On the Scientific Value of the Transit of Venus

Well, this is it. On Tuesday Venus will pass in front of the Sun, along with everything that entails for my job of trying to present it to the public. To be honest, I'm most excited simply that it will be over, so life can return to a semblance of normality and not crazy over-working (I was originally scheduled to work 64 hours this week over a period of 5 days, until I was able to point out that for 2 of those days I was redundant and unnecessary).

Today I thought I'd quickly explain why previous transits of Venus were such objects of scientific interest, to the point that multiple nations sent scientific expeditions on hazardous voyages around the world. It all has to do with the size of the Solar System

Back in the early 1600's a brilliant astronomer by the name of Johannes Kepler formulated three descriptions of planetary motion that have come to be known as Kepler's Laws. A full description of them would take another post, so it will suffice to say that they describe planetary motion in the Solar System to a very high degree of accuracy (one made even better when Newton introduced his theory of gravity to explain why the laws worked the way they did). The third law, in particular, relates the square of the time it takes a planet to orbit the Sun to the cube of the semi-major axis of its orbit. To a good approximation, given that all planetary orbits are pretty close to circles, what this says is that if you know the orbital period of a planet, you can figure out how far it is from the Sun. Figuring out the orbital period is a bit of work, but nothing that the astronomers of the day couldn't handle, and they were excited to find out just how big this Solar System of ours is.

Unfortunately, there's one catch: the way the law is formulated gives the distance from the Sun to the planets in terms of the distance between the Sun and the Earth. Since that wasn't known to begin with (that's part of the reason for wanting to find it, after all), it seemed that astronomers were stuck (sure, they could use the law to say that Jupiter is 5.204 times farther from the Sun than Earth is, but without absolute numbers it's a somewhat hollow achievement).

This sorry state of affairs remained until a Scottish mathematician named James Gregory suggested that observations of the time taken for Mercury to cross the Sun's face as seen from widely separated points on Earth could be used to figure out how far away the Sun was. The young astronomer Edmond Halley (better known for being the first person to predict the return of a comet, which still bears his name) tried to do this for a transit of Mercury in 1676 but was frustrated by the fact that only one other such observation existed, and didn't think that two data points were accurate enough. He suggested that more accurate calculations could be done using a transit of Venus instead, but he unfortunately would not live to see the next one in 1761.

His suggestion, however, did not go unheard (being the second Astronomer Royal to the British crown may have had something to do with it), and when the next pair of transits rolled around astronomers around the world were ready. Expeditions from England, France, and Austria traveled around the world for the 1761 transit, and Captain Cook made his first voyage to the Pacific to observe the 1769 one.

This was, in a very real sense, the first major example of international co-operation in history. It's something we don't even think about today, but it helped set the stage for the atmosphere of cordial co-operation that exists in science throughout the world today (with the occasional bit of friendly rivalry thrown in). We don't find it strange today that scientists from all over the world freely publish the results of their experiments which may have required millions of dollars and hundreds of man-hours to find, but it didn't necessarily have to be this way. Science and knowledge could have been (and have been at points in history) very territorial things, hoarded for national gain (think of the secrets of Greek fire, known only to the Byzantines). Instead we have a world where anyone can pick up the latest issue of a scientific journal and peruse its contents freely (in the sense of “personal freedom”, not the “no-cost” sense), and I like to think that astronomers may have had something to with that.

To cut a long story short, the expeditions, although many of them did observe Venus, were not successful in their main quest to determine the Earth-Sun distance. The reason has to do with the “black drop effect”, wherein Venus appears to elongate as it approaches the edge of the Sun's disk (from either side). Unfortunately, precise timing of exactly those moments was the critical information needed for the calculations to work. This effect was at first (and for a long time) thought to be proof that Venus had a atmosphere, but in reality it has more to do with imperfections in observing equipment and turbulence in Earth's atmosphere (Venus does have an atmosphere, of course, but that's beside the point, as the black drop effect also shows up during transits of atmosphere-less Mercury to a lesser extant).

Anyway, when the transits of 1874 and 1882 came around, astronomers tried again. Although the black drop effect was still in, well, effect, the data generated from all previous observations was enough to get a pretty good value of 149.59 million kilometers (92.95 million miles) using statistical methods, very close to the modern day value of 149,598,261 kilometers (92,956,048.8 miles).

And where are we today? With the advent of radar and other modern advances we can now calculate the distance to the Sun to about \(\pm\)30 meters (~100 feet), and transits of Venus are no longer necessary to tell us how big the Solar System is. They are now interesting for entirely new reasons that could hardly have been foreseen by those astronomers of old. Now, observations of transits of Venus have the potential to help with the burgeoning field of finding planets around other Stars, especially small, rocky planets like Earth and Venus.

At this point, it's interesting to speculate on what may happen between now and the next transit of Venus in 2117. Given how much the world has changed in the last 130 years, I don't think I want to make any predictions, but it would be interesting to see how our knowledge of exoplanets will grow in the meantime, and how much can be learned from studying this current pair of transits. Exciting stuff!