 Another reference frame is provided by the cosmic microwave background (CMB). The Milky Way is moving at 552 ± 6 km / s with respect to the photons of the CMB, toward 10.5 right ascension, − 24°declination (J2000 epoch, near the center of Hydra). This motion is observed by satellites such as the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP) as a dipole contribution to the CMB, as photons in equilibrium in the CMB frame get blue-shifted in the direction of the motion and red-shifted in the opposite direction.
