> Now, how big are the light echoes? Bill had used the value of 450 and 1000
> light years.This was not the radius and I used his values for my example
> rather than looking it up. In 1988 the light echoes were 60 and 90
> light-years across. But they expand (due to the geometry) at 1-3 light
> years per month (Bill, this is not faster than light travel). Today the
> outer light echo easily could have up to 400 light years diameter.
That may have answered my question. If the outer rings are expanding
then I understand what you are saying. I did not pick that up from
Goldsmith; in fact he did not mention the expansion of the rings.
Goldsmith does show two photos of the rings, one dated 13 February,
1988, and the other dated 16 March 1988. The position of the outer ring
relative to foreground or background stars in each photo appears to be
absolutely identical. Is that because an angular distance of one to
three light years is miniscule at a distance of 169,000 light years? If
the outer ring were 90 ly across in 1988 as Glenn says, I would not
expect to see any difference in one month.
If what I understand is correct, then I've got it now, and I thank you
gentlemen for your time!