Re: To put it another way

From: Brian Tung (brian_at_isi.edu)
Date: 10/12/04 

Date: Tue, 12 Oct 2004 17:45:44 +0000 (UTC)

Martin R. Howell wrote:
> One of my favorite little astro thingies goes like this:
>
> If the Milky Way were reduced in scale such that the Sun assumed the size
> of a basketball, then it would take a page the size of the North American
> continent on which to image the rest of the galaxy.

I don't think that can be quite right. If the rest of the galaxy (which
is about 100,000 light-years across) is the size of North America (about
8,000 km across, generously speaking), then a basketball (about 25 cm, or
0.00025 km, across) would represent a volume about 1/320 of a light-year
in diameter, give or take. That is about 200 AU. I think if you take
a sphere centered on the Sun, and reaching out as far as the aphelion of
the furthest known KBO, that might be in the vicinity of 200 AU.

I remember the analogy you're talking about, but I don't remember exactly
how it goes, either.

Working in good old English units: Since the number of inches in a mile
is very nearly the same as the number of AU in a light-year, if we model
the Sun and Earth as being an inch apart, Neptune would be about 30 inches
away, and alpha Centauri something over 4 miles away. The galaxy would
be 100,000 miles across, which would stretch nearly halfway to the real
Moon. The Virgo cluster would be about as far as Mars is at opposition,
and the nearest quasars maybe as far as Uranus (I forget how close the
closest ones are, precisely).

Brian Tung <brian@isi.edu>
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