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Blowin' in the Wind, Issue #102 Aphelion and Perihelion - April 3, 2013
April 03, 2013
Hello ,

Aphelion and Perihelion: our Orbit

Just what are Aphelion and Perihelion? Perihelion occurs when the earth is closest to the sun. Similarly, aphelion is when the earth is furthest from the sun.

The closest we ever get to the sun is just over 147 million kilometers, about 91 million miles, and the furthest is a little more than 152 million kilometers. This difference in the distances, which is roughly 3%, does not determine our seasons. It also results in an eccentricity in our orbit of a little over 1.6% at the present time. Perihelion happens during the Northern Hemisphere winter. And aphelion occurs in June. With winter in the north, when we are closest to the sun while the folks in the southern hemisphere enjoy summer, this brings up a question: If we are closest to the sun in January and getting more radiation in energy per square metre of land earth surface, does it moderate our winter/winter cycle somewhat?

Here's a couple more...Do we end up with milder winters and slightly cooler summers in the north than we would otherwise have and does the current setting make the southern hemisphere winter colder? While we do get at little less solar energy because of the increased distance in our summer up north, are the winters more extreme down south?

How is climate affected?

Climate is never straightforward and easy to address just by looking at one or two inputs, and these questions cause us to examine one great example of the effects of complex multi-variable problems. Something offsets the eccentricity cycle for this case in point. The southern hemisphere has more water overall, which has a tremendous specific heat, that is heat storage capability, and it takes more heat to change its temperature, than it does to warm up the air above dry land.

It counters the more extreme differences in seasonal energy influx in the south with a moderating influence of the water on the climate. As a result, we see more maritime, and less extreme, climates all the way through mid-latitude southern-hemisphere land and ocean surface.

In other words, until you reach the coast of Antarctica, there are no climates south of the equator which are classified as continental, and these are the areas which have the greatest differences between summer and winter temperatures. By the way, Antarctica itself is too cold all year for a continental classification is labeled with an ice cap climate, with the warmest areas known as tundra.

More about Antarctica

We know Antarctica is much colder than the North Pole. Is it because of the earth-sun distances? No.

Most of that difference is due to the high altitudes completely surrounding the South Pole. This keeps the icy continental surface held up in cooler upper atmospheric air all year around. Also, during their summers, a large white ice sheet reflects more of its incoming solar radiation.

Climate mathematicians make use of a parameter called albedo to describe the reflection associated with this ice. And Antarctica’s ice sheet is much larger than the one covering Greenland. This enhances the temperature differences quite greatly.

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