OFFLIST: Galactic Year: (was: Re: Reviving an old tradition)
|Date:||Saturday, February 11, 2006, 20:10|
--- In email@example.com, Henrik Theiling <theiling@...> wrote:
>>>>The modern/space-age section would include terms like [snip]
>>>>galactic year [snip]
>>>What's a galactic year?
>>The time it takes for the galaxy to revolve once about its axis.
>>Some hundreds of millions of years, I forget exactly. --larry
Unfortunately this "definition" defines nothing, because galaxies are
not rigid objects and do not "revolve" as a unit about their axes.
(If they did, "rotate" would be the word to use.)
(By analogy if you tried to define a "solar day" as the length of
time it takes the Sun to "rotate" around its axis, you'd get
different answers at the solar poles than at the solar equator. The
sun's equator spins around its axis faster than its poles do; this
tangles up its magnetic field lines something fierce, and
straightening them out involves lots of prominences and sunspots and
Different layers of a galaxy revolve in different periods; the stars
closer in take less time to revolve around the galactic center, the
stars further out take more time to revolve around the galactic
(BTW the orbits are ordinarily elliptical rather than circular, and
they precess at different rates, too. If the long axes of these
elliptical orbits are all parallel, the galaxy looks neat and
organized; a while later, after the ones closer in have turned at a
different rate than the ones further out, the galaxy looks spiral.)
You could define a "galactic year" as the length of time it takes
_our_ star to revolve around _our_ galaxy's center. That would have
a definite meaning at each star, but would probably change when you
moved to a different star -- probably not by much as long as the two
stars were close to each other and in the same "Population".
>Simple learn the Galaxy Song from Monty Python's Meaning of Life by
>We're thirty thousand light years from galactic central point,
>we go round every two hundred million years.
_We_ go around once every two-hundred-million years.
Stars closer in to the center take less time to go around;
stars further out from the center take more time to go around.