Quote:
Originally Posted by Robert Tulip
I am always eager to see references.
|
I haven't found an accessible copy of Frank Bash's original paper on-line. The figures I've given you come mainly from him,
via Henbest and Couper's
The Guide To The Galaxy. (Recommended, though now just a little dated.)
Quote:
Originally Posted by Robert Tulip
So the solar system keeps in a precessing plane with the universe while it orbits the galaxy? For a minute I had in mind it spinning on a string entrained from the galactic centre. With the apparent movement of the galaxy around the earth over 240 million years, the galactic centre must drift north or south across the ecliptic every 120 million years.
|
The plane of the solar system will stay in the same orientation unless some force acts to change it. I certainly doubt if it will sail around keeping itself edge-on to the galactic centre. So a very long-lived observer would see the galactic centre cycle from ecliptic north to ecliptic south and back again, yes.
Quote:
Originally Posted by Robert Tulip
Is the galactic centre ever near one of the earth's celestial poles?
|
In 8,000 years or so, the celestial poles will have precessed so that they're lying close to the galactic plane, though not actually on it. That will repeat every 26,000 years. So we could imagine that there might come a time, quarter of a galactic orbit from now, when precession would bring one pole close to the galactic centre.
Quote:
Originally Posted by Robert Tulip
If the plane of the zodiac against the galaxy is fixed like the tilt of the orbit of the earth against the sun, does this mean there are 'seasons' in the galactic year, and we are now at an equinox, just as the sun crosses the equator at the equinox?
|
Certainly the position of the galactic plane in the sky goes through cycles, with the precession period and the galactic orbital period.
Quote:
Originally Posted by Robert Tulip
On this model, in 'summer/winter' in 60 million years, will the galactic centre have moved into the south celestial hemisphere or the north?
|
From my diagram, the galactic centre looks to be moving south of the ecliptic at present. It's already in the southern celestial hemisphere.
Quote:
Originally Posted by Robert Tulip
Does this mean our whole spiral arm is flapping up and down with this period?
|
No, for several reasons. The arm is a statistical structure: just a density wave that doesn't share the motion of its component stars. And the arms are delineated by bright, short-lived stars which pass through their entire main sequence career in a few million years, far shorter than the vertical oscillation period. I'm also not sure if the vertical period is the same for all amplitudes of oscillation (like a pendulum), or if we can expect a mixture of oscillation periods.
Grant Hutchison