Celestial objects’ speed through space is measured relative to other things. Often that speed is based on the reference of the cosmic microwave background radiation. Cosmic microwave background radiation is photons, that are remaining from the first light that was able to travel freely through the universe. Light is traveling at C (and experiences no time). Eli5 How is that light/photon cloud a standard for a motionless frame of reference when its photons are moving at the speed of light?