Which Way is Earth Moving in Space?

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?

M31 Andromeda Galaxy

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.

CMBR Credit: WMAP

But these tiny fluctuations aren’t visible right away.  The raw images from CMBR probes show what we call a dipole, as seen here.

CMBR Dipole. Credit: COBE

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.

 

13 thoughts on “Which Way is Earth Moving in Space?

  1. Braden Hepner

    I’ve been led to believe that all galaxies are moving away from one another, as illustrated by Brian Greene’s pennies-on-an-inflating-balloon analogy, where as the balloon expands, the space between the pennies (galaxies) increases? If that’s true, we wouldn’t be moving toward anything, but becoming more isolated in space. What am I missing?

    1. It is correct that we are becoming more isolated in space. However, we are still close enough to other galaxies for gravity to matter. Andromeda is the only large galaxy moving toward us due to gravitational pull, and in fact the Milky Way will collide with Andromeda in 3.5 Billion years. But in the distant future, galaxies will move further apart, galaxy clusters will spread out after all gravitational interactions have resolved, and nothing will be close enough for gravity to overcome the expansion of space.

  2. Jim

    In the first diagram above, you show the solar system moving counterclockwise around the Milky Way, but the Milky Way is rotating clockwise. The solar system is moving in the direction of rotation of the Milky Way, not counter to its movement.

    1. Thanks for the comment. How do we determine clockwise and counterclockwise in space? We need to decide which way is up, which is arbitrary in the universe. If you look at a spinning disk (like the galaxy) from ‘above’ it may be rotating clockwise, but look at the same disk from the other side and it will appear to be rotating counterclockwise.

      In the particular image, it seems the direction is purely for aesthetics, and most images of the Milky Way (artist’s conceptions since we can’t go out and photograph it from outside) show Solar System motion with this convention.

      Hope this is helpful!

  3. Jim

    If you pour milk into a cup of coffee, and stir it clockwise, you will see the “arms” of the swirling milk trailing, or following, the direction of rotation, not “digging in” to the direction of rotation. https://www.quora.com/Which-direction-does-the-Milky-way-rotate

    The first diagram shows the Milky Way rotating clockwise, but the solar system (with north as “up”) moving against the Milky Way’s direction of rotation. This may be “aesthetic” but it is incorrect.

    In the Celestial Coordinate System, there are conventions for which is “up” and which is “down.” It is not purely arbitrary. https://en.wikipedia.org/wiki/Celestial_coordinate_system
    although up and down do not truly matter in space, they do matter in earth-based astronomy and celestial navigation.

    1. Ah I see what you’re point is about the image then. I was responding to your use of the terms ‘clockwise’ and ‘counterclockwise.’ Specifically correcting the phrase “the Milky Way is rotating clockwise” in general, as I hadn’t noticed the direction of the spiral arms in the image. All coffee and milk aside, yes the solar system follows the rotation of the Milky Way.

      Your initial comment had nothing to do with celestial navigation. It only said that ‘the Milky Way is rotating clockwise,’ a statement which makes no sense without a reference frame. So naturally I responded to that. Thanks for explaining your original comment.

  4. olivier leduc

    just a question that appeared to me in my shower… are we going towards the pole star or running away from it? Thanks!

    1. Good question! We are moving away from Polaris since it’s at the pole right now, but which direction is the question. When a spinning top precesses, the precession is in the same direction as the spin. This means that the north celestial pole is moving clockwise from Polaris, around a circle centered on a point known as the ecliptic pole, which is in the constellation Draco.

      https://upload.wikimedia.org/wikipedia/commons/1/16/Precession_N.gif

    2. Jim

      If you mean, is the distance between the earth (or our solar system) and the pole star (currently Polaris) decreasing (“going towards”) or increasing (“running away from”), the answer is that we are “going towards” Polaris, i.e., the distance between us and Polaris is decreasing at 17 km/sec. https://en.wikipedia.org/wiki/Polaris – see “radial velocity” under Astrometry on right-hand column.

      If you mean, is the Pole Star drifting away from its position relative to the North Celestial Pole, or is it getting closer to the NCP, the answer is, it is drifting away. This is due to precession, or the wobbling of the earth’s axis – like a spinning top. in about 14,000 years the pole star will be Vega. It takes about 26,000 for the earth to complete one precessional cycle. The earth spins counterclockwise (with north as UP), but it precesses in a clockwise direction. If you have a top, you can observe this for yourself http://spaces.imperial.edu/russell.lavery/ast100/Lectures/Page03/Fix03_15.jpg

  5. Just asking

    Quote from this article: “Sun moves towards the constellation Hercules.”

    Hercules is visible on the northern sky. That means Earth travels towards Polaris.

    The distance between Polaris – Earth – Cruz (Southern cross) is more or less fixed.

    A star we see is deemed to be the emission of light at the time when a star was at the location where we detect the light today. Therefore, the physical location of Polaris must be 323-434 (published distances, with a “minor” inaccuracy of 33%) light years north of the location we see, in total sync with earth movement. The same would have to be true for the southern cross.

    That would, right now, put the physical location of most of the stars of the southern cross between Polaris and Earth, and the rest in close proximity to Earth. That is in gross contradiction to why we see the emitted light of a star, as light would always be detectable first at the closest proximity.

    Why can’t we see these stars, even so, they have to be right now very close to Earth?

    In the “ever expanding” universe,
    The Moon closes in on the earth,
    Earth closes in on the sun,
    Sun closes in on the constellation of Hercules,
    Hercules closes in on the center of the galaxy Milky Way,
    Milky Way closes in on the galaxy cluster Virgo,
    Virgo is closing in on the galaxy cluster Andromeda,
    Andromeda is closing in on the Great Attractor in the contracting Laniakea Supercluster.

    All of this measured by light or radiation emitted hundreds, thousands and even millions, if not billions of years ago, without consideration of (unproven big G) gravity and matter in the proximity of any star bending light and the implications of decaying speed of light, without having the slightest clue, what the speed of light might have been at any point in the distant past. Decaying speed of light introduces a margin of error at a magnitude, that makes all measurements irrelevant. Measured with varying methods, which are not calibrated against each other and former inaccurate method results are not remeasured and updated against the entire model of the universe. Everything that is not a star (self illuminating celestial body = sun) is deemed to be a mysterious dark matter, while most of it would have to be non-self illuminating celestial bodies. Never heard anybody making that conclusion, even so, that very concept is applied to everything inside our solar system.

    It is probably safe to assume, that scientism isn’t doing us any good and should not be confused with knowledge.

    Claimed age of the universe: 13.7 Billion years
    Claimed size of the universe: 93 Billion light years
    Anybody got a problem with light and matter traveling 46.5 billion light years in just 13.7 billion years?

    1. You’re making a lot of incorrect assumptions that lead to a lot of big contradictory conclusions. I’ll link you to a good explanation of comoving coordinates to explain why the universe is 93 billion light years across even though it’s only 13.8 billion years old. It’s a great video well-presented. I hope you try to learn more and gain a better understanding of the universe, and how it has taken many hundreds of years of theory and observation to reach the current understanding we have.

      https://www.khanacademy.org/science/cosmology-and-astronomy/universe-scale-topic/big-bang-expansion-topic/v/radius-of-observable-universe

      1. Braden

        Thanks. Makes sense. Fascinating.

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