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Why Are There Seasons?

October 1, 2013

Why are There Seasons?

Earth has four seasons – Spring, Summer, Fall, and Winter – that’s widely known. But what causes these seasons?

First let’s put a myth to bed: Summer occurs when the Earth is closest to the Sun. Not true. The Earth orbits the sun in an ellipse, kind of like an oval.  On July 5th the Earth is at the Aphelion, furthest from the sun and on January 4th it’s at the Perihelion, or closest to the sun.

NASA

Perihelion vs. Aphelion

On the left side of the image, is the relative size of the Sun at the Perihelion or when the Earth is closest to the Sun on January 4. On the right side of the image the Sun is at the Aphelion or Earth's furthest from the sun on July 4.

While Earth’s orbit around the Sun does play a role in the changing of the seasons, it’s not because the Earth receives more solar energy when it’s closer to the Sun or less energy when it’s further away.

The main reason we have the seasons is due to the Earth’s tilt. 

Billions of years ago Earth was walloped by a massive space object, causing the ejection of billions of tons materials and eventually leading to the formation of the Moon.   That massive collision did more than form the moon, it pushed the Earth off kilter by 23.5 degrees. 

Wikipedia/Dna-webmaster

The Earth's Tilt

The Earth is tilted 23.5 degrees. That tilt is a major contributing factor to seasons on out planet.

Those 23.5 degrees coupled with the Earth’s orbit around the Sun allows for uneven heating of the planet. 

So, in August, in the northern hemisphere, the Earth is tilted toward the sun, which means longer days and summer weather. 

Conversely, August in the southern hemisphere is characterized by shorter days and winter weather, because that part of the Earth is tilted away from the Sun. 

Fast-forwarding to February, the northern hemisphere is leaning away from the sun and in the grips of winter. Meanwhile, the southern hemisphere is getting prolonged periods of sunshine and is baking under summer heat. 

NOAA

The Change in Seasons

The above image shows northern hemispheric seasons as they are related to position in orbit around the Sun and the Earth's tilt.

The official changing of the seasons is marked by four astronomic events: The vernal (spring) equinox, summer solstice, autumnal equinox and the winter solstice.  Solstices mark the highest or lowest angle of the sun and the equinoxes mark the mid-point of sun in the sky.


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