MadSci Network: Astronomy

Re: Is the Sun leaving the Milky Way

Date: Wed Nov 5 11:24:40 2003
Posted By: Gavin Ward, , Physics, AEA Technology
Area of science: Astronomy
ID: 1067522528.As

Hi Peter,

You pose an interesting question which brings up some interesting concepts such as "relative motion" and "frames of reference".

You may already be familiar with these ideas. For example, if you were walking back (at about 3 mph or 5 km/hr) along a slowly moving railway train (moving at about 3 mph or 5 km/hr) the train would appear to be moving in your FRAME of REFERENCE but the outside world would appear to be stationary in your Frame of Reference. An observer standing outside watching sees the train moving past in their FRAME of REFERENCE but you appear stationary to them.

Let's look at some of the relative motions affecting the earth. Standing at the equator you are moving around the centre of the earth at about 1000 mph (1600 km/hr) as the earth spins on its axis. Earth is moving around the sun at about 66,000 mph (106,000 km/hr) in its nearly circular orbit.

Actually there's a slight wavy-ness or wobble to this path as the earth is influenced by the moon and the pair of them rotate around their common centre of mass once every 27 days or so. So already a point on the surface of the earth is tracing out a rather complicated path though space! (If you were to look down on it from above - in the sun's frame of reference - i.e. assuming the sun was stationary).

The sun is not stationary; it moves around the centre of the Galaxy - our galaxy the Milky Way.

So on top of all the motion we mentioned earlier we're whizzing around the centre of the Milky Way at about 250 km/sec (900,000 km/hr) which is a lot faster. It so happens that we are headed in the direction of where M13 is now. M13 is a Globular Cluster - a clump of old stars gravitationally associated with each other (just like we're associated with the moon) and M13 is within our Galaxy. See for some pictures of M13.

We are all (M13, other Globular Clusters and the sun) orbiting around the centre of our Galaxy the Milky Way and the sun is NOT leaving the Milky Way. There will be some relative motion between each other as we are not all on the same orbit, and not on circular orbits, so it is quite to be expected that we'll be moving towards (or away from) other objects in the Galaxy. This is just like traffic moving along a curved road has relative motion which, for a while, may give the appearance that some vehicles are getting closer to each other and may even appear to be on a collision course for a short while. But if they are in different lanes they won't collide.

M13 is a member of the Milky Way galaxy and is just visible to the naked eye. M13 is 21,000 light years away (in a different direction to the centre). The centre of the Galaxy is 26,000 light years away - we are about two thirds of the way out from the centre.

Relative motion can be very complicated and this becomes apparent when animators try to mimic it in cartoons. They have a very hard job making it look natural for ordinary down-to-earth situations. So for one last fascinating aside, the Disney corporation spent a lot of time doing original research into how natural objects move and used this research in the movie Pinocchio.

Pinocchio contains a number of amazing ground-breaking animation sequences lasting just a few seconds. They are so short and so natural that the viewer doesn't even notice them (which is why Disney movies are of such high quality and you can't quite put your finger on why). The sequence I'll refer you to is when Pinocchio is sitting on a trapeze, in a birdcage, suspended from the roof of a moving wagon. How complicated is that? Pinoccio is moving, the trapeze is rocking, the bird cage is waving about and the wagon is jostling on a bumpy road - and it all looks so natural. This is one of the most complex sequences in conventional (non-computer) animation so when you next see it in Pinocchio, you will also think of the relative motion of the sun and the stars whizzing through space around the centre of the Galaxy.

Gavin Ward

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