"If you use the Starry Night to go back 100,000 (actually the limit is
99,999 years, but who is counting)"
Larry, you can go back 100,000 years with Starry night - it goes back
to 99,999 BC (or is that -99,998). So, you can go back more than
102,000 years including the magic 100,000 years ago...
But it would be hard to know for sure at a glance if they are right ;-)
Kindest wishes
Doug
-----Original Message-----
From: lebofsky <[email protected]>
To: Chris Peterson <[email protected]>
Cc: meteorite-list <[email protected]>
Sent: Mon, Sep 12, 2011 12:55 pm
Subject: Re: [meteorite-list] 8000BC Big Dipper Petroglyph: Evolution
of star positions
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, [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
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