Answer:
The earth travels around the sun in a (parabolic) path known as the ecliptic.
The axis of the earth's rotation is tilted about 23 deg to the ecliptic.
At the winter solstice (around Dec. 21), the sun will appear to be at its farthest south in its orbit. (This marks the beginning of winter for observers in the northern hemisphere and the beginning of summer for an observer in the southern hemisphere)
The tilt of the axis of the earth's rotation causes the change in the daylight hours.