aphelion.gifThe Earth was at aphelion yesterday. Aphelion is the greatest distance the Earth gets from the Sun. From now until January 2nd, when perihelion occurs, we'll be getting a little bit closer to the Sun every day. Like the rings of dust that circle Saturn the Earth's orbit is not perfectly circular. Instead we have a slightly eccentric orbit. Gothamist and the rest of the Earth were 152.6 million km away from the Sun yesterday and will be 147.5 million km away come January. Since we are slightly farther from the Sun in July, the Earth receives a little less sunlight than in January.

Why is it warm in July and cold in January? You might say it is only warmer in the Northern Hemisphere, and you would be partially correct. However, the average global temperature is also greater in July than in January. This takes a bit of explanation.

Seasons are controlled by the 23.5 degree tilt of the Earth with respect to our orbit around the Sun. The tilt means the hours of daylight are greater in the summer hemisphere and the Sun's rays are more directly overhead. Here's an experiment you can do (as always Gothamist encourages adult supervision when conducting scientific experiments): You need a flashlight, a piece of paper or cardboard, a foot or so of string, tape and a pencil. Tape the string to the flashlight and the cardboard. Hold the flashlight directly above the cardboard with the string taut. Trace the outline of the brightest light made by the flashlight. Now, keeping the string taut, move the flashlight down so it shines on the cardboard at a 45 degree angle (which is close to the angular change we go through from summer to winter), half-way between vertical and horizontal. Trace the outlight of the light again. Although the same amount of light shines on the cardboard in both instances, the light is much more concentrated when the source, the flashlight, is directly overhead. That change in intensity is what drives the seasons.

If you look at a world map you'll notice a lot more land area in the Northern Hemisphere than in the Southern Hemisphere. Land heats up and cools down much faster than water. As a result, the Northern Hemisphere summer is much warmer than the Southern Hemisphere summer. By the same reasoning shouldn't the Northern Hemisphere be much colder than the Southern in the winter? It would be were it not for icy cold Antarctica sitting above the South Pole. Antarctica makes the Southern Hemisphere winters really, really cold. The end result being that the Earth as a whole is warmer in July than in January.

One final note. The dates of the aphelion and perihelion are not fixed. Over thousands of years the Earth precesses in it's orbit. The aphelion moves backwards from July through June all the way through January and eventually back to July. That change in sunlight when the aphelion is in January is a primary instigator of the Ice Ages, but that is a story for another time.