The milky way, that is? Don’t we know?
I’m assuming your question is in reference (however tangentially) to Chronos’s Staff Report on the shape of the galaxy.l
In a sense the question is not meaningful. Our galaxy is disc shaped. If you were outside the galaxy, looking at it, you will either see a disc or an edge.
If you see it as a disc. you will view it as rotating clockwise from one side of the disc or anti-clockwise from the other side.
If you see it edge on, it depends on your orientation which part seems to be moving towards you, and which is receding. Suppose the top is moving towards you. Turn sideways or upside down and the top becomes left or bottom.
So, if I say that the galaxy is turning clockwise or anti-clockwise - what does that mean, even here on Earth?
Of course the question is meaningful! Sure, you have to define a reference point. That’s also true when we talk about the direction of revolution for the Solar System, the Earth, and even the clock on your wall. If I’m not mistaken, it is typically assumed that the vantage point is “North”, above the Galactic disk, looking “South”.
Anyway, I’m not sure what technique was used when they first determined the direction, but one method they could use would be to measure the doppler shift of stars closer to the galactic center. Since those stars will be revolving more quickly than us slow pokes in the outer areas, you could see that the stars to the right of the center are approaching and the stars to the left are receiding. Or vice-versa - I’m not sure which way it’s revolving myself.
Ahh, but which side is “above” the Galactic Disk?Whichever one you pick, I’ll run around to the other side (be patient, it might take me awhile) and claim that the side I
'm on is the “above” side and you’re “below” me.
At the very least, one can point to some point in the sky, and say that the Galaxy rotates in a certain direction as seen from that point. One might ask, for instance, “In what direction is the point from which the Galaxy will appear to rotate counterclockwise”, and then call that direction “Galactic North”.
Now, as to how that’s determined: Newton tells us that things in closer to the center of the Galaxy should be moving faster than things further out, and we can easily measure the radial velocities of various parts of the Galaxy (at least, to the extent that we can see them at all) using the Doppler shift. If we look at something closer than us to the center, then the direction that part is moving relative to us is the direction that the Galaxy is rotating.
Actually, that’s one of Kepler’s Laws. Or it would be if the galaxy was close enough to a point-like object for Kepler’s Laws to apply.
From what I understand, they first found the direction and speed the Sun travels. They did this by measuring the proper motion and doppler shift of a goodly number of stars in all parts of the sky. If you plot the positions of those stars on graph paper with an arrow showing the direction and magnitude of their proper motion, it becomes very obvious what direction the Sun is traveling. Do something similar with doppler shifts and you get an idea how fast the sun is moving.
In the actual case, they did a lot of mathematical calculations to get more precise numbers than they could get from graphs. Once they found the Sun’s motion, they could subtract it from that of the other stars to get those other star’s real motions. It turned out that almost all nearby stars are going at roughly the same speed in about the same direction. And that told us which way the Milky Way was rotating.
That won’t help much, because all of the individual stars we see are also part of the Galaxy, and so also rotating with it. And if we use instead objects outside our Galaxy, we have no way to correct for the motion of the Galaxy as a whole.
Kepler gave us his Laws for a central mass, but Newton told us why those laws work. Since we now know why Kepler’s laws work, we can extend them to other mass distributions. We can also look at other spiral galaxies, and notice that in all of them, the closer stars to the center move more quickly.
WillGolfForFood - the definitions of up and down, North and South, clockwise and counter-clockwise, etc are purely arbitrary. To communicate effectively, scientists establish conventions. Sure, you could run to the other side of the Galaxy and claim that YOU are really the one who is “above” the galactic plane. And all the scientists will pat you on the head, tell you that you did a good job of demonstrating the arbitraryness of the terminology, and then go back to what they were doing.
Nobody is bothered by the arbitraryness of it. They just want to make sure that the conventions are established in a non-ambiguous way. E.g. if I define North as “toward the top of the map”, it will be misinterpreted if somebody has a map with Antartica toward the top.
You start with any one of several non-ambiguous (but arbitrary) ways of defining which of the Earth’s poles is North. You can use the Earth’s magnetic field, star constellations, continent placements, etc. The rest of the definitions fall out of that: “Above” means “more Northerly”. Direction of rotation is determined from above looking down. If you need a non-ambiguous definition of “clockwise”, you can define it as the opposite direction of the Earth’s rotation (as seen from above.)
Here’s a little more on direction of rotation:
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980225a.html
P.s. - I hesitated before listing magnetic fields as a non-ambiguous way of defining North. But I decided that one could define Magnetic North using electric current. I.e. a wire with current flowing through it can be used to establish magnetic North. But then the problem becomes defining electrical positive and negative. That can be accomplished with Chemistry. E.g. the Carbon rod in a Carbon/Zinc battery is defined to be positive. Once you’ve defined electrical polarity, it becomes possible to define magnetic North, and then geographic North, and then “above” and “below” the Galactic plane.
Magnetic fields are a lousy way to define north and south, because they’re subject to change and even reversal. North for any object is defined as the direction from which the object is viewed as rotating counterclockwise (clockwise and counterclockwise are already unambiguous). The most common system of celestial coordinates, right ascension and declenation, is based (in part) on terrestrial north. The question of “which way is the Galaxy rotating” then can be phrased as “what are the Right Ascension and Declination of galactic north?”.
Ah, well - at least I get patted on the head a lot.
That’s one way of defining north. Intuitive, mathematically useful, simple. Able to be determined even when you are not on Earth.
The IAU, though, defines north as that pole which points to the north ecliptic hemisphere. Those objects that rotate clockwise about their north pole are said to be rotating retrograde. How a group of scientists managed to come up with this definition is beyond me, but if you think this is a silly definition, I won’t argue with you.
The galaxy, by this definition, rotates retrograde.
See, that’s what I get for assuming that scientists are logical. While it’s undeniable that the Earth and the Galaxy rotate in opposite directions, it seems a bit silly to call the Galaxy the backwards one. Ah, well, no sense in arguing with the IAU.
That’s not scientific illogic; that’s human ego, putting the Earth at the center of the Universe!
Once again, the arbitrariness of conventions. You can either assign planets a North Pole based on which is viewed as rotating CCW, or you can define the North Pole as the one pointed toward the Galactic North. Either or, as long as you’re consistent within the system.
Of course the IAU def makes it very difficult when the planet has a skewed axis - the poles are pointed toward the galactic equator. Now which is North?
At the risk of taking a joke too seriously, scientific conventions are often established before there is enough information. For example, when electric current was first being investigated, it was known that electricity “flowed”, but it wasn’t obvious which direction it went. One charge was arbitrarily designated positive and the other negative. Current was said to flow from + to -. It was subsequently discovered that electric current physically consists of electrons, which move from - to +. Whoever first established the convention (Ben Franklin?) had a 50% chance of getting it right, and he lost.
Fortunately it doesn’t really matter. One can also think of current as being the “lack” of electrons that flows from + to -. When talking about semiconductors, one sometimes talkes about “holes” that do that. But the simple truth is that for 90% of electrical work, the physical reality doesn’t matter. All that matters is that the established convention is followed by everybody. (Remember positive ground cars?)
So, is Galactic North so named because of an egostistical earth-centric attitude? Not really. Terestrial North was defined long before humans had the slightest clue as to the nature of the universe (or even the nature of their own planet). When people wanted to talk about things in the sky, they just naturally extrapolated outward.
When the nature of the Galaxy became clearer, it made no sense to change the established conventions just to be more P.C. Better to stick with an accident of history that everybody already agrees on, rather than trying to get everybody to change to a new and theoretically more proper convention. (Why does that last sentence make me think of the Metric System in the U.S.?)
Except either way of defining Galactic north would be consistent with the established conventions, depending on which way one defined the conventions. Is terrestrial north “The direction from which Earth is seen to rotate counterclockwise”, or “The direction from Earth to [symbol]a[/symbol] Ursa Minoris”? I would argue that the former more accurately describes the convention for terrestrial North, but Galactic north is defined in terms of the latter.
This reminds me of a SF story I read which took place on aplanet where the sun rose to the West. This seemed silly to me, since “West” is pretty much defined to be the direction in which the sun sets.