At 12:00 am on Jan 1, 2015, the earth is in a specific point in its orbit around the sun. Relative to the sun, the earth returns to that point exactly one year later. But, during that same time, the sun has been moving within the Milky Way galaxy, so those two points in space are some distance apart? I tried to simplify the question by simply asking about the sun (ignoring the fact that the galaxy is moving, also, ad infinitum. Was that correct? If so, what is the distance the sun has traveled in a year’s time?
According to this:
Assuming that’s correct, simply multiply the above by 8765.81, the number of hours in a year and you’ll get the number of miles per year the sun travels around the galactic core.
This topic on galactic orbits has always fascinated me. The idea of our solar system orbiting the Milky Way galaxy and oscillating up and down as it does is mind-blowing considering the length of years involved. I never get bored reading about galactic orbits and look forward to reading what others here have to say or add to this topic.
OK, Duckster’s link says 45,000 mph, and XT’s says 486,000 mph. How can there be two speeds so far apart? I’m confused.
Now when you answer, bear in mind: I’m not a smart man. Use little words. Words a 40-year-old with an English degree can understand.
I have a cite that says 40,000 mph. Not entirely safe for work;[spoiler]https://www.youtube.com/watch?v=buqtdpuZxvk[/spoiler]
They are both right.
The first figure, 43,000 miles per hour (70,000 km/hr) is relative to the local standard of rest. That is to say, the speed of the Sun compared with all the other stars in the local part of the galaxy. This is like comparing the speed of your car on the freeway to the average speed of the other cars; if you are going 5mph faster than the other cars, then that is your relative speed compared to them.
The second figure, 483,000 miles per hour (792,000 km/hr) is the absolute speed compared to the centre of the galaxy. If you are in a car on the freeway this would be your actual speed compared to the road.
http://astrosociety.org/edu/publications/tnl/71/howfast.html
Note as well that the Galaxy is moving as well relative to the background radiation at 1.3 million miles per hour (2.1 million km/hr); we can only detect this by looking at the Doppler shift of the cosmic background, so for most purposes this movement can be ignored.
IS there any easy way to use both the galaxy’s movement and the Sun’s movement together? Or would the galaxy’s own movement dwarf the solar movement?
I’m not sure what you would want to use it for, but it seems to me that the galaxy’s movement with respect to the cosmic background is not that much greater than the rotation;
483,000 miles per hour (rotation) is about a third of the speed of the movement of the galaxy (1.3 million miles per hour). But the angle between the axis of rotation and the direction of movement of the galaxy is quite steep - more than 60 degrees.
So the movement of the Sun would look like a very skewed and quite stretched helix through space.
Not that I am a physicist, but is it ever valid to talk about “absolute” speed ?
Speed is always relative to something, shirley… ?
So, it seems that I, along with a whole bunch of other people, have moved over 4.2 billion miles from where we were just a year ago! With all that traveling under our respective belts, since we’re heading in the same direction, wouldn’t it be grand if we all got along a little better?
Thank you all for using little words.
Yes, in this case relative to the center of the Galaxy, as eburacum45 said.
I once saw a gray scale map of the cosmic background radiation in which the different grays marked the wave length. One thing that struck me was that, in addition to the graininess that marks the density fluctuations that eventually led to galaxies, it was not anisotropic. The average gray was different in different directions. This was explained in the caption as representing our movement relative to the CBR. Does that not give an absolute standard of motionlessness?
I think it sort of does, in our Hubble Volume at least. That is how the 1.3 million mph figure is calculated (you need to extract the rotation of the Sun around the Galaxy to get the movement of the Milky way against the CMBR).
But we don’t know how big the complete universe is, so we can’t really describe anything within it as absolute without comparing it to something else.
If Sol’s system was/is in the same plane as the entire galaxy rotation, would not the time rate difference be measurable/noticeable while we on earth are on the retrograde side of Sol’s rotation at the same time we are on the retrograde side of the galaxy’s retrograde side compared to when we are on the advancing sides of both if we were on the other side of the galaxy?
I do not know any other way to explain my question other than to compare helicopter blade rotation in comparison to direction of flight with a small rotating system attached to the end of the blade in that same plane spinning out on the end when it is all compared to the movement of the helicopter & it’s movement compared to the ground.
Does time of a life, say 100 years compare exactly to 100 years on a system that is 90 degrees to the galaxy rotation?
Viewing from above may make sorting out the rotations I am trying to explain easier?
Anybody? 
I thought maybe a physicist might chime in, rather than an amateur worldbuilder like me - but it seems to me that your experience of time in your own frame of reference will always be unaffected by your motion compared to any other location. Someone observing you from Sirius, or the galactic centre, or from the Virgo Supercluster, might observe some distortion in the rate of time passing in various different parts of your orbit around the Sun (or around the Galaxy) but you wouldn’t notice any difference.
There will be some slight variation in the timings of the signals received from pulsars due to the orbit of the Earth around the Sun, but I’d be interested to know how much of that is due to relativity and how much is just due to Doppler effects.
OK, I’m not getting this. If I’m on a seat on a Ferris wheel, my speed relative to the wheel’s axle is zero - I’m neither approaching it nor receding from it. My motion is relative to objects not on the Ferris wheel, like the ground.
I’d expect the speed of the sun around the center of the galaxy to be defined relative to a line between the center of the galaxy and a distant galaxy, with the distant galaxy chosen so that the sun’s pretty close to the line. What’s the rate of change in the sun’s position relative to that line? seems to me to be the appropriate measure.
Your radial velocity might be zero, but your tangential won’t be
It certainly gives a standard of motionless, and for some purposes it’s a very convenient one, but it’s not absolute. We can perform the observation here, and come up with a standard, and scientists billions of lightyears away could also perform the observation, and come up with a standard, but the two standards would not agree.
Oh, and when you said “not anisotropic”, you were using a double negative. You meant “not isotropic” or “anisotropic”.