Travelling light

In the 1990s, the Hipparcos satellite enabled scientists to use triangulation techniques to calculate the distance of the Pole Star, Polaris, from Earth. Their answer was 434 light-years (a light-year is a tad under 6 trillion miles). But in 2012, astronomer David Turner analysed the spectrum of its light and concluded that the star was ‘only’ 323 light-years away. Polaris fluctuates in brightness, however, and the Hipparcos team think that Turner might not have taken this adequately into account, using a higher value which would have made the star appear closer. Someone has set out to decide who’s right:

I am a light ray, and I’m well on my way
From Polaris towards your blue planet.
I have used the ‘straight’ lines that space-time defines
On my journey, since first I began it.

When shall I arrive? Well, I’d better contrive
To work out the timings involved:
I’m travelling at c1 on my trajectory –
But one problem still has to be solved.

I need your assistance to find out the distance
From Polaris to Earth: but oh dear!
It seems you’re not sure what it is any more,
And the error’s not  just a light-year.

The problem’s Polaris, for this sort of star is
The sort whose brightness keeps changing.
If you measure it wrong, it’ll not be too long
Till your sums will all need rearranging.

If Dave Turner’s correct, well then I’d expect
To arrive a whole century sooner.
But I trust Hipparcos2 to measure my star, ’cos
It’s been such a great cosmos-tuner:

It’s clocked the positions, with milliarcsec3 precisions
(And motions and parallaxes, too),
Of 60K-score of our galaxy’s store
Of stars (which leaves billions to do!)4

If Hipparcos is right, and the time of my flight
Is four-three-four years, it’ll show
That Dave Turner was wrong (as I guessed all along).
If you doubt it, ask me – I should know!

1 ‘c’ is the usual symbol for the speed of light in a vacuum, which is about 670,616,629 mph.
3 A very precise measure of angle: there are 3.6 billion milliarcseconds in just 1 degree!
4 Wikipedia reckons the estimated total number of stars in our Milky Way galaxy to be “between 200 and 400 billion”

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