BRIAN: All right.
So we have a great set of telescopes, two telescopes that Fraunhofer made.
And to get the most out of a good instrument,
you need to have a great experimentalist.
And it turned out Fraunhofer gave-- his telescopes were
used by two great early astronomers.
And none more important than this astronomer, Bessel.
And so, Bessel was not just an experimentalist.
He was also a mathematician.
There's a very famous set functions-- Bessel functions--
that he was involved with it.
I won't say invented them.
But he was involved with their tabulation and things.
And so, he was able to get the most out of this telescope that he was given.
PAUL: He used to say that every telescope had to be built twice.
First of all, it was actually built when it was originally manufactured.
But secondly, you had to-- when you observed
with it-- understand every single piece of it,
and all the things that could go wrong.
So for example, whenever he got delivery of a new telescope,
he wouldn't start taking data for possibly two years.
He'd spend two years trying it out, measuring everything
that could possibly go wrong, all the systematic errors.
He worked very hard at these telescopes.
In this telescope, for example, he had to work out
exactly what angle corresponds to the shifting of the two
halves of the front lens.
He had to work out how the whole thing would bend, depending on gravity.
As the temperature changed, the screw that
controlled the two halves of the mirror would
change because of expansion and contraction.
He calibrated all that.
In fact, he often spent a lot of his time
going to look at other people's data, and recalibrating it.
He was able to show, for example, one of his assistants always measured
the stopwatch 2 seconds too late.
Because he looked at how the timing of the different
stars in different parts of the sky, and found on one side
it was always 2 seconds off from the other side.
So he actually spent a lot of his time going through other people's data
from other astronomers, and analyzed in great detail what we now
call data reduction, what we both spend a huge amount of our time doing.
When you observe, you're only half-started.
Most of the work is afterwards, trying to find all these possible systematic
errors, and weeding them out, and fixing them.
BRIAN: So this is exactly what a modern scientist of the day would use.
He only spent two years working on his telescope.
Our own SkyMapper-- we've had to work six years to get all its issues,
and we're continuing.
Part of my daily work is to go in and try
to weed out all those systematic errors, in the same way that Bessel really
started doing.
But when you do it, then you get good quality data.
And I can tell you that good quality data,
no matter how smart you are as a mathematician,
you always do better if you have good quality data.
PAUL: And so, at long last, he measured parallax.
He actually was able to see a star wiggling.
He didn't trust his own--he published his own data.
He wasn't the first to measure it, but he was the first person
to measure it and be believed.
Because he was so careful and precise.
It's much like with your discovery of dark energy.
There were plenty of other clues before, but you were the first-- actually
the data was so good that people actually believed it.
When he measured it, he thought, I'm not quite sure.
So he then closed down the observatory, took the telescope
to pieces, rebuilt it again, and then measured it again
to make sure it was still that after everything
had been rebuilt from scratch.
BRIAN: That's being very dedicated.
PAUL: And it was still there.
And this is the level of dedication that made him famous.
And he was really believed.
And he was one of the great astronomers of the time, brought
great prestige to Prussia back then.
So he'd found parallax.
But what we're concerned with is another discovery he made a few years later.
He was out looking for parallax around more stores.
And he recognized-- this is a picture I took three nights ago.
Here's Orion.
And up there is the brightest in the sky-- Sirius.
BRIAN: The dog star, yes?
PAUL: The dog star.
And he decided to try and look for a wobble in this.
And he found a wobble.
But not the wobble he expected.
BRIAN: A much bigger wobble than he was expecting.
PAUL: And of wobble not with a one-year period, as the Earth goes
around the Sun, but a 50 year period.
BRIAN: Right.
And it was a very large wobble that was on order of 7 arc seconds, which
would be-- we now know is an order of magnitude
more than the parallax measurement of this star.
And so you can see Sirius is moving in a nice ellipse.
And the reason why stars move in ellipses
is typically because they're in orbits around other stars.
But in this case, the star isn't very bright.
Sirius is very bright.
And whatever it's orbiting-- not very bright.
PAUL: Yes.
Sirius weighs about twice the mass of the Sun.
It's a luminous star.
And clearly, you can tell there must be something maybe-- if it's over here,
it must be over that way.
As they go round, they're opposite each other the whole time.
So there's something there.
Must be quite a lot, because it's making Sirius move a lot.
It couldn't be a planet.
It's got to be much heavier than a planet.
But we're not seeing it.
BRIAN: So Paul, you should be able to use that orbital motion
and calculate the star that we cannot see.
PAUL: OK.
Let's go and do that.