I'm trying to measure my 120v, 60hz mains.
Don't do that without a special isolation transformer.
Yes, I'm aware of the safety risks involved.
Apparently you aren't.
If you want to measure mains voltage in a safe, reliable and repeatable way, use something like this:
http://www.yuanxing.net/1-2-1.asp
The OP is not connecting the probe ground clip to
anything,& is in fact,
removing the ground lead entirely,relying upon the Oscilloscope's earthing via its power cord to provide the return path.
This method is quite safe,providing certain sensible precautions are taken,
(1) Clip the probe onto the Active connection with the power at the socket off,step away from the connection,then turn the power on.
In countries which don't have switches,read this as plugging the power cord in----in any case,it is a good idea to leave the power lead unplugged,until you are ready.
(2)Following your test,switch off, the Mains supply to the point your probe is clipped to,before touching anything.
Experienced Techs working on linear power supplies in equipment,often "have a look" at the incoming Mains to the device,just to check it is there.
One side of the transformer primary should show 120v/230v w.r.t earth,the other side.maybe a few volts.
If both sides show 120/230v,the Neutral connection is broken,if they both say "a few volts" the Active is broken.
If both say "zero" both sides of the Mains are missing.