the sun, determined? The answer to that question involves some history that will be replayed this Tuesday.


Distances can be measured accurately by two methods: triangulation and parallax. Both techniques are based on the geometry and trigonometry taught in high school. They involve, as the names imply, triangles, and they need the length of one side of those triangles. To use the methods astronomers need special circumstances. The problem they faced down through history is the fact that you cannot tell how far away something is looking straight at it.


The rare time when the necessary requirements are fulfilled occurs when a planet passes between our Earth and the Sun. The only two planets that can do so are Mercury and Venus, because their distance from the Sun is less than ours. Those events are called passages or transits.


And they are indeed rare. Although the Transit of Venus, which will begin at 6:05 p.m. on June 5, also occurred just eight years ago, it will not happen again until 2117, more than a century in the future. (The odd periodicity of these transits is in years: 105.5, 8, 121.5, 8.)


For accurate measurement it was important to have observations taken some distance apart. For this reason, beginning with the 1761 transit, observers traveled to distant parts of the earth. (Even then they had to hope for cloudless skies.) That year the best measurements were taken in South Africa by Jeremiah Dixon and Charles Mason. Do you recognize those names? We know them for the Mason-Dixon line that they later surveyed between Maryland, Pennsylvania and Delaware.


In 1769 more good measurements were possible, some in Tahiti by Captain Cook's expedition. Those measures produced an AU of 95 million miles. Further sightings in 1874 and 1882 reduced this million-mile error significantly to 92,951,000 miles, but still over 4,000 miles from today's figure.


It was not until modern radio telemetry and radar equipment became available to make extremely accurate measurements and computers were able to process them that this figure was reduced to the one used today.


The accuracy of the AU today is within 100 feet. That may not seem so close when we have GPS devices locating us on the Earth within 10 feet, but proportionally the AU measure is about 1000 times more accurate.


Although the AU problem has been solved, transits continue to be of importance to astronomers. For example, the tiny reduction in light arriving at the Earth from the sun during the transit is providing a comparison for use with other stars to predict the existence of planets outside our solar system.


If the weather cooperates, there are three locations where you can join Buffalo Astronomical Association members beginning at 5:30 p.m. on June 5 to view this phenomenon through specially equipped telescopes: Buffalo Museum of Science, 1020 Humboldt Parkway; Williamsville Space Lab Planetarium (for directions) and Penn Dixie Paleontological and Outdoor Education Center (for directions). There is a $5 charge at the museum for special glasses; the other sites are free.