Why do stars move faster in the celestial equator?

[ZiHao]

Stars seem to 'travel' faster on the celestial equator than the stars situated at the poles. Why is this? Is this just a perspective problem?

[Cornelius Varley]

5 minutes ago, ZiHao said:

Stars seem to 'travel' faster on the celestial equator than the stars situated at the poles. Why is this? Is this just a perspective problem?

It depends on how you define "faster". All stars rotate around the celestial poles at the same angular velocity (15 degrees/hr), those further away from the poles may appear to be moving "faster" but this is only because they are further from the centre of rotation. 

 

[Neil H]

2 hours ago, Cornelius Varley said:

It depends on how you define "faster". All stars rotate around the celestial poles at the same angular velocity (15 degrees/hr), those further away from the poles may appear to be moving "faster" but this is only because they are further from the centre of rotation. 

 

So it's like a record on a player outer looks fast then the centre but it's all the same speed I never thought about like that 

[Captain Scarlet]

If you imagine a ball spinning about its vertical axis, say, on a table. An ant sitting on that ball and looking up will see the same patch of ceiling not moving very far or very fast. But if the ant looks "sideways" at the walls, everything will be moving apparently much faster.

So it is with us standing on the earth. Look up towards the Pole star, and nothing moves very much, because that is more or less the Earth's axis of rotation. But looking towards the equator, or at the "walls", the stars move much faster.

Cheers, Magnus

[symmetal]

5 hours ago, Neil H said:

So it's like a record on a player outer looks fast then the centre but it's all the same speed I never thought about like that 

The outer grooves of a record are moving faster than the inner grooves if you consider linear velocity. Only the angular velocity is the same. From the perspective of the stylus the outer grooves pass underneath much faster than the inner grooves. The recorded wavelength is therefore longer on the outer grooves. One cycle of a 1kHz tone recorded on the outer groove occupies a significantly longer groove distance compared to the same tone on the inner groove. Therefore it's easier for the stylus to track outer grooves.

This is why record companies tried to put quieter tracks on the inner groove. Loud music is much more likely to distort playing on an inner groove unless you have an expensive tonearm and cartridge.

By comparison CDs speed up the rotation of the disc as the laser head moves towards the centre to avoid the digital 'pits' on the disc having to be closer together towards the centre. 

LPs have constant angular velocity and variable linear velocity.

CDs have constant linear velocity and variable angular velocity.

The night sky movement is similar to an LP in as much as the distance from the rotational axis determines its linear velocity. (It's really tangential velocity but at any moment in time the linear and tangential velocity are the same.) 

Alan

[Neil H]

Wow so not like a record 🙄

[Kyle Allen]

Have you ever seen those images of star trails centred on the North or South poles? The stars appear to trace sets of concentric circles of increasing size as they get further from the poles. Every star takes the same amount of time to do one rotation around the pole (the rotation is of course caused by Earth’s rotation) but stars further from the poles have further to travel as they must trace a bigger circle so, as others have said above, they have a faster linear velocity even though their angular velocity is the same. Hopefully this makes sense! 😀

[Neil H]

Yeah a clear as mud lol 

[Cornelius Varley]

12 minutes ago, Kyle Allen said:

Have you ever seen those images of star trails centred on the North or South poles? The stars appear to trace sets of concentric circles of increasing size as they get further from the poles. Every star takes the same amount of time to do one rotation around the pole (the rotation is of course caused by Earth’s rotation) but stars further from the poles have further to travel as they must trace a bigger circle so, as others have said above, they have a faster linear velocity even though their angular velocity is the same. Hopefully this makes sense! 😀

Like this ?

 

[Neil H]

Brilliant photo👍

[Kyle Allen]

11 minutes ago, Cornelius Varley said:

Like this ?

 

Yes, that’s what I was thinking of 🙂