Seasons: They Have a Reason

The seasons have been a staple of life on earth, influencing the moods and activities of many -- but rarely do we think about why they occur.

The seasons are fascinating, something we take for granted but yet something that has a major impact on our day to day life. A common misconception regarding the 'reason for the seasons', is that it is caused by the distance of earth's rotation around the sun, however the earth's orbit is nearly circular, with a 3% difference from closest to farthest approach to the sun, being nearly negligible in terms of differences in energy recieved. Additionally, if the seasons were merely based on various distances from the sun, the seasons would remain consistent across hemispheres, but they don't.

Rather the underlying cause for the seasons is the tilt of the earth's axis, which allow for the light from the sun to hit the earth's surface more or less directly. Check out this diagram, to help visualize why the tilt of the axis is the reason for the seasons.

Source: http://scinote.tumblr.com/post/109822921304/the-spin-of-the-earth-the-seasons

As one can see, during the summer the northern hemisphere recieves the most energy from the sun, and is consequently the warmest. This is based upon the tilt of the axis.

The severity of the seasons is dependent upon the degree of tilt. If another planet had a more drastic tilt, the seasons would fluctuate much more. As a result of this, the axial tilt of a planet can determine the chance for life on that planet. Scientists believe that reduced seasonality can be linked to the evolution of complex organisms. Rene Heller, a postdoctoral research associate at the Leibniz Institute for Astrophysics, notes that, "axial tilt, or obliquity, is a crucial parameter for climate and the possible habitality of a planet". If this tilt, is lost or altered it could potentially eliminate, or significantly reduce the habitability of a planet. Over the course of a planet's history, many circumstances can affect its axial tilt. This includes: the impacts of celestial bodies, gravitational pulls from neighboring stars and planets, as well as other factors.

To further exemplify the importance of obliquity, we could look at a potential planet in the "goldilocks zone", but one with an obliquity similar to that of Uranus. For this planet, the north pole would face the star they orbit for a quarter, then shift completely away the next quarter. One can imagine the difficulty of survival of complex life with such a large obliquity, causing rapid and extreme fluctations in climate. However, it is also important to maintain an axial tilt. If it erodes, this causes the equators to feel the full force of the sun's radiation, while the poles would recieve even less seasonal shifts than they already do. As a result, there would be bands of differening temperatures based on the planet's latitude. While in theory this could still enable life to persist, it could also spur the collapse of the planet's atmosphere.

This severity of seasons upon earth is also based upon latitude, but to a much lesser extent because of its pronounced obliquity. The equator is least affected by changes in the directness of solar radiation, and thus has the least extreme seasons. On the other hand, at the poles the effect is most pronounced, with dramatic shifts in daylight depending on the season.

As one can see, seasons have a bigger impact on daily life besides the need to shovel your driveway, or turn on the AC -- in fact they are arguably vital to human existence. If you would like more information about the seasons, check out these links!

- http://spaceplace.nasa.gov/seasons/en/

- http://www.morehead.unc.edu/Shows/EMS/seasons.htm

- https://www.youtube.com/watch?v=aSNs15sEINM

- http://www.astrobio.net/news-exclusive/high-planetary-tilt-lowers-odds-for-life/