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The first parallax measurements

Published on Jun 28, 2007 at 1:01 am. 1 Comment.
Filed under astronomy, stars.

Early last week, I wrote about parallax and distance measurements. This is a follow-up post to that one.

Stellar parallax is very small, and thus correspondingly difficult to measure. The closest star has a parallax of 0.772 arc-seconds (that is nearly 1/4700 of a degree). That is a very tiny angle to measure, and so it is no wonder that it took so long for astronomical technology to advance to where the measurements could be made. The capability to make such small measurements finally came in the early Nineteenth Century.

One problem for astronomers trying to measure parallax is that the stars are vast distances away. The farther a star is from us, the smaller the parallax, and the harder that parallax is to measure. Even today, most stars are simply too far away to reliably measure parallax. In the early Nineteenth Century, it was worse. The technology was such that only a handful of the nearest stars had big enough parallaxes to measure. But, there are a lot of stars in the sky? Which ones would be the best candidates to study and to attempt to measure? The measurements would be time consuming, and an astronomer would not be able to measure many stars, so he’d have to pick a star and stick with it. But, if the selected star were too far away, then he’d never be able to measure parallax.

At first, astronomers had thought all stars to be similar, so the brighter stars were presumed to be the nearer ones. But, that idea had begun to fall by the wayside by the Eighteenth Century. Astronomers realized that brightness may not correlate at all with nearness (and it largely doesn’t). Eager attempts to measure parallax inevitably resulted in failure. The stars were simply far more distant than anyone had been prepared to imagine. But, along the way, there were a lot of interesting discoveries. For example, the search for parallax led to the discovery of the aberration of starlight.

But, there was another important factor, besides brightness, that astronomers looked to when trying to decide on a target star: its proper motion. Edmund Halley discovered that some stars had apparently shifted position over historic times. The stars are not fixed in space relative to one another. This apparent shift of the stars, as seen from Earth, is their proper motion. Assuming that most stars are moving at similar speeds, then the nearer stars might appear to have higher proper motion than the more distant ones. You can see this same effect by looking out the window of a car as you drive down the highway. The nearer objects seem to be going past the window far more quickly than the more distant objects. But, that only really holds true when you are looking at stationary objects. Other cars driving in the same direction as you may be far closer than cattle or trees alongside the road, but they will not appear to be moving very quickly with respect to your window because they have very nearly the same speed as your car. Likewise, stars quite near the Sun might not be seen to have a high proper motion if they share the Sun’s motion through the galaxy. Still, this seemed to be a far more promising correlation with distance than the brightness measure.

So, the hunt was on. Numerous astronomers were eagerly working to make the first parallax measurements. Among these were Thomas Henderson, working in South Africa, Friedrick Wilhelm Struve, and Friedrick Wilhelm Bessell (both in Europe). Henderson actually got the jump on the others, making measurements of Alpha Centauri. In 1833, he packed up and went back to England, along with his data. He was in no hurry to reduce his data, so it languished for years. When he finally did get to looking at his measurements, he found that there did seem to be what may have been a parallax shift in Alpha Centauri, but he did not trust his data. He had only 19 measurements, far too few to be certain or conclusive in his findings. Furthermore, the instrument that he had been using had been damaged in shipping to South Africa. He had painstakingly applied corrections to the measurements, but he realized that other astronomers would cast doubt on his findings. He decided to wait for better measurements made with another instrument by his successor at the far southern observatory. Alpha Centauri is indeed the nearest star (actually it is a triple star system, and the closest of the three, Proxima Centauri, is the nearest star other than the Sun), and it really did have a large enough parallax to measure. And, as it turns out, Henderson’s corrections to his data were approximately correct. However, he didn’t know all of that, so he held off publishing his findings until he had more data sent to him.

In the mean time, Bessell had acquired a spectacular and very precise instrument ideally suited for the task. It had originally been designed for measuring the sizes of features on the Sun, but he expertly adapted it to measure the distances between stars. Giuseppe Piazzi had shown the star 61 Cygni to have a particularly high proper motion. In fact, it was dubbed the “flying star” and at the time held the record as the star with the highest proper motion (a record that it was to eventually lose to Groombridge 1830, and then to Barnard’s Star). This made it an excellent target star. However, after only a few months, Bessell gave up the endeavor because he found the comparison star that he’d selected to be too dim to follow in poor sky conditions. Other concerns took him away from the task for a number of years.

Then, in 1837, Struve announced that he’d measured the parallax of the star Vega. The number that he gave was 0.125″. Bessell poured over Struve’s data, but was not convinced that it was really believable. He feverishly resumed his measurements of 61 Cygni. For the next year, any clear night that he could observe the star, he did, often a dozen times per night, making measurements. After a year, he had hundreds of positions determined using thousands of individual measurements. His data showed no doubt that 61 Cygni moved back and forth as the Earth moved around the Sun. He had found clear evidence of parallax. Bessell computed the parallax of 61 Cygni to be 0.314″, and he published his results in late 1838. Soon afterwards, Struve revised his parallax measurement of Vega to a value nearly double what he had originally found. That huge change cast serious doubt as to the reliability of his measurements. So most astronomers, including Struve himself, ceded the first parallax measurement to Bessell. With Bessell and Struve’s measurements available, Henderson finally published his own findings for Alpha Centauri.

Interestingly enough, despite Struve’s uncertainty in his own measurements, his original value for Vega’s parallax is amazingly close to the modern accepted value of 0.129″. Bessell’s parallax for 61 Cygni is also not far off of today’s accepted value of 0.285″. It really is hard to say who should get credit for the first parallax measurements. All three, Struve, Bessell, and Henderson, were working at about the same time. Henderson didn’t believe his measurements, so he didn’t publish them right away, and thus is seldom given credit. Struve published his measurements a year before Bessell, but his measurements were deemed somewhat uncertain, a fact that he most clearly stated himself. Struve, himself, gave Bessell credit for the first unambiguous measurement of stellar parallax. But, I think that all three deserve some mention.

If you want to read more about this episode in the history of astronomy, an excellent resource is Alan Hirshfeld’s book Parallax: The Race to Measure the Cosmos.

-Astroprof

Finder chart for 61 Cygni created using Starry Night Pro software.