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!