### How many times round the world is a light year?

*This question was inspired by a student in one of my lessons on Standard Form*. He asked "how far around the world would I have to go before I travelled a light year?"*

To make this simple, and because we're only looking for a ball-park figure, I'm going to use heavily rounded numbers in this post!

**What is a light year?**

See this post for a slightly more in-depth look at what a light year is, but a light year is a unit of distance that equates to about 9.5 x 10

^{15}metres.

That's a '95' with fourteen zeroes after it: 9,500,000,000,000,000 metres, or nine-and-a-half quadrillion metres.

I've tried to give that number some context or clarity by stating it in different ways, but in reality it probably doesn't mean much to you: it sure as hell doesn't to me. It's just too big. In the post linked to at the beginning of this section I offered some further examples, but they all use other units that are far too big to comprehend. So lets try bringing it down to size a bit: if we look at it in kilometres, it's only a paltry 9,500,000,000,000 (nine-and-a-half trillion) of those. The image above (Credit: NASA/CXC/SAO) is of Proxima Centauri, the closest star (other than the Sun) to Earth, which is about four light years away.

**How far is it around the world?**

Earth's radius (the distance from its centre to the crust) is, on average, a bit less than 6,500 km. If we assume that the Earth is a perfect sphere we can use that trusty formula from GCSE maths, C = 2 x Ï€ x r to find the distance around its equator, where r = radius of the Earth: 2 x Ï€ x 6,500 = 40,000 km (ish).

**So how many times would we have to travel around the world before we could say we'd travelled a light year?**

- A light year measures around 9,500,000,000,000 km.
- One trip around the world is about 41,000 km

Divide one by t'other and we'll find out how many times a trip around the world would fit into a light year:

9,500,000,000,000 / 41,000 = 2.31707 x 10

^{8}

That's about 200,000,000 (two hundred million). So we'd have to book two hundred million round-the-world trips to be able to say that we'd travelled a light year.

I wonder how much that'd cost...

* 'Standard Form' is a way of writing numbers that takes up less space (most of the time). For example, the number 5,000,000,000,000,000,000,000,000 can be written 5 x 10

^{24}, which takes up much less space on twitter. If you'd like a better post on Standard Form, you'd be better off asking for one over at MathsQS.

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