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, [email protected] 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
______________________________________________
Visit the Archives at http://www.meteoritecentral.com/mailing-list-archives.html
Meteorite-list mailing list
[email protected]
http://six.pairlist.net/mailman/listinfo/meteorite-list
