charles@utastro.UUCP (Charles Sandel) (08/20/84)
It's worth a trip to the country to see
the summer Milky Way. More about it --
right after this.
August 18: The Milky Way
If you go into the country this month,
plan for a moonless night when the air is clear and still.
Look overhead -- and there you'll see a hazy starlit band arcing across the
sky -- the glittering arc of the summer Milky Way.
If you have binoculars, you can prove to yourself that the haze is really
stars -- countless stars blurred together over colossal interstellar distances.
These stars only appear close together because they're very far from Earth.
When we look at the Milky Way, we're looking into the plane of our own galaxy.
We see the Milky Way as a starlit trail spun across the sky -- but it's
really a huge star-island with a compact center and winding spiral arms.
The Milky Way is a so-called spiral galaxy -- one of billions in the universe.
In summer, our night sky is turned so that we face toward the center of our
own galaxy.
The galactic center lies due south in August,
in the direction of the constellation Sagittarius.
In a dark sky, you can see that the starlit trail
widens in the south into a broad boulevard of stars.
It's tempting to imagine that these very stars reside in the galaxy's core.
But they don't.
The actual center lies some 30 thousand light-years from us and is wholly
obscured by impenetrable clouds of gas and dust.
When we look southward in the August sky, we're really seeing stars in the next
spiral arm inward from ours, called the Sagittarius Arm.
Script by Deborah Byrd.
(c) Copyright 1983, 1984 McDonald Observatory, University of Texas at Austin
--
*>> Charles Sandel <<*
uucp: {ut-sally, ut-ngp, noao, charm}!utastro!charles
arpa: charles@utastro.UTEXAS.ARPA charles@ut-sally.UTEXAS.ARPA
at&t: (512) 471-4461 x439wmartin@brl-tgr.ARPA (Will Martin ) (08/20/84)
> In summer, our night sky is turned so that we face toward the center of our > own galaxy. The above quote caused me to begin wondering... Is this situation (that Earth is on the side of the Sun toward the galactic center during Northern Hemisphere summer) something that is pure chance, or is there a reason for it? Is this one of those variable astronomic phenomena that move in long cycles, like the pole star changing as the Earth's axis precesses? If so, do we just happen to be around when this is the case, and in 50,000 or 100,000 years our solar system will have rotated as a whole, and Earth will be on the galactic-center side of the Sun during Northern Hemisphere Winter instead of Summer? (That year-figure is a random guess; what is the actual cycle length, if this is true?) Or is this situation sort of "fixed" with respect to the galaxy? By the way, is there any force or influence that encourages the plane of the planetary orbits to fall in line with the galactic plane? Or is it random chance that our orbit plane is not at right angles to the galactic plane, or some other arbitrary angle? Please post any explanation or discussion. Thanks, Will Martin