Two Ropes Riddle

Here ya go... a little systems logic riddle

You have two ropes. One is 23 feet long and the other 5ft. Each rope takes exactly one hour to burn from one end to the other. However, the ropes do not burn at a uniform rate. This means that cutting one rope in half will not guarantee you 30 minutes of burning time.

Using a lighter, what system of lighting these ropes would you use to determine exactly 45 minutes?