starting at zero, the sine wave goes up to ~200v, back to zero, down to ~-200 and back up to zero. that is one complete cycle. The average voltage is 120v. 60hz is 60 complete cycles, or 120 zero crossings per second.
The way that solid state dimmers work is that the SCR (or triac) delays the turn on point for
some time after the zero crossing, reducing the average voltage, at the expense of a clean sine wave. better dimmers add a inductor to clean up the sine wave.
