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.
But...
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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