Caption: The cosmic microwave background (CMB) dipole anisotropy in a whole-sky sky map in Mollweide projection or some similar graphical projection.

Features:

1. The sky map shows the CMB dipole anisotropy temperature anomaly (i.e., difference from mean) of the cosmic microwave background (CMB) temperature T = 2.72548(57) (Fixsen 2009).

   The spectral colors (violet, blue, green, yellow, orange, red) represent the low to high temperature anomaly.

   The sky map is a bit dated since it shows data from the COBE probe (1989--1993). However, the 2018 value for the time-constant amplitude of the anomaly is 3.36 mK = 0.00336 K (Planck 2018 results. III. (2018, p. 22)). Superimposed on the time-constant CMB dipole anisotropy is a small time-varying anisotropy of amplitude of is 271 μK = 0.271 mK= 0.000271 K (Planck 2018 results. III. (2018, p. 21)) which is NOT represented in the figure.

   Note anomaly (time-constant and time-varying) is everywhere very small compared to the (mean) CMB temperature T = 2.72548(57) (Fixsen 2009).

2. The CMB dipole anisotropy is NOT intrinsic to the CMB as is the CMB intrinsic anisotropies (which are due to cosmological effects). It is caused by the Doppler shift of the CMB due to the translational motions of Sun and Earth relative to the local-to-Earth comoving frame that participates in the (mean) expansion of the universe. Note that CMB by modern cosmological theory is NOT Doppler shifted in comoving frames.

   The translational motion of Sun gives the time-constant CMB dipole anisotropy and the translational motion Earth (which is due to the Earth's orbital motion) gives the time-varying CMB dipole anisotropy.

   In analyzing the CMB intrinsic anisotropies, the CMB dipole anisotropy is subtracted off the data as a first step.

   Note the Doppler effect (or shift) is covered in IAL 7: Spectra: The Doppler Effect.

3. Using the Doppler effect formulae (plus other information as needed), we can determine the velocities of motions relative to our local comoving frame and nearby local comoving frames. For example:
   1. The Solar System barycenter (i.e., center of mass of the Solar System) is moving at 368(2) km/s in some direction (see Wikipedia: CMB dipole anisotropy; Caltech: Description of CMB Anisotropies).

   2. The Milky Way barycenter is moving at 552(6) km/s in the direction 10.5 hours right ascension (RA), 0.24° declination (Dec or δ) in equatorial coordinates (epoch J2000) which is toward near the center of constellation Hydra (see Wikipedia: Milky Way: Velocity).

   3. Local Group of Galaxies barycenter is moving at 627(22) km/s in some direction (see Wikipedia: CMB dipole anisotropy; Caltech: Description of CMB Anisotropies).