Science communication

2018.04.16

The reason for seaons

As the Earth moves around the sun, it rotates on a tilted axis; this is the cause of seasons

The period of daylight has grown noticeably longer in Stockholm since the beginning of the year and flowers have started to emerge. While enjoying my “spring fever”, I wondered how many others appreciate why we have seasons.

I did a quick survey of non-scientist friends and family (sample size = 4). The answers I got to the question “Do you know why there are seasons?” included “Not really” and “It has something to do with the Earth moving around the sun”.

Hence, this blog post.

It’s about the tilt

Yes, the reason for seasons has something to do with how the Earth moves around the sun, but not in the way you might think. Our distance from the sun does change throughout the year (the Earth’s orbit isn’t a perfect circle), but this isn’t enough of a change to result in the seasons that we experience. Rather, the seasons are due to the Earth’s 23.5° tilt as it rotates on its axis.

Because of the tilt, the angle at which sunlight hits the Earth (called the angle of incidence) changes throughout the year. During summer in high latitude areas, the sun is relatively high in the sky and its energy is more concentrated over a given area. This concentrated energy results in increased temperatures and plant production. Contrast this situation with the winter, when the sun appears low on the horizon. The sunlight that strikes earth is spread out over a larger area, and as a result, its energy is weaker. 

This is one of those cases where a picture is worth a thousand words.

earth tilt

During the year, the angle of sunlight that strikes different parts of the world changes because of the Earth's tilt. During winter, the angle is such that the sunlight is spread out over a larger area, making it weaker. Source https://www.weather.gov/cle/seasons

Given the tilt, the further you are from the equator, the more likely you are to notice the change in seasons. For example, there is not as much difference between seasons in Coasta Rica compared to, say, Iceland.  

During the vernal (or spring) and autumnal equinoxes, there are about 12 hours of daylight and 12 hours of darkness at all places of the Earth. The word “equinox“ comes from the Latin words aequus (equal) and nox (night). On these days, there is relatively no tilt with respect to the sun.

During the winter and summer solstices, the Earth’s tilt is at its maximum relative to the sun. In the Northern hemisphere, the winter solstice marks the day with the least amount of daylight and the summer solstice marks the day the most amount of sunlight. The summer solstice has special meaning in many places. In Sweden, it is a big deal.

If that didn’t clarify things, a video is probably worth even more words than a picture.

Seasons and culture

The seasons and changes in seasons are found throughout the arts and literature as motifs, metaphors, and plot devices. My personal favorite is the violin concerti, The Four Seasons, by Italian composer Antonio Vivaldi. Originally published in 1725, Vivaldi gives musical expresses to the seasons and their differences in a magnificent way.

Recently, Marie Samuelsson composed The Five Seasons as a continuation of Vivaldi’s Baroque work. The Five Seasons reflects Samuelsson’s impressions of how climate change is disturbing the seasons with extreme events, like floods and drought. I had the opportunity to attend the first performance of this work at Östersjöfestivalen last year. You can hear the striking contrast between The Four Seasons and The Five Seasons here.

Michelle McCrackin

Limnologist
michelle.mccrackin@su.se