zog741 wrote:
-BO- wrote:
How about that fixed star question in CD last night perhaps 6c? The reason a star appears to be fixed is because it's so far away from Earth? Really?
Yes, that seems exactly right. What other answer did you have in mind?
I did not play this CD by the way; I came in late for BB. But what you report as BT's answer sounds fine.
-- RWM
I agree with Zog and Buzztime; distance is exactly why the stars appear fixed. That, and the stars of the Milky Way galaxy close enough for us to see revolve more or less uniformly over the course of 200 million years
I'll use Sirius for my example, since it's close, significant, and observable by almost anyone south of the Arctic Circle. Sirius lies 8.6 light years away, or 5.879 trillion miles, or 63,250 AU, whichever you prefer. The angle formed by Earth-Sirius-Sun measures around 0.000906°. Sirius' parallax is roughly 0.377 arcseconds, hence six months of the Earth's movement around the Sun yields a change in the position of Sirius equal to 1/6 the size of Neptune's disk -- which I can barely resolve in binoculars -- and another six months causes the star to swing back to almost the original position. Obviously, only professionals using sophisticated equipment are going to notice changes so slight. Hell, most general-use charts and catalogues are only updated every 50 years
Even if we assume the Sun is moving relative to Sirius rather than the galactic center, and we plug in Zog's 12 miles/second (yielding an additional 378.7 million miles, or 4 AU), it still would not be enough to change the position of Sirius in the sky by more than one Neptune disk per year, or the angular diameter of the Moon per 1000 years