[ExI] Asteroid on track for possible (probability of 1:25) Marshit

[Alejandro Dubrovsky]

On Thu, 2008-01-03 at 14:26 -0600, Bryan Bishop wrote:

> I have yet to understand just how it is that astronomers are able to 
> specify so precisely what stars they are observing, since the planet is 
> revolving around the local star, and the star is revolving around the 
> galaxy and everything is shifting, and I've never seen a positional 
> database on the internet before. I would think we would need to 
> calculate observation-altitudes, telescopic angle, the precise time and 
> position, etc. etc. But instead, apparently the relative position of 
> the stars is stable enough to be conveyed as "eh, look left to that big 
> bright one each night" ??


At a parsec (three and a quarter light years) an object would have a
parallax of one arc second from one side of the sun to the other on the
earth orbit, around 300 million kms.  One arc-second is not much.
Pretty much everything is much further than a parsec away so in general
the revolving around the sun bit won't throw you too far off the mark. 

The sun moving through the galaxy bit is dealt by specifying coordinates
not only by the right ascension/latitude pair, but also by the year
standard for which the coordinate is valid, which gets updated every 50
years.  Most coordinates nowadays I think use 2000, old catalogues use
1950, etc.  Since the movement of the sun in the galaxy in 50 years is
small, it can probably be assumed to be linear, and you can probably
interpolate nearby star positions quite accurately like that.  (The
'probably's there are because I'm making those bits up)

In large telescopes the final bit of accuracy is provided by guide
stars, relatively bright far away stars that everyone can agree are at
some designated coordinate The telescope can lock on to the guide stars
during the observations and keep a fairly stable and well known frame of
reference as the earth rotates.  

I can't remember the names of many databases online.  SDSS is a huge
survey of the northern sky (http://www.sdss.org/).  USNO-B is an older
one from the southern sky.  

(I am not an astronomer, I just worked for them for a period.  The above
could be way off)