Monday, March 10, 2008

Astronomical distances are .... (well) astronomical

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How can someone begin to describe astronomical distances?

We sometimes use fancy compound terms like outer space, deep space, interstellar space, and intergalactic space.
Image used with permission of Jim Boles.
You can be factual like NASA's page on cosmic distances or Wikipedia's page on astronomical distances.

You can use humour to make your point, like Douglas Adams in the The Hitchhikers Guide to the Galaxy:
Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is.
You can even be darkly poetic, like Michael Moorecock's "Dark Corridor" introduction to Hawkwind's "Space is Deep":
Space is infinite, it is dark
Space is neutral, it is cold
Stars occupy minute areas of space
They are clustered a few billion here
And a few billion there
As if seeking consolation in numbers
But none of these help you visualize how big space is!

Think of the Earth as a shiny blue marble (standard marbles are 1.25 cm or 1/2" in diameter - smaller than the one in the photo). Below are some approximate comparable scale sizes for objects /distances in our solar system:
  • The Moon - 3.5mm or 0.13"
  • Mercury - 5mm or 0.2"
  • Mars - 7mm or 0.37" - a chickpea
  • Venus - another marble (a bit smaller than Earth)
  • Low Earth Orbit Satellites - basically the same size as the marble
  • Uranus -5 cm or 2" - an orange
  • Neptune - another orange (a bit smaller than Uranus)
  • Communications satellites (geosynchronous) - 8.25 cm or 3.25" - a hockey puck
  • Saturn - 12 cm or 4.75" - grapefruit
  • Jupiter - 14 cm or 5.5" - a slightly bigger grapefruit
  • Saturn's visible rings - 26.8 cm or 10.5" - an old LP record
  • The Sun - 1.36 m or 54" - a small weather balloon or large exercise ball
Okay, that was fairly easy. So you might be considering a scale model of the solar system using fruit, vegetables and some odds and ends from a garage sale. Now you need some real estate.

Wait for it! Still using our marble for Earth, the Moon would be 0.75 m or 29" away. Or about an adults' arms length. Below are the approximate distances of things from the Sun using the same scale (I'll give up on Imperial measures here):
  • Mercury - 136 m - a bit larger than a football field
  • Venus - 213 m
  • Earth - 293 m - a bit shorter than the Eiffel Tower
  • Mars - 488 m - a bit shorter than the CN Tower
  • Jupiter - 1.6 km
  • Saturn - 3 km
  • Uranus - 6 km
  • Neptune - 9 km
  • Pluto - 14 km at its' farthest
That is a very long hike from our little blue marble!

Now if you wanted to place these to scale and allow space for the full orbits of all the planets and Pluto (it was demoted to a dwarf planet), the Sun would need to be in Mel Lastman Square in North York at the center of a 28km circle running from Lake Ontario to north of the David Dunlap Observatory in Richmond Hill!
If you're wondering what that looks like, take a look at this map of the Solar System in Toronto.
And it gets worse:
  • Proxima Centauri the nearest star at around 4.2 light years would be 77,000 km away from our marble. On scale, the marble is about 1/5 of the way to the moon!
  • Betelgeuse a red supergiant about 430 light years away would be 7,883,333 km away. On our scale, that's about 1/8 of the way to Mars at it's closest or about 1/20th the distance to our Sun.
  • The Milky way galaxy is about 100,000 light years across. So our marble is now about 1.8 billion km away or outside the orbit of Saturn.
Galaxies are millions of light years apart, the Local Group of galaxies containing our own Milky Way is about 10 million light years across. On the scale of our marble, the local group only gets us about 1/200th the way to the nearest star! The Universe itself is billions of light years across.

I'll stop now before my head hurts trying to visualize all of this in terms of our little blue marble.

Astronomical distances operate on levels. Just when you think you've gotten used to one of them, you'll see another and be humbled all over again.

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