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Saturday, 7 January 2017

What is the Milky Way? And how is pizza involved?

Are you looking for a  gift with a difference? How about art by a mad scientist?

Given how many people live in and around cities these days, you may not have seen this:

Image courtesy of Stephane Guisard

I mean the Milky Way not the cactus.

The reason why a lot of people won't have seen it (including me - at least for a while) is that it doesn't really look anything like that bright. That picture is taken with a camera that was left to gather light for minutes or hours, turning a faint band of fuzz running across the sky into a glistening river of starlight.

To see anything like that picture you'll have to go well away from civilisations lights - then, in night sky, you'll see a band of stars. If you went into space, well away from Earth and the Sun**, you'd see that band actually running all the way around the sky. 

But, even if you did, the Milky Way still wouldn't be giving you the whole story.
It looks like a band of stars, but it’s actually a disk of them – it's a galaxy, a two hundred thousand light year wide disk of stars. The reason why it looks like a band running across the night sky is that we’re inside the disk – Earth is located about two thirds of the way from the middle, which it circles every 250 million years. 

Above: A map of the Milky way, showing the position of Earth.

Not convinced? To get a visual and tasty idea of how this works, cook yourself an extra large pizza. 

If you have no pizza in this is an excellent opportunity to order a take away pizza – tell your waistline a mad scientist made you do it – but ask them not to slice it. 

Once you have your large pizza take it into the kitchen*. Now slice out some of the pizza – enough that you can get a camera, iphone, or your head, inside the pizza. I’d go for a couple of traditionally shaped wedges taken centre to rim, but if you’re hell bent on scientific accuracy (and a monster) you could slice an appropriately sized circular chunk out, centred on a spot two thirds from the middle. Take the remaining pizza, and put your image-making thing inside the gap in the pizza. 

A cat is not an imaging device, although this is hilarious.

If you are using your own eyes and went for the scientifically accurate circular chunk removed two thirds from the centre... you are probably now wearing the pizza on your head. But as funny as that is it doesn’t affect the outcome.

Anyway, get your eyes or lens precisely on the level of the pizza, and take a picture (or just look through the scalding sauce now dripping onto your face): You can see that the disk shaped pizza is a band stretching across the 'scamera picture. 

Now imagine that your pizza is two hundred thousand light years wide and faintly glowing, and you’ll understand how the gigantic, disk shaped Milky Way galaxy can look like a band of light. 

You also have a fair bit of mess, but at last you have (slightly mutilated but hopefully still edible ) pizza. 

* This is not actually necessary, but ...I'd recommend it. 
**When I say get away from cities... that's a bit excessive, but you could. 

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