Mysteriously Still Stars

[trek_mambo]

I stood the other day gazing up at the night sky. All the recollections of years past came to mind. Suddenly I had a profound thought. I remembered for a second the joy of spinning on that spinning wheel they have in public parks. I remember my brother and friends spinning the wheel faster and faster until all I could see was a mash of colours smeared as if abstract art. It seems either my eye could not keep up with the objects around me.

I thought... Doesn't the earth also rotate around it self, then around the sun and around the galaxy relatively if not faster than me having been on a spinning wheel? I mean We see the sun going around and the moon going around... but what about the stars in the sky which in relativity we are moving ridiculously fast... Why is it when we look at the night sky we do not see the same mash of colors instead of distinct little dots in the sky almost as if the stars are not moving at all?

This bothered me deeply. The sun moves because we are gravitationally closer bound to it.... but the stars are distant after all and in theory should we not thus see them as smudges rather than dots?

[The_Librarian]

Excellent question, and one worthy of a good answer.

[Random717]

We aren't turning/moving quickly enough... it's almost impossible to track a stars real time movement with the naked eye. If you stare at city lights on the horizon they appear as dots, even if you start moving your head really slowly, there is still a period in which they remain focused before disappearing into streaks. I guess the earth is moving at that really slow speed...

[Claymore]

You're correct in principle. Take a night-time long exposure of the night sky, and you'll see that the stars do move across the sky in a circular pattern; thing is, it's not rapid apparent movement, so you won't see motion unless you're examining the same place for at least several minutes.

[Mr TB]

We aren't turning/moving quickly enough... it's almost impossible to track a stars real time movement with the naked eye. If you stare at city lights on the horizon they appear as dots, even if you start moving your head really slowly, there is still a period in which they remain focused before disappearing into streaks. I guess the earth is moving at that really slow speed...

Seemingly closer to the truth your answer I think...

Standing on the spinning wheel... how far are you from its axis?... spinning.
At what speed should the earth rotate around its own axis for you to experience the same sensation you are experiencing on the spinning wheel.
I.O.W. what distance are you standing from the axis of the earth...
and on goes the light... momentum that is the answer...

[Pyro]

I think you are comparing rotational speed with linear velocity.

If you're driving past a field, you'll have trouble following trees right next to the car, yet it's very easy to follow trees that are a couple of hundred metres away. They're both the same speed relative to you, yet the angle through which they move is not the same. A tree close to you might force you to turn your head almost 180 degrees in a few seconds, while you can easily track the faraway tree for a looooong time...

Those stars far away are actually moving so fast that their colour changes. Show me a merry-go-round that can make things change colour

[The_Librarian]

And now for something interesting.

(X)<---------------<-------------[You]

In a normal, everyday situation, you can see the object at (X), for example a tree that's 20 metres away from you (example).

Now, consider the following:

(X)<------------------<-----------[You]
.
.
.
.
(Y)




Light takes a while to travel. So when you see the star at spot (X), it is already at spot (Y).

[Mr TB]

I think you are comparing rotational speed with linear velocity.

If you're driving past a field, you'll have trouble following trees right next to the car, yet it's very easy to follow trees that are a couple of hundred metres away. They're both the same speed relative to you, yet the angle through which they move is not the same. A tree close to you might force you to turn your head almost 180 degrees in a few seconds, while you can easily track the faraway tree for a looooong time...

Those stars far away are actually moving so fast that their colour changes. Show me a merry-go-round that can make things change colour

I wanted to edit the point I made when I mentioned momentum...
momentum refers to linear motion am I correct?

I actually wish to refer to angular velocity, accelaration and momentum...

The earth and spinning wheel rotate around its own axis.
The person experiencing the sensation is a certain distance from such axis.
How close are the objects to the person.
What effect does the rotational velocity/acceleration has on the physical condition of the person.

There is actually quite a few things to consider in your question T_M

[plazma]

Parallax, or more accurately motion parallax, is the change of angular position of two observations of a single object relative to each other as seen by an observer, caused by the motion of the observer. Simply put, it is the apparent shift of an object against the background that is caused by a change in the observer's position. The term is dervived from the Greek παραλλαγή (parallagé), meaning "alteration".

Parallax is often thought of as the 'apparent motion' of an object against a distant background because of a perspective shift, as seen in Figure 1. When viewed from Viewpoint A, the object appears to be closer to the blue square. When the viewpoint is changed to Viewpoint B, the object appears to have moved in front of the red square. It is most commonly used in astronomy.

http://en.wikipedia.org/wiki/Parallax



And therein lies your answer. Our viewpoint is so very, very tiny, when looking at stuff on the scale of galaxies and clusters.

[Mr TB]

And now for something interesting.

(X)<---------------<-------------[You]

In a normal, everyday situation, you can see the object at (X), for example a tree that's 20 metres away from you (example).

Now, consider the following:
(Z)
+
+
+
+
(X)<------------------<-----------[You]
.
.
.
.
(Y)


Light takes a while to travel. So when you see the star at spot (X), it is already at spot (Y).

And if it was travelling in the direction (Z)...

[Messugga]

Plasma got a tad technical there. More than needed, anyway. Here's a simple example. Imagine a car driving past you at say...100km/h, about 1m from your eyes. Your eyes aren't moving to the side, nor is your head moving. The car would be observed as a blur in that you wouldn't really be able to tell anything about what's going on inside. Now, look at the same car, in the same way, traveling at 100km/h, but from 100m away. Save for the fact that your eyes probably aren't sharp enough to really pick up the finer details, you'd still be able to get a pretty good impression of what it looks like, right? Same thing with the stars, only, they're several hundred, if not thousand lightyears away from your eyes. It has to do with angular velocity of the object. I'm a bit rusty with the exact terms involved, but I think it gives you a pretty good, practical picture of why you're observing what you do.

[plazma]

http://en.wikipedia.org/wiki/Focus_%28optics%29

And that is Plazma with a Z, Mesugga. )

[Pyro]

Ah yes - Angular velocity - That's the term I was looking for. I'm extremely rusty on these things

You basically describe motion by position, velocity, and acceleration. Then you have linear and angular versions of these 3, but you don't normally say linear velocity, or linear acceleration. You just say velocity and acceleration. Normally velocity and 'speed' is the same thing.

Momentum is velocity (squared) times mass. It's basically how hard it is to stop something in motion, or how much movement force it has.

If you spin, everything around you has the same angular velocity. The parralax effect describes the effect when two objects move at the same velocity, but have a different angular velocity relative to your viewpoint.

The whole speed of light issue is more easily understood if you compare it to the speed of sound. If you see a plane pass overhead, you hear the sound coming from somewhere, but that point seems to be behind the actual plane. Light just travels much faster, so the difference in where it is, and seems to be, is much smaller.

[Messugga]

Pyro, speed and velocity most certainly isn't the same thing, normally. The speed of electrons oscillating in an atom differs by a huge margin from their velocity

[Grim|Reaper]

speed has no direction...which is a scalar quantity
velocity has direction....which is a vector quantity