The tilt of the Earth's axis. The axis the Earth spins on is not straight up and down when you look at the orbit that the Earth takes around the sun. It is tilted at 23 degrees. If you look at a globe, you'll see something called the arctic circle. Winter happens in the northern hemisphere where the axis is pointed away from the sun. If you are north of the arctic circle, you will have a period where the sun never rises in the winter. As you head south, the day period gets longer and as you go south of the equator, you find it has gotten longer still until you are at the south pole and find that it has 24 hour a day sunlight for a period.

 

Bottom line, the north pole points away from the sun in northern hemisphere winter and the closer you are to it, the shorter the days.