BBC: Finding your way in space

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, burt before mobile phones where invented arriving at your destination wasn't so easy. At sea, sailors used the stars and Sun to nivigate- 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 usea 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 for 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 Armstron 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 giant bar magnet in it, and your compass needle will point north towards Eath's magnetic pole. And we've arbitrarily defined north as, that's what we're going to say 'up' is, liike, 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 hemispher. But from space, Earth can jus tas 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: