by neufer » Fri Sep 27, 2013 12:35 pm
Nitpicker wrote:
At a guess, it is perhaps "Annual Proper Motion in arc seconds" as measured from the centre of M81, not as measured from our sky.
A tenth of an arc second is a 13 millionth of 360º
so that would mean galactic orbits taking ~ 26 million years.
That is STILL way too fast by an order of magnitude!
(However, Adriaan van Maanen was, in fact, "measuring" proper motions as best he could from a hundred times further away and got motions that were too fast by three orders of magnitude.)
Nitpicker wrote:
I've never heard of Proper Motion being relative to anything other than the Solar System, but things may have been different in 1921.
Things were indeed different a hundred years ago... astrometric measurements were
much cruder.
The upcoming Gaia space observatory represents
the first time humans will be able to observe proper motions BEYOND our own galaxy:
http://en.wikipedia.org/wiki/Gaia_%28spacecraft%29 wrote:
<<Gaia is a space observatory to be launched by the European Space Agency (ESA) in November 2013. The mission aims to compile a 3D space catalogue of approximately 1 billion stars, or roughly 1% of stars in the Milky Way. Successor to the Hipparcos mission, it is part of ESA's Horizon 2000 Plus long-term scientific program. Gaia will monitor each of its target stars about 70 times to a magnitude 20 over a period of 5 years. Gaia will create an extremely precise three-dimensional map of stars throughout our Milky Way galaxy and beyond, determining the positions, distances, and annual proper motions of 1 billion stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag.>>
[quote="Nitpicker"]
At a guess, it is perhaps "Annual Proper Motion in arc seconds" as measured from the centre of M81, not as measured from our sky.[/quote]
A tenth of an arc second is a 13 millionth of 360º
so that would mean galactic orbits taking ~ 26 million years.
[color=#0000FF]That is [b][u]STILL way too fast[/u][/b] by an order of magnitude![/color]
(However, Adriaan van Maanen was, in fact, "measuring" proper motions as best he could from a hundred times further away and got motions that were too fast by three orders of magnitude.)
[quote="Nitpicker"]
I've never heard of Proper Motion being relative to anything other than the Solar System, but things may have been different in 1921.[/quote]
Things were indeed different a hundred years ago... astrometric measurements were [b][u]much[/u][/b] cruder.
The upcoming Gaia space observatory represents [b][u]the first time[/u][/b] humans will be able to observe proper motions BEYOND our own galaxy:
[quote=" http://en.wikipedia.org/wiki/Gaia_%28spacecraft%29"]
<<Gaia is a space observatory to be launched by the European Space Agency (ESA) in November 2013. The mission aims to compile a 3D space catalogue of approximately 1 billion stars, or roughly 1% of stars in the Milky Way. Successor to the Hipparcos mission, it is part of ESA's Horizon 2000 Plus long-term scientific program. Gaia will monitor each of its target stars about 70 times to a magnitude 20 over a period of 5 years. Gaia will create an extremely precise three-dimensional map of stars throughout our Milky Way galaxy and beyond, determining the positions, distances, and annual proper motions of 1 billion stars with an accuracy of about 20 µas (microarcsecond) at 15 mag, and 200 µas at 20 mag.>>[/quote]