Environment and Ecology

The Earth’s Environment

If the Earth is closest to the Sun in January, why is it so cold in the Northern Hemisphere at that time?

While the Northern Hemisphere is experiencing cold in January—and the Southern Hemisphere its warm summer—the Earth is closest to the Sun in its orbit. When the planet is farther from the Sun—and although it seems contrary to what we would think—the Northern Hemisphere is warmer. Scientists know that the average temperature increases about 4°F (2.3°C) when the Earth is farther from the Sun (the Northern Hemisphere summer), even though the sunlight is less intense from that distance. This is not only because of the tilt of our planet’s axis (see above), but also because of a characteristic of our planet—the distribution of the continents and oceans. In particular, the Northern Hemisphere contains more land, while the Southern Hemisphere has more oceans. Heating up the land is easier than heating the oceans; thus, when it’s January, it’s more difficult for the Sun to heat up the southern oceans—and we experience overall colder temperatures even though our star is closer.

Another factor when it comes to warmer Northern Hemisphere temperatures versus the Southern Hemisphere is that the duration of the summers in each hemisphere differs as the planet orbits the Sun. Following Johann Kepler’s (1571–1630) second law of planetary motion, a planet moves more slowly when it is farther away from the Sun (called aphelion) than when it is close (perihelion). In fact, the Northern Hemisphere’s summer is two to three days longer when the planet is farther from the Sun, giving the Sun more time to bake the continents.

This diagram, which is obviously not to scale, shows how the tilt of the earth on its axis results in different seasons as our planet orbits the sun over the course of a year.


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