Our planet orbits the Sun. 365.25 days to go full circle (ellipse actually) and bring the seasons to Earth. But the Sun is not really stationary, it’s actually moving through space. It’s orbiting the center of the Milky Way, along with the rest of the galaxy. It actually has a periodic motion as it moves around the Galaxy, slowly moving up above the galactic plane then being pulled back down below by the disk stars.
Currently, the Sun is moving toward the constellation Hercules at a speed of around 72,000 Km/h. It is also moving up to the top of the galactic plane, but will soon reach it’s maximum height. I say soon, but it will actually take around 14 million years.
But this isn’t all of the motion, is it? Is our galaxy moving in space? What do we compare it’s movement to if it is moving?
The answer is that our galaxy is moving in space at a speed of about 2 million km/h toward galaxies in the constellation Hydra. It is moving together with the local group of Galaxies, including Andromeda, which is also slowly colliding with the Milky Way. But again, how do we measure this?
The answer is surprising, but fascinating. The Cosmic Microwave Background Radiation, the leftover radiation from the recombination era when the Universe was only 380,000 years old, has very small fluctuations in it that are clearly visible in images, such as this one below.
But these tiny fluctuations aren’t visible right away. The raw images from CMBR probes show what we call a dipole, as seen here.
Since the Earth is moving in space, any light that reaches it will have a Doppler shift. The light from something we are moving toward (or is moving toward us) is slightly bluer, while the light from receding motion is redder. So when we see the dipole in the CMBR, we are seeing getting the Doppler shift from Earth’s movement against the rest-frame of the Universe!
It’s the shift that comes from Earth orbiting the Sun, plus the movement of the Sun around the galaxy, plus the movement of the Galaxy in the local universe. We can shift the dipole to give us the rest-frame CMBR that we see above, and in the process we measure the speed of the Earth through the universe. It’s like being able to measure a single water molecule as it moves through a balloon. The balloon isn’t moving, but the water sure is.
Does the rest frame of the universe have any real significance? Yes and No. It is interesting to think of the universe having a preferential frame of reference, and it may lead to useful discoveries in the future if we find advanced ways of probing the universe. But from a Physics point of view, Einstein’s relativity says that the laws of physics remain unchanged regardless of the reference frame. In simpler terms, it doesn’t really matter.