Article I, Section 8, of the Constitution, which enumerates the powers of Congress and provides Congress with the authority to make all laws necessary and proper to carry them out; also referred to as the elastic clause.
There is an exact numerical answer to this question. By definition, 1 light year is exactly 9,460,730,472,580,800 meters. By definition, 1 mile is exactly 1,609.344 meters. So, 1 light year is exactly 5,878,625,373,183 and 849/1397 miles, by definition.
According to Eyewitness's Astronomy, a light year travels at up to 5.9 mph (per SECOND) and 9.5 kph (per SECOND) so multiply that by 60, times that by 60, multiply that by 24, and that by 365, and you should have a pretty close estimate, as to how far away a light year is. Miles: 176,601,600 (5.9*60*60*24*365).