Hello.
This is 6 Minute
English from BBC Learning
English.
I'm Sam.
And I'm Rob.
How good are you at
finding your way from
A to B, Rob?
Can you read a map?
Come on, Sam, this is
the 21st century!
Everyone uses GPS and
mobile phone apps to find
their way around
these days.
True, but before mobile
phones where invented
arriving at your destination
wasn't so easy.
At sea,
sailors used the stars
and Sun to navigate - to
work out which direction
they wanted to travel.
And navigating on land
was almost impossible
without a compass - an
instrument for finding
directions that uses a
magnetic needle which
moves to always
point north.
But, as we'll be hearing
in this programme,
navigation at sea is easy
compared to finding your
way in outer space.
After
all, what's up and what's
down for astronauts who
are floating in zero
gravity?
In space is there
a true north, like here
on Earth?
And how is
everything complicated by
the fact that all the
stars and planets
are moving?
Some big questions there,
Rob, but first I have a
question of my own.
You asked how astronauts
know which way is up,
so who better to ask
than the first person
in space?
But who was
that?
Was it:
a) Neil Armstrong?
b) Yuri Gagarin?
or
c) Valentina Tereshkova?
Well, Neil Armstrong was
the first man on the
Moon, but I don't think
he was the first person
in space.
So I think it's
b) Yuri Gagarin.
OK, I'll reveal the answer
later in the programme.
Now let's get back to
Rob's earlier question
about whether there's
such a thing as north
in space.
And to answer
that it's first useful
to know how north is
found on Earth.
Listen as astrophysicist
Ethan Siegal as he
explains why a compass
always points north
to BBC World Service
programme, CrowdScience.
... because Earth
behaves like it has a
giant bar magnet in it,
and your compass needle
will point north
towards Earth's magnetic
pole.
And we've
arbitrarily defined
north as, that's what
we're going to say
'up' is, like, the
North Pole - that's
as 'up' as you can go.
Planet Earth is like
a giant magnet.
Because the needle
of a compass is
magnetised, it's
attracted to the
magnetic pole - the
points near the North
and South Poles
where the Earth's
magnetic field
is concentrated.
This explains how we
find north, but
Ethan points out
that the decision to
call north 'up' and
south 'down' is
arbitrary - decided by
random chance, not
based on any
particular reason.
When we look at a
world map, we think
of north as 'up', the
USA in the northern
hemisphere is above
Brazil, in the southern
hemisphere.
But from
space, Earth can
just as easily be
seen the other way
up, with Australia,
South Africa and
South America at the
top.
Both views
are equally true.
Wow, that's a
mind-blowing thought!
But even though we
can argue which
direction is up, it's
still true that we
can use a compass to
navigate on Earth.
However, this simply
isn't true in space.
Here's astrophysicist
Ethan Siegal again to
tell BBC World
Service's CrowdScience why:
The problem with
navigating in space is
that the magnetic field
flips irregularly every
few hundred, or few
thousand light years.
There's no central
object like the black
hole at the centre of
our galaxy - it
doesn't dominate the
whole galaxy, it
doesn't make a
magnetic field that
you can feel out
here 25, 27-thousand
light years from the
centre.
So, magnetism
is not a good guide
to navigating in space.
A light year sounds
like a measurement
of time, but in fact
it measures the
distance that light
travels in one year -
which, given that
light can travel 7.5
times around the Earth
in one second, is a
very, very long way -
around 6 trillion
miles, in fact.
Well, the problem is
that every few hundred
light years the
magnetic field flips -
turns over or moves
into a different
position.
So, a
compass, which depends
on magnetism, is no
good for navigating
in space.
So how do spacecraft
know where they are,
and which way to go?
The answer is both
simple and very
clever - they use
specialised heat
sensors to detect
the position of the
Sun and use that to
guide their way.
So simple yet so
ingenious!
I'm sure
it would have
impressed the first
person in space,
whoever they were.
Ah yes, in my question
I asked who the first
person in space was.
And I said it was
b) Yuri Gagarin.
I've got to be right,
haven't I?
It was right, of
course!
Soviet
cosmonaut Yuri Gagarin
became the first man
in space in 1961,
with Valentina
Tereshkova following
in his footsteps
to become the first
woman in space
two years later.
OK, let's recap the
vocabulary from this
programme on how
to navigate - or
find your way -
in space.
On Earth you can
use a compass - an
instrument with a
magnetic needle
that moves to point
north, that is
towards to the
magnetic pole - a
point near the
North or South
Poles where Earth's
magnetic field
is strongest.
Saying that north
is 'up' is arbitrary -
done randomly, not
according to any
particular reason
or principle.
A light year is a
unit measuring the
distance that light
travels in one year -
around 6 trillion miles.
And finally, to flip
means to turn over
or move into a
different position.
Once again, our time
is up.
Goodbye for now!
Bye bye!