Motion of the Milky Way

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Our small planet is part of a complicated hierarchy of structure in the heavens:

  • The Earth rotates once per day, so a person standing on the equator moves at 1700 km/h, relative to the centre of the Earth
  • The Earth orbits the Sun at 100,000 km/h, relative to the Sun (in a non-rotating frame of reference)
  • The Sun orbits the centre of our Milky Way galaxy at 800,000 km/h
  • The Milky Way is approaching the centre of our “Local Group” of galaxies at 200,000 km/h. (This is my rough estimate, based merely on the fact that Andromeda and the Milky Way are approaching one another at twice this speed, and these are the dominant two members of the galaxy group.)
  • The Local Group is falling towards the Virgo Cluster at around 400,000 — 1,000,000 km/h, the “Virgocentric flow”. (This is after subtracting the Hubble flow. Note the Local Group and Virgo Cluster are both contained within the Virgo Supercluster, an even larger structure.)
  • The Virgo Supercluster is moving towards the “Great Attractor” region at 1,000,000 km/h, according to an older source. (The Great Attractor is due to the Hydra-Centaurus Supercluster, or the even larger Laniakea Supercluster which encompasses all of the above and more. The Norma Cluster marks the centre.)
  • The Laniakea Supercluster is moving towards the Shapley Supercluster.
Map of the sky showing the "hot" and "cold" spots of the cosmic microwave background (CMB). This unevenness ("anisotropy") is due to the motion of the Solar System, as the Earth's motion relative to the Sun has already been subtracted. This is from the COsmic Background Explorer (COBE) satellite in the early 1990s.
Map of the sky showing the “hot” and “cold” spots of the cosmic microwave background (CMB). This unevenness or dipole is due to the motion of the Solar System, where the Earth’s motion relative to the Sun has presumably already been subtracted. This is from the COsmic Background Explorer (COBE) satellite in the early 1990s, the first detailed map. In most pictures of the CMB this anisotropy has already been subtracted out, leaving much finer hot/cold dimples.

Going back a step, an alternate method is to measure the cosmic microwave background (CMB). This radiation is nearly uniform in all directions, but shows a hot and cold spot (see Lineweaver 1996  for history). Since this is 100 times more pronounced than the finer fluctuations, it makes sense to interpret it as a Doppler effect due to motion. Hence, the Solar System’s motion is calculated as 1,300,000 km/h in the direction of the constellation Leo. By subtracting off the Sun’s estimated motion, the Local Group has a velocity of 2,200,000 km/h in the direction of the constellation Hydra. This is relative to the “CMB rest frame”, assumed to coincide with the Hubble flow, which is the average motion of matter at large scales and is thought of as being “at rest”. However understand this “rest frame” is just a natural and convenient choice, and not the centuries-old concept of “absolute rest” held by Isaac Newton.