

An 18 Billion Mile Journey is almost complete. - sprout
http://scienceblogs.com/startswithabang/2010/08/an_18_billion_mile_journey.php

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petercooper
_Which means, for the first time since its discovery, Neptune is about to
return to the same position in space that it occupied the day it was
discovered. And what date will that be?_

It will return to the same position within its orbit relative to the sun, but
surely not the "same position in space"? (The solar system has moved a little
further around in its orbit within the Milky Way too.)

I'm aware I'm questioning a qualified _astrophysicist_ here - something I am
not - so I'll blindly assume it's something to do with default frames of
reference until corrected otherwise ;-)

~~~
celticjames
Good source for a layman's explanation of how we measure where things are in
the sky is Astronomy Cast:
[http://www.astronomycast.com/astronomy/ep-171-solar-
system-m...](http://www.astronomycast.com/astronomy/ep-171-solar-system-
movements-and-positions/)
[http://www.astronomycast.com/astronomy/ep-170-coordinate-
sys...](http://www.astronomycast.com/astronomy/ep-170-coordinate-systems/)

~~~
nooneelse
Astronomy Cast is great. I wish I had a similar "not just what we know, but
how we know what we know", friendly conversation type podcast for more
subjects.

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mturmon
It may be interesting to note that one way objects are discovered now is by
automated sky surveys ("event factories" or "robotic telescopes") like this
one:

<http://voeventnet.caltech.edu/feeds/Catalina.shtml>

The essence is to make multiple passes over the sky, comparing current images
with past images. Large image-differences represent moving objects, which are
looked up in a database. If they are new, they are entered into an event
queue, represented above. In principle, other robotic telescopes can scoop up
promising events and follow up on them.

