How Earth ACTUALLY Moves Across the Galaxy

**Matt O'Dowd** (0:00)
You said this place was steps from the water.

**SPEAKER_2** (0:02)
We just haven't found the steps yet.

**SPEAKER_3** (0:05)
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**Matt O'Dowd** (0:08)
Enough to get lost?

**SPEAKER_2** (0:10)
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**Matt O'Dowd** (0:15)
Just steps from the water.

**SPEAKER_2** (0:16)
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**Matt O'Dowd** (0:59)
Perhaps you've seen videos of how the planets of the solar system move through the universe in this cool helix.
But not only are these misleading, Earth's real motion, your real motion through the universe is way more complicated and way more interesting.
This video of the planets moving in a corkscrew pattern isn't exactly wrong, but it's often presented in the most misleading sense by suggesting that the old idea of planets orbiting the sun is wrong and that we really move in a cool vortex. This is misleading because it suggests that one frame of reference, the one where we see this helix, is fundamentally better than any others. Today, I'm gonna say a word or two against this sort of reference frame chauvinism, and then talk about how Earth really moves through the universe, which involves a surprising amount of awesome astrophysics. To start, I want you to imagine that you're in a ship, moving steadily on a perfectly smooth ocean in an enclosed cabin. The portals are closed, so there's no way you can tell how fast you're moving, or even that you're moving at all. This is the thought experiment that Galileo Galilei described in his 1632 book, Dialogue Concerning the Two Chief World Systems, to express what we now call Galilean Relativity. It's the idea that there's no single best frame of reference from which to define the concept of stillness. There's no absolute rest frame. All non-accelerating frames, also called inertial frames, can be taken as motionless as far as the laws of physics are concerned.
So, the view of the solar system tracing this helix through space is a valid frame of reference. But you know what else is a valid frame of reference? This one, where the planets orbit the sun just like you were always taught. At least on timescales of less than a few million years. OK, meme debunking achieved. Let's move on to the interesting astrophysics and talk about how Earth is really moving and what we can learn from that. Given this Galilean relativity of motion, is it even meaningful to say how the Earth is moving? Well, there may be no such thing as a single fundamental definition of motionlessness, but that doesn't mean that all reference frames are equal. We choose a reference frame or a rest frame that's most useful for the question at hand. For example, if your brain is subconsciously computing the trajectory of a ball flying through the air so you can catch it, you're not going to take into account the fact that the surface of the Earth is moving at 1,000 kilometers per hour or so due to its rotation. You can treat your patch of the Earth's surface as inertial for that calculation. If NASA wants to send a robot to Mars, it probably doesn't want to use the frame of reference of Cape Canaveral or even of the Earth's center to do its celestial mechanics. It uses an inertial frame of reference of the solar system.
But NASA gets to ignore the fact that the sun is racing around the galaxy at 230 km per second. So if we want to answer the question, how is the Earth really moving, we need to choose useful frames of reference.
Let's start with the frame of reference of our familiar descriptions of the motion of the planets, one roughly fixed to the sun.
One thing is inaccurate about this depiction, and that's the idea that the planets are orbiting the sun while the sun is fixed relative to the planets. The planets tug on the sun just as it tugs on the planets, causing the sun to move. The best reference frame to describe the motions within the solar system is that of the barycenter, the center of mass of all of the objects in the solar system. This is the real point that everything in the solar system is orbiting.