For the last six months or so, Earth has been pulled closer and closer to the Sun. As of January 5, we’ll be a full 5.1 million kilometers closer to the Sun than we were last July.

But don’t worry, this isn’t the prelude to a 2012 Mayan-calendar disaster. Our planet is merely reaching its annual journey to perihelion—the point in the Earth’s slightly elliptical orbit when it’s nearest to the Sun (hēlios in Greek). After reaching the perihelion, the Earth’s orbital momentum will fling it back away from the Sun like a yo-yo until it reaches the mid-summer aphelion.

Does the close proximity of the Sun have anything to do with global warming (and our snow-scarce winter so far)? No. While this momentum does lead to some interesting scientific phenomena, none of them have anything to do with global warming.

But first, some background for you science geeks. Due to the Earth’s perihelion, we’re currently about 3.5 percent closer to the Sun than we will be next July. That means the Sun currently appears about 7 percent brighter, averaged globally. While the sun is brighter, the Earth is actually slightly cooler, since most sunlight currently falls on the Southern Hemisphere, where the reflective oceans keep the Earth from absorbing as much solar energy as during our summer. The Earth also moves faster along its orbit during this time of year, hitting a top speed of almost 19 miles per second before it begins to slow down as we climb out of the Sun’s gravity well. As a result, the northern winter is nearly 5 days shorter than the northern summer.

These effects vary with the eccentricity, or shape, of the Earth’s orbit. Today, with just a 3.5 percent difference between our perihelion and aphelion, Earth’s orbit is roughly a circle. But over the next 95,000 years, Earth’s orbit will become squished to an ellipse with a 13 percent difference in distance before spending another 95,000 years rounding back out to its present shape.

This variation in Earth’s orbit over time is one of three Milanković cycles. During World War I, Milutin Milanković theorized that variations in the Earth’s orbit determine climatic patterns. Along with orbit shape, the tilt of the Earths’ rotational axis changes between 22.1 and 24.5 degrees on a 41,000-year cycle and the axis itself precesses—or wobbles—on a 26,000-year cycle.

But was Milanković right? Can these variations help explain climate change? Historically, yes. The complex interplay of these multi-thousand year Milanković cycles—which vary the amount of solar radiation absorbed by the Earth—has combined with plate tectonics to trigger changes in the Earth’s climate in the past.

But what about today’s climate change? Is it caused by the shape, axial tilt, or wobbliness of the Earth’s orbit? No. We only have ourselves to blame for today’s global warming.

As powerful as these natural effects are, they are dwarfed by human-caused emissions of greenhouse gases. In the words of researcher Richard Alley, winner of the Stephen H. Schneider Award for Outstanding Climate Science Communication, we are currently changing the climate “a million times faster than nature.”

Photos: redOrbit, Star Tribune

As of January 5, we'll be a full 5.1 million kilometers closer to the Sun than we were last July. So does this close proximity have anything to do with global warming (and our snow-scarce winter so far)?