A relative of a relative posted a video on Facebook the other day that I’m half-heartedly working to debunk. The gist of it is that it’s wrong to think of the solar system as planets just circling around the sun, because the sun is also moving, so instead of circles (or ellipses) in a plane, the planets are actually describing three-dimensional vortexes, which has a bunch of nonsense mystical significance.
My specific question regards the direction of the sun’s movement. I’m assuming that the video’s authors are referring to the motion of the sun around the center of the galaxy, though they don’t say explicitly. In the video, they depict the direction of the sun’s movement as perpendicular to the plane of the elciptic, and I think that’s wrong. It makes for a nice visual, with the sun shooting “upwards”, with the planets all swirling around the equator, but I’m pretty sure it’s incorrect.
Isn’t it true that the plane of the galaxy is more or less coplanar with the ecliptic? And therefore, wouldn’t a more accurate depiction look more like a dancer with a big hoop skirt, moving around side to side in the same plane as the planets? I think that’s right, but I don’t want to commit myself publicly without checking first.
As bizerta notes, it doesn’t look that way, when you see the Milky Way in a dark sky. Various Googling suggests the angular difference is 62.6 degrees.
But all motion is relative, and the choice of your coordinate system is arbitrary. So if you want to, you can surely find one in which the motion of the planets is helical.
No, the solar ecliptic is oriented about 60° to the galactic plane of rotation, which is obvious once you think about the nearly overhead position of the Milky Way (i.e. the cross section of the galaxy we can observe) in the night sky. Although the Sun does move within the plane of the ecliptic around the common barycenter (center of gravity) of the solar system, this is largely driven by the Sun-Jupiter attraction with minor variations depending on the alignment of the other planets with respect to Jupiter. The Sun also oscillates slightly up and down in the ecliptic also, although not enough that you would really notice by any but the most careful observations of both the Sun and other planets (again, primarily Jupiter which is inclined only 1.3° to the ecliptic).
And it is wrong to think of the ecliptic as a fixed and static condition; the plane of the ecliptic is simply the plane which is described by the Earth’s rotation about the Sun, from which observations of other planets and celestial objects are made from the Earth’s surface or Earth-orbtiing satellites. That makes a convenient coordinate frame for us, but was with the apparent retrograde motion of planets, it is entirely due to our moving in a non-inertial reference frame. The appropriate frame is actually mated to the so-called invariant plane in which inclination is measured from a plane that is oriented with the primary axes of the moments of inertia for the weighted average of all components of the solar system. Since this distribution changes with various planetary alignments it varies over time, but again is largely dictated by the orientation of Sol and Jupiter.
This is true, but all of the other, more natural, definitions of “the plane of the Solar System” correspond almost exactly with the ecliptic, so it makes very little difference which one you use.
OK, thanks. So not particularly coplanar, then, but it’s also definitely not perpendicular, either, so the depiction in the video is still off. I did explain a bit about the whole “all motion is relative” thing, but I don’t think I accomplished much.
Thanks for scratching that particular intellectual itch.
Not necessarily. Although the Sun, and other stars, are, on average rotating about the galactic center, there is a large component of randomness in the motions of individual stars (largely due to “local” gravitational interactions between stars, but also, I believe, things such as stellar explosions of various sorts, which sometimes have a directional component that can send the remnant star jetting off in a particular direction). I do not, offhand, know which direction the direction the Sun is currently headed in (relative to the galaxy as a whole), but it is very unlikely that it is on a simple and stable orbital path around the center, and it may well be moving quite rapidly in some other direction.
Coordinate system choice isn’t arbitrary when we’re talking rotation. In all the commonly used relativities, coordinate systems are inertial, with no acceleration. That excludes rotating ones.
If you imagine the moon’s path around the sun (without the earth in the picture), it basically describes a near-perfect circle with a slight up-and-down wave. There is no ‘looping’ because the moon’s orbit around the sun is so much larger than the orbit around the earth, and the speed of the solar orbit is so much greater (approx 30 times).
The Bad Astronomer addressed this last year…
I’ve been getting lots of tweets and email from folks linking to a slick-looking video, a computer animation showing the motion of the planets around the Sun as the Sun orbits around the Milky Way Galaxy. It’s a very pretty video with compelling music and well-done graphics.
However, there’s a problem with it: It’s wrong. And not just superficially; it’s deeply wrong, based on a very wrong premise. While there are some useful visualizations in it, I caution people to take it with a galaxy-sized grain of salt.
Why? The basis of the claim is that the planets aren’t orbiting the Sun heliocentrically, but are instead a vortex going around the galaxy.
Normally I wouldn’t bother debunking stuff like this; wacky claims are made all the time and usually disappear on their own. But in this case I’m getting a lot of people telling me about it, so clearly it’s popular—probably because it seems superficially right, and it has very nice graphics. I’m also seeing it spread around by people who do understand science, but missed the parts of it that are way off. With stuff like this, it always pays to dig a little deeper.
Dr Plait’s analysis is remarkable, and takes great care over something that is essentially nonsense.
I don’t know who made the animation, but they have made many errors; one that I notice is that two of the planets appear to have roughly the same orbital period. This is quite wrong. As Dr Plait points out, the tilt of the ecliptic means that about half of the planets are ahead of the sun at any one time; but only when considered from some reference points.
Right - but you don’t need a rotating frame of reference. The right inertial one yields helical paths for the planets. (Not that there’s much value in such a choice.)