Today at 5:47 p.m. UTC will mark the moment of the winter solstice in the northern hemisphere -- the shortest day and the longest night of the season. Earth's seasons, the the transitions beteween them, the summer and winter solstices and the spring and autumnal equinoxes, are cause by the tilt of the earth's axis (currently 23 degrees 44 minutes) with respect the the plane of our orbit around the sun. In our case tonight, the north end of the axis is pointed farthest away from the sun, as seen in the image above. The axis tilt during an entire yearlong orbit may be seen below.
The axis tilt is more or less stationary in terms of human lifespans, though over geologic time it does undergo precession or wobble, with each cycle taking about 26,000 years. This is why our "north star" (the star toward which the north axis end appears to point) is currently Polaris, but in another thousand years will be Gamma Cephei.