Quick safety question probing 120VAC
Use a quality, high impedance, differential probe if your scope doesn't have the capabilities of differential inputs.  This is an external device that provides isolation, typically up to 500 or 1000 VDC, and yet delivers a low level (.01 or .001) representation of the input signal.

If you scope does have the capability of differential inputs, Select "A + B", and use those two input channels to measure your signal.  Note that you do NOT use common and the probe tip, but only the two probe tips.

Dealing with HV as I do, I much prefer just using the differential isolator for safety sake... It prevents even accidentally touching the (typically) grounded ring at the end of the probe to a live circuit, letting out the magic smoke.