Later scientists found that, because of wobbles in the planetary orbit, they needed a more exact measurement of a second, and in 1967 brought in Universal Co-ordinated Time (known as UTC for some reason, rather than UCT). This defines a second by relating it to radiation emitted from a caesium-133 atom. UTC has officially replaced GMT, but for all but the most exact time measurements they are the same, and most people still refer to GMT. Ironically, UTC is co-ordinated from Paris, although it is based on Greenwich, not Paris time.