[USMA:29417] Re: Prefix use outside SI

[Bill Hooper]

The light year (ly) was defined before the metric system had extended 
it's range of prefixes to large enough values to cover the distances 
encountered in astronomy. Astronomers may be faulted for not replacing 
it with appropriate SI units now, but there was sound reason for 
creating a very large unit of distance at the time when it was created. 
(The relation between the distance in light-years and how far back in 
time we see the object is of interest, but I'm not sure it is 
over-riding.) Perhaps a better, pre-metric unit could have been created 
at the time, but that is neither here nor there. Replacing it with the 
petametre (Pm) is the only issue we need to be concerned with now.

The light-year is equal to 9.461 Pm. Anything measured in light-years 
may just as easily be measured in petametres. There apparently are 
distances of interest that are currently expressed in the thousands and 
millions of light years. These could be expressed in exametres (Em, 
where 1 Em = 1000 Pm) and zettametres (Zm, where 1 Zm = 1000 Em). But, 
as it is true with any other widely varying measurements, for 
comparison sake it is oftern desirable or necessary to use the same 
unit for all of a particular set. Therefore, in discussing relative 
distances to stars, other parts of our galaxy and other "nearby" 
galaxies, the petametre might be used for all of them, despite the fact 
that the distance to other galaxies goes into the millions of 
petametres and more.

   Distances to ...
the nearest star, 38 Pm
the center of our galaxy, 280 000 Pm
the Andromeda galaxy, 19 000 000 Pm = 19 million Pm
I have not heard of any astronomical distances that were expressed in 
light-years with SI prefixes attached (e.g. megalight-year).

Regards,
Bill Hooper
Fernandina Beach, Florida, USA
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Make it simple; Make it Metric
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