Precise can be an imprecise term

Recently I read an article about some conclusions regarding the track of the Andromeda Galaxy through space relative to our own galaxy.  The basis for the conclusions were what the article called “precise” measurements of the distances to stars in Andromeda.  What does precise mean when measuring the distance to and location of stars in another galaxy?  A little research established that the various methods that have recently been used to establish the distances to stars in Andromeda had a margin of error that averaged out to plus or minus .06 mly.  That sounds good, right?  Only mly stands for mega-light year.  One million light years.  .06 mly is sixty thousand light years.  A light year is equal to roughly six trillion miles or a bit less than ten trillion kilometers.  Sixty thousand light years, then, is about 36 followed by thirteen zeros miles, or 6 followed by fourteen zeros kilometers.  That’s the average margin of error, best case, if our measuring methods are as good as we think they are (and one of those measurements used cepheid variables, the once “standard candles” used a rulers to measure interstellar distances that were recently discovered not to be so standard after all, meaning the ruler we had been using to measure the universe was actually a rubber ruler).  Our own Milky Way Galaxy containing hundreds of billions of stars is estimated to be about 100,000 light years across, so that’s another way to visualize the margin of error in distances to stars in Andromeda.  So, precise?  It depends upon what you mean by “precise.”  I’ll probably take those recent conclusions with a grain of salt.  (How big is a grain of salt?  Well, that depends…) 

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