Page 29 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

NEED TO KNOW

HOW FAR

ARE STARS?

For measuring the big distances to the
stars, astronomers use something called
parallax. This involves watching a
close star’s movement against the fi xed
background of much more distant stars.
It’s like holding up your index fi nger at
arms‘ length and viewing it against the
background through your left eye and
then your right – there is a position shift
between each view because your fi nger
is much closer than the background.
This is how it works in space, too,
except our two views are six months
apart, when Earth is on either side
of its orbit around the Sun. This is the
widest possible separation we can get
between our telescopic ‘eyes’, and even
then the shifts are so small that this
Big distances to the stars technique only works accurately out to
are measured using the a few hundred lightyears.
parallax method

DISTANCES

IN SPACE

HOW CAN WE EVEN ATTEMPT TO DESCRIBE
THE MIND-BOGGLING DISTANCES BETWEEN STARS?

Stepping into the garden on one of those crisp big distances need a big scale: several of them
REFLECTING clear nights, I see sparkling stars and a crescent in fact. We would use centimetres or inches to
RADIO WAVES Moon about to set near the western horizon. measure the length of a bookcase (for astronomy
Saturn’s up there, Jupiter’s over here and there books, of course), but that would not make any
The Lovell Radio goes a shooting star. Fantastic. sense when trying to explain the distance from
Telescope at Jodrell Bank What is just as amazing is the fact that I Glasgow to Belfast. So kilometres or miles are
in Cheshire was used to
measure the distance of cannot tell which of these objects is the closest our Earthly scale of things.
Venus by bouncing radio or furthest away from me. Knowing that the star
waves off the planet. Aldebaran is over nine million times more distant INCREASINGLY UNWIELDY
Just like the technique for than the Moon does not really compute anyway Journeying into space, with the close Moon at
the Moon (see p30), it – these distances are too big for my brain. around 384,000km (250,000 miles) or the nearby
involves measuring the As far as locating objects in the sky is Sun at 150 million km (93 million miles), you
time it takes for a burst
of radio waves to come concerned, we project them onto an invisible can see that these units become more and more
back. Of course, Venus great glass thing around us called the celestial unwieldy the further we go. Saying, “Oh, that’s
is farther away than the sphere. It’s useful and it works, and most of the 83 thousand million squillion zillion km away” is
Moon: the lunar laser is time we’re really not bothered how far away not that useful and takes up a lot of time, when
back in three seconds, something is anyway. But what if we are? you could just say, “Oh, that’s three zarquons.”
while the journey to Venus The fi rst thing to realise – and I mentioned the Admittedly, I made that last unit up, but I’m sure
and back is four minutes.
problem with thinking about the sizes – is that that you get the point.

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