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

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Mon, 12 Sep 2011 09:54:35 -0700
Message-ID: <2723de68de72ce2929bca2f641ae6a92.squirrel_at_webmail.lpl.arizona.edu>

Hi Chris:

VERY Sorry!

Just pointing out (it was sort of in the back of my mind at the time),
that Starry Night DOES have a special routine for looking at the
constellations and asterisms back in time. That was all!

If you use the Starry Night to go back 100,000 (actually the limit is
99,999 years, but who is counting), the diagram that Robert Juhl showed of
the 7 bright stars of the Big Dipper is sort of correct. But this is
100,000 years ago, not 8,000. I was somewhat confused by Robert's comment
that Wu Jiacai used "different assumption." What other assumptions are
there that would significantly change the proper motion of the start over
this period of time (I think the program actually takes the distances to
the stars as well as their direction of motion, so some stars get closer
and move faster while others get farther away and appear to move slower).

Again, Chris, I apologize, I only meant that the program does let you
"look back" 100,000 years.

Larry

> 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
>
Received on Mon 12 Sep 2011 12:54:35 PM PDT