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Is July hot because we're closer to the Sun?

In fact the Earth is at its farthest point from the Sun in early July and closest to the Sun in early January.

The difference in distance from the Earth to the Sun between these two extremes is about 3 million miles (5 million kilometers), or 3.3 percent, which makes a difference in radiant heat received by the Earth of nearly 7 percent. Thus for the Northern Hemisphere one might assume that this difference tends to warm the winter and cool the summer.

The seasons, however, are not caused by the change in the distance of the Earth to the Sun. Rather, they are due to the 23.5-degree tilt of the Earth's axis, which causes various parts of our planet to be turned toward or away from the Sun at various times of the year. Summer in the Northern Hemisphere is when that half of the planet is positioned for longer days and maximum sunlight.

The preponderance of large landmasses in the Northern Hemisphere has an effect, too, tending to make winters colder and summers hotter than those of the Southern Hemisphere. Land warms and cools more quickly than water. The Southern Hemisphere has a far greater amount of ocean coverage, which moderates temperatures, helping to make the winters a bit milder and the summers a bit cooler.

Here's another way to think about this: In the Northern Hemisphere, when we think of "cold outbreaks" during the wintertime, we think of cold surges of air coming down from Canada or Siberia, where the frigid air builds up. There are no such similarly large land regions in the Southern Hemisphere that can produce comparably large outbreaks of cold air.

There are quirks in all this, however. When we compare temperatures of the North Pole versus the South Pole, the South Pole wins because Antarctica is a land-locked continent as opposed to the North Pole, where a similar solid landmass does not exist.

But Antarctica is completely surrounded by water, so any frigid air that might originate from it is significantly modified when is spreads out toward Australia, South Africa and South America. [Mysteries of the Sun]

Next Page: Hey! That's no North Star!