If you have been watching sunsets over the past few months, you may have noticed that they have been slowly moving southward along the horizon. Over the next few nights, the sun will set at its most southerly point and will appear to linger for a few days. This event marks the winter solstice. Solstice literally means "sun stand still." The exact moment of the solstice varies from year to year. This year, the earliest solstice since 1896 occurs on Friday, Dec. 21 at 6:12 a.m.
The tilt of our planet's axis causes the sun to appear to change its path across the sky, as Earth orbits the sun. In the summer, the sun takes a long high path across the sky, and we receive lots of sunlight. During winter, the sun is low in the sky, and its trek from sunrise to sunset is short. Winter days in Pittsburgh have about 9 hours of light and 15 hours of darkness.
The solstice is the day when the northern hemisphere and Pittsburgh receive their smallest share of the sun's radiation and energy. Our ancestors feared the loss of sunlight. They knew how much we depend on the sun. They marked the winter solstice as a special event. From this point forward, the days begin to get longer and the nights shorter.
Because Earth's North Pole is tilted away from the sun, sunlight now strikes us at a long slanting angle. This spreads the sun's warmth over a larger area. As a result, we get far less energy from the sun in December than in June. It is this drop in solar energy, not the distance of the Earth from the sun, that causes the seasons to change. If Earth's axis were not tipped to the plane of its orbit, we wouldn't have seasons, trees would not shed their leaves and animals wouldn't hibernate.