[meteorite-list] 8000BC Big Dipper Petroglyph: Evolution of star positions

[Chris Peterson]

Hi Larry-

I'm not quite sure what you are disagreeing with. My use of the term 
"asterism" as opposed to "constellation"? Maybe I was unclear in some 
way about what I said, because I can't find a disagreement between what 
you said and what I said.

To be clear, I was discussing how asterisms change with time, due to 
proper motion, not how constellation boundaries change. I was merely 
pointing out that most star charting software has the primary purpose of 
showing how the sky looks on a specific date, as determined by looking 
at the orbit of the Earth, the rotation of the Earth, precession, the 
observer's position on the Earth, etc - and this can only be accurately 
determined over a few thousand years. Some sky charting software also 
adjusts the positions of the stars based on proper motion, but even 
though that can be done over millions of years, many programs don't 
support this because they deliberately limit the valid epochs to those 
for which accurate charting is possible.

Chris

*******************************
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

On 9/12/2011 8:53 AM, lebofsky at lpl.arizona.edu wrote:
> Hi Chris:
>
> I rarely disagree with you, but I do this time (sort of).
>
> There IS an option in "Starry Night" to look at the constellations over
> time (using proper motion). The Big Dipper (an asterism, not a
> constellation), looks very similar in 8,000 BCE to what it looks like
> today. Chris: it is called constellations over time.
>
> And "using a different method" (or whatever the statement was) to say what
> the Big Dipper looked like then makes no sense. I doubt that the
> petroglyph could have been used to depict what the Big Dipper looked like
> 100,000 years ago.
>
> Larry
>
>> That's because precise calculation of the positions of the planets-
>> including Earth- is only possible for a few thousand years. Beyond that,
>> the chaotic nature of orbital dynamics in a multiple body system becomes
>> dominant. No software, professional or amateur, can provide an accurate
>> topocentric sky map for more than a few thousand years either way from
>> the present.
>>
>> That is quite different from estimating the shapes of asterisms over
>> time. In most cases, the proper motion of the brighter stars is well
>> known, and makes it possible to know what constellations will look like
>> over periods of millions of years. But since the purpose of sky charting
>> software is primarily to produce accurate topocentric star maps, they
>> generally limit themselves to a much shorter period. They won't let you
>> look at the Big Dipper 100,000 years ago, not because they can't
>> accurately render it, but because they can't accurately position the
>> entire asterism in the sky.
>>
>> Chris