December 28 – Happy Birthday, Thomas Henderson

nPostednDecember 28, 2013

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Henwas the first to measure the parallax of a star—but he didn’tnpublish for several years, apparently because he lacked confidence innhis measurements, and by the time he did, someone else had alreadynpublished and got the credit!

nThomasnHenderson was born on this date in 1798 in Dundee, Scotland. He ntrained as a lawyer and worked as such for several differentnnoblemen, but his hobbies—the things he did for fun—werenmathematics and astronomy. n

nHenwas good enough at his hobbies that he became a professionalnastronomer, earning a position at an observatory in South Africa andnlater becoming the first Astronomer Royal for Scotland.

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nWhat’sna parallax, you ask?

nThenparallax of a star is the apparent change in its position caused bynthe observer’s own changed position.

nInnastronomy, we needed to develop yardsticks of sorts to figure out hownfar away space objects are. One “yardstick” that only works withnthe very nearest stars is parallax. We take a measurement of a starnagainst the background of more distant stars at one point of thenEarth’s orbit around the Sun, and then we measure that same star’snposition six months later, when the Earth has reached the farthestnpossible point from that of the original measurement. Using the knownndistance of the two points in the Earth’s orbit and the measuredndistance of the star’s position, an astronomer can figure out thenapproximate distance between the star and the Earth-and-Sun (solar)nsystem.

nHendersonncomputed that Alpha Centauri was about 3.25 light years away. Thisnmeasurement was 25.6% too small, but it is considered a fairly goodnfirst stab at the distance. Alpha Centauri, as it turns out, is thenlargest star in the star system that is closest to Earth. n

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.

nInthought it was interesting to note that, in the 1830s, there was an“space race” between scientists—everyone wanted to be the firstnperson to measure the distance to a star using parallax. Some eagerlynpublished their measurements, but were later discredited by othersnand so are not considered “first.” And fear of that sort ofndiscrediting is what made Henderson hesitate to publish.

nBut…

nHow’dnyou like to go down in the history books as the first person to do itnbut the second to publish?

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nAlsonon this date: