“Speed of light constant for every distance in universe. Actual light speed in kilometers or miles only has meaning for us,” Orlav reiterated. “We recognize those measurements here on earth. We agreed that a 'second' equals about nine billions of waves emitted during radiation of Cesium 133. It is how we have, as you say, 'atomic clock'. This type of atomic radiation occurs all throughout universe. A Cesium 'second' is universal constant. Before 1967, second was defined as time for light to travel from our sun to agreed-upon point on Earth. Earth's orbit not completely smooth, so not so good model,” he concluded. “If we don’t have exact measurement of atom that decays same way for millions of year for all of universe, we would have different ideas of what a second is, planet by planet, da?”
There were some thoughtful reflections as the class digested this.