Greetings, astronomy fans! As you may have heard, next Tuesday evening (as seen from the east coast of the U.S.), we’re going to witness a rare and historical astronomical event: the transit of Venus.
If you somehow missed the news, the basic picture is that Venus will pass in front of the disk of the sun, and provided you don’t actually look directly at the sun (guidelines here), you will see the shadow of Venus.
Why is this such a big deal? It has to do with two very important — and intimately related –- numbers that we didn’t have a handle on until relatively recently: the speed of light, and the distance from the earth to the sun.
The first measurements of the distance to the sun were done entirely from geometry, and the distance to the sun, now known — a distance of about 93 million miles, and known, a tad unimaginatively as, an “Astronomical Unit” – was first measured in multiples of the earth’s radius.
The ancients – the ones who weren’t foolish enough to think that the sun orbited around the earth – tried a number of relatively unsuccessful approaches to figure out this fundamental step in the distance ladder. We don’t actually know how unsuccessful they were, because it’s tough to figure out exactly how to convert between ancient units. Working in the 3rd century BCE, Aristarchus of Samos, in one of the best estimates until modern times, was off by something like a factor of 15.
It wasn’t until nearly two millennia later, in the late 1700’s, that the French astronomer Jerome Lalande used the Transit of Venus to accurately measure the distance to the sun.
Every century or so, the planets line up just perfectly so that the planet Venus passes precisely between the earth and the sun. Transits are useful because they appear slightly different to people on different parts of the earth. Two observers – aligned East-West from one another – will see the transit start at very slightly different times. It works exactly the same as your eyes .
Your left and right eyes see ever so slightly different images and from that, your brain does a calculation of depth and distance. Blink your eyes back and forth, and you’re going to see things move slightly, with closer objects moving more. Put into mathematical terms, your brain is really figuring out all distances as a ratio of the distance to the separation of your eyeballs.
With the transit, it works much the same way. Station two observers at different points of on the earth, and triangulate. There is a small complication. We don’t get the distance to the sun so much as the distance to Venus. Fortunately for us, using pure geometry, we can figure out all of the distances in the solar system in terms of ratios. This was how Kepler was able to come up with his laws of planetary motion more than 150 years before an absolute distance scale was established for the solar system.
Because of relative tilts of earth’s and Venus’s orbits, you get one shot to observe a transit of Venus, and then another 8 years later. After that, you’re out of luck for about 120 years. This transit will be seen on June 5-6, 2012, so if you miss it, you’ve probably missed it for your lifetime. I was lucky enough to see the 2004 transit (which was at an ungodly hour in the morning, if I remember correctly), and I’m hoping to catch this one as well. For those of you in the Philadelphia area, I hope you’ll stop by the Drexel Observatory and take a look (weather permitting, of course).
Because of how the distances are calculated, it’s important to get more than one measurement. Edmund Halley — made famous for the comet that he did not discover — tried to take a set of observations of the transit of Mercury in 1676, but was largely unsuccessful. Unfortunately for him, he was 37 years too late for the 1639 Venus Transit, and nearly a century too early for the next one. Fortunately, Lalande had access to the 1761 and 1769 transit data, and from that was able to get a very good measure of the distance to the sun, accurate to within a few percent.
While it’s true that we weren’t able to measure the distance to the sun in meters or earth radii or any other usable units until the 1770’s, as it turns out, we knew the approximate distance to the sun in light-minutes almost a century before that. Back in the 1670’s, a Danish Astronomer named Ole Rømer noticed something very odd about the recently discovered moons of Jupiter. It may not have escaped your notice that orbiting bodies form a convenient clock. Our orbit around the sun marks a year. The orbit of the moon around the earth historically defined a month. The same can be said for any other orbiting bodies. They should run like, well, clockwork.
Rømer noticed that when Jupiter was nearest in its orbit to the earth, the moons seemed to be running about 22 minutes ahead of when Jupiter was furthest away. Rømer concluded (correctly, as it happens) that light takes a certain amount of time to get to us from Jupiter, and when it’s further away, it will take longer than when it’s closer. Since at closest approach Jupiter is two astronomical units closer than at furthest approach, Rømer found that it takes light about 11 minutes to travel an Astronomical Unit.
Astronomical measurements are tough, and were even tougher in the 17th century, since telescopes were still in their infancy. As it turns out, the actual light travel time to the sun is closer to 8 minutes, 19 seconds. Still, Rømer was in the right ballpark, and ultimately set the groundwork for relating space and time.