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In this video: http://www.youtube.com/watch?v=0jHsq36_NTU#t=55 at 0:55, it is claimed that "the sun is [...] dragging the planets in its wake".

Is this true? My understanding is that the sun and the planets are all moving together through the solar system at the same velocity, and there is no friction involved, so there's no dragging or indeed motor force required.

What's really going on?

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  • $\begingroup$ The video has very pretty graphics, however it doesn't really say anything. It says heliocentric motion and helical motion are different, but the difference is in perception only. $\endgroup$ – Mike Dunlavey Sep 13 '14 at 1:40
  • $\begingroup$ Thanks for this observation. ... I haven't thought about that aspect of things. My question is solely focused on the statement about dragging. Dragging implies a force in the direction of travel. I don't see any reason why there would be such a force. .... Indeed the planets could be dragging the sun! $\endgroup$ – GreenAsJade Sep 13 '14 at 2:51
  • $\begingroup$ The intent of this site is to give better answers than any other site on the internet. That's difficult in the case of this question as Phil Plait has done a rather thorough debunking of this pretty but terribly wrong video at his Bad Astronomy blog. See No, Our Solar System is NOT a “Vortex”. $\endgroup$ – David Hammen Oct 31 '14 at 14:56
  • $\begingroup$ It's the intent of this site to be the best site for Physics answers. It's a bit hard to be the best site for answers if you only answer questions that aren't answered elsewhere... $\endgroup$ – GreenAsJade Oct 31 '14 at 21:17
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My understanding is that the sun and the planets are all moving together through the solar system at the same velocity, and there is no friction involved, so there's no dragging or indeed motor force required.

Drag might be a poor word choice in this context. It doesn't refer to friction or air drag. And certainly nothing to do with a motor. I think the author is using drag in the "carry along with it" sense of the word.

What the author might be trying to say is the planets orbit the Sun while the sun moves through the galaxy. So the planets also must move through the galaxy in a way that closely mimics that of the Sun.

Similarly, the author might also be saying that the Sun exerts a force on the planets that causes them to "stay close" to it. I suppose this could be interpreted as correct since if something exerts a force on only the Sun, the planets will likely keep on orbiting it as long as the force isn't too large. But in reality, the gravitational force by the rest of the galaxy has the same effect on the Sun and on the planets, so this effect likely isn't as big as one might expect.

To make this last point more clear, consider our moon's motion in our solar system. It goes around the Earth, which in turn goes around the Sun. (Therefore the moon's path in the solar system has a sort of Spirograph shape.) But if the Earth weren't there, the moon could orbit the Sun just fine without the help of Earth; it just wouldn't have the smaller revolutions due to its motion around the Earth.

I'm sure celestial mechanics are cringing at my terminology. Sorry.

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  • $\begingroup$ The sun "drags" the earth and the moon around it. It exerts a force on them that makes their (coupled) motion go in an orbit. Similarly, the galaxy "drags" the solar system around it. However, the sun does not (I am asserting) drag the planets around the galaxy. The solar system as an entity has an orbit within the galaxy, but the only influence of the sun on the planets is their orbit around it. The sun is not responsible for "dragging" the planets through the galaxy "in it's wake" ... is it? $\endgroup$ – GreenAsJade Sep 13 '14 at 7:07
  • $\begingroup$ That's the viewpoint I'm positing here to some extent. Perhaps others will disagree. $\endgroup$ – BMS Sep 13 '14 at 16:20

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