Why is the average temperature of earth is greater in July (Aphelion) than in January (Perihelion) ?
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Earth does travel around the Sun, but the path is not a perfect circle. It's an ellipse, slightly lopsided. One end is a bit closer to the Sun than the other.
On January 4th, 2006, our planet is at the closer end--a point astronomers call "perihelion." We'll be closer to the Sun than we are at any other time of the year.
"All planets in our solar system travel around the Sun in elliptical orbits. It's Kepler's 1st Law," says University of Florida astronomy professor George Lebo. "The eccentricity of Earth's orbit is 1.7%. In January when we're closest to the Sun (perihelion), the distance is 147.5 million km vs. 152.6 million km in July--the farthest point (aphelion)."
A nearby sun means more sunlight for our planet. "Averaged over the globe, sunlight falling on Earth at perihelion is about 7% more intense than it is at aphelion," says Roy Spencer of NASA's Global Hydrology and Climate Center (GHCC).
Then why is it so cold outside?
"Seasonal weather patterns are shaped primarily by the 23.5 degree tilt of our planet's spin axis, not by aphelion or perihelion," explains Lebo. "During northern winter the north pole is tilted away from Sun. The Sun hangs low in the sky, and days are short. That's what makes January so cold." (Note: Seasons are reversed in the two hemispheres, north and south. So January is generally warm in the south.)
There's more to the story: Says Spencer, "the average temperature of the whole earth at perihelion is about 4oF or 2.3oC lower than it is at aphelion." (See the global temperature data at the GHCC web site.) Our planet is actually colder when we're closer to the Sun. Strange but true.
On January 4th, 2006, our planet is at the closer end--a point astronomers call "perihelion." We'll be closer to the Sun than we are at any other time of the year.
"All planets in our solar system travel around the Sun in elliptical orbits. It's Kepler's 1st Law," says University of Florida astronomy professor George Lebo. "The eccentricity of Earth's orbit is 1.7%. In January when we're closest to the Sun (perihelion), the distance is 147.5 million km vs. 152.6 million km in July--the farthest point (aphelion)."
A nearby sun means more sunlight for our planet. "Averaged over the globe, sunlight falling on Earth at perihelion is about 7% more intense than it is at aphelion," says Roy Spencer of NASA's Global Hydrology and Climate Center (GHCC).
Then why is it so cold outside?
"Seasonal weather patterns are shaped primarily by the 23.5 degree tilt of our planet's spin axis, not by aphelion or perihelion," explains Lebo. "During northern winter the north pole is tilted away from Sun. The Sun hangs low in the sky, and days are short. That's what makes January so cold." (Note: Seasons are reversed in the two hemispheres, north and south. So January is generally warm in the south.)
There's more to the story: Says Spencer, "the average temperature of the whole earth at perihelion is about 4oF or 2.3oC lower than it is at aphelion." (See the global temperature data at the GHCC web site.) Our planet is actually colder when we're closer to the Sun. Strange but true.
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Answer:
The average temperature of Earth is greater in July because it is the month of 'aphelion' which means that the Earth is nearest to the Sun in July whereas it is farthest from Sun in January.
Hope it helps
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