Why winter days are shorter and summer days are longer?

The reason why winter days are much shorter is the tilt of the Earth's axis to its orbiting plane.

This tilt equals 23.5 degrees and makes the Earth expose its northern hemisphere to the Sun on one side of the orbit, so it is summer in USA and Europe. When our planet proceeds to the other side of its orbit, the summer comes to its southern hemisphere, to countries like Australia and Brasil.