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Re: OT: Celestial maps

From:Mark J. Reed <markjreed@...>
Date:Tuesday, January 15, 2008, 14:06
On Jan 14, 2008 11:04 PM, Herman Miller <hmiller@...> wrote:
> Many of the bright stars zip across the sky pretty quickly. I've got > Starry Night Pro running now, which only goes back to 99998 BC,
That's an odd date. I very much appreciate systems that are up-front about the limits of their range (having seen too many whacked-out astrological theories "supported by astronomical data" via extending calculations far past their actual precision), but 99998? Why not 99999? Or even 100000 BCE (=-99999 CE)?
> Going back millions of years
Minor note: the plural is only barely appropriate to Charlie's question, since the Pleistocene begins less than 2 million years ago. As to the question about the linear calculation, it is my understanding that the "proper motion" of a star is linear by definition. Since over the course of 200 million years the star will make a circle in the sky and come back to where it was(?), the actual path can't be linear forever, but we lack the tools to chart the whole thing. Besides the proper motion, there is also the radial velocity to consider; a star may be brighter or dimmer in the night sky of long ago because it has moved closer or further away. (?) Since the orbital period of the galaxy is about 200 million years, does that mean the night sky from Earth would have looked pretty much the same 200 million years ago as it does today, at least as far as the fixed stars that haven't gone supernova or something? -- Mark J. Reed <markjreed@...>


Lars Finsen <lars.finsen@...>