TORONTO, Jan 20 (Reuters) - The Canadian dollar weakened to a session low against its U.S. counterpart on Tuesday after Canadian factory sales data fell 1.4 percent in November, double the expected decline. The Canadian dollar touched C$1.2022 to the greenback, or 83.18 U.S. cents, more than half a cent weaker than Monday's finish at C$1.1947, or 83.70 U.S. cents. (Reporting by Solarina Ho)