As a result the sound waves arrive at the front side about 6 seconds earlier than equivalent waves from sunspot-free regions, in a total travel time of about 3 hours. The change in speed becomes evident when sound waves shuttling back and forth get out of step with one another. MDI data for 28-29 March 1998 revealed, on the far side, a sunspot group that was not plainly visible on the near side until ten days later. Observations for 24 hours were more than sufficient to detect the sunspots, which means that routine monitoring is a realistic possibility.