#elapsed_time__elapsedtime  the time that elapses while some event is occurring
  supertype:  #time_period__period__period_of_time  an amount of time; "a time period of 30 years"; "hastened the period of his recovery"
  subtype:  #one-way_light_time__onewaylighttime__OWLT  the elapsed time it takes for light (or radio signals) to travel between the Earth and a celestial object
  subtype:  #round-trip_light_time__RTLT  the elapsed time it takes for a signal to travel from Earth to a spacecraft (or other body) and back to the starting point

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