December 28 – Happy Birthday, Thomas Henderson

Posted December 28, 2013

He was the first to measure the parallax of a star—but he didn't publish for several years, apparently because he lacked confidence in his measurements, and by the time he did, someone else had already published and got the credit!

Thomas Henderson was born on this date in 1798 in Dundee, Scotland. He trained as a lawyer and worked as such for several different noblemen, but his hobbies—the things he did for fun—were mathematics and astronomy.

He was good enough at his hobbies that he became a professional astronomer, earning a position at an observatory in South Africa and later becoming the first Astronomer Royal for Scotland.

What's a parallax, you ask?

The parallax of a star is the apparent change in its position caused by the observer's own changed position.

In astronomy, we needed to develop yardsticks of sorts to figure out how far away space objects are. One “yardstick” that only works with the very nearest stars is parallax. We take a measurement of a star against the background of more distant stars at one point of the Earth's orbit around the Sun, and then we measure that same star's position six months later, when the Earth has reached the farthest possible point from that of the original measurement. Using the known distance of the two points in the Earth's orbit and the measured distance of the star's position, an astronomer can figure out the approximate distance between the star and the Earth-and-Sun (solar) system.

Henderson computed that Alpha Centauri was about 3.25 light years away. This measurement was 25.6% too small, but it is considered a fairly good first stab at the distance. Alpha Centauri, as it turns out, is the largest star in the star system that is closest to Earth.

Here is a non-astronomical example of parallax.
A nearby object such as your thumb will seem to
change position in reference to farther-away objects
such as buildings when viewed first from your left
eye and then from your right eye.
I thought it was interesting to note that, in the 1830s, there was a “space race” between scientists—everyone wanted to be the first person to measure the distance to a star using parallax. Some eagerly published their measurements, but were later discredited by others and so are not considered “first.” And fear of that sort of discrediting is what made Henderson hesitate to publish. 


How'd you like to go down in the history books as the first person to do it but the second to publish?

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