OPENING
Welcome to Episode 37 of The STEM Sessions Podcast, published on December 12, 2025. This is the podcast that says it’s OK to do your own research on STEM topics even if you don’t have any fancy credentials. I am your host, Cody Colborn.
INTRODUCTION
Let’s start with a bit of news about the podcast
- In 2026, I’ll be mixing interviews into the line
- I have several recorded and several on the books
- Topics include native gardening in southern california and the history and science of board games like backgammon
- It hopefully adds a fresh dynamic and variety
As for this episode, while preparing my notes for upcoming Socal Science Hikes winter solstice night hike
- One topic is solstice marking the changing of the seasons
- Fell down the rabbit hole of how we define seasons
- It’s a weird combination of scientific complexity and ambiguity
- And I thought it would be make a good episode
One last thing, I really really wanted to use Slayer’s Seasons of the Abyss for this episode’s music, but alas, copyrights
This is The STEM Sessions Podcast Episode 38 – What do Seasons even Mean?
MAIN BODY
Living in North America, I’ve known there to be for season: winter, spring, summer, fall
- Growing up in Montana, winter meant cold and snow, spring meant the snow was melting and plants started to get green, summer meant hot and working outside, and fall meant harvest
- Winter typically started around Thanksgiving and lasted until mid April, spring was April until early June, summer took us through September, and fall was October and November
- My grandpa used to say Montana actually has only three seasons: winter, august, and September
- Account common expression was Montana had only two seasons: winter and road construction
- That last one is the most accurate in hindsight
After moving to phoenix, Arizona, the seasons shifted
- Summer was May through October
- Fall was November
- Winter was December through February
- And spring March and April
- Plants greened up in the fall and winter instead of spring, and started turning brown in the spring instead of fall
Then I moved to Southern California, and the season shifted again
- Summer was now July through October
- Fall was November
- Winter was December through February
- And spring was March through June
So seasons have always been arbitrary in mind
- Their beginnings and ends shift year to year and location to location
- Just a way to communicate colloquially
Surprisingly, at least to me, this idea of the year being divided into four seasons goes back to at least the first century BCE, when Roman scholar Marcus Terentius Varro wrote down his suggestion for when each season should begin
- Those starting dates are roughly six weeks earlier than what we recognize today
Four seasons also show up in other cultures, but while common, the concept wasn’t universal
- Medieval Scandinavia, as a counter example, divided the year into two seasons: winter or vetr and summer or sumarr
- The division being mid April and mid October
Today, we’ve settled on four seasons: winter, spring, summer, fall
- Evenly distributed throughout the year
- But different user groups have different starting dates for each season
What most of us learn is called the astronomical seasons
- Seasons start and end on the astronomical events of the solstices and equinoxes
- Winter begins on the winter solstice, around Dec 20 depending on the year
- Spring on the vernal equinox around March 20
- Summer on the summer solstice, around June 20
- And Fall on the autumnal equinox, around Sept 20
On one hand, it makes sense to use solstices and equinoxes as the divisions
- They are measurable, repeatable events – closest things we have to a natural clock or calendar
- Always been significant to our species
- Ceremonies, holidays, and traditions were built around them
- In some ways, agriculture was built around them
- But even as a kid, using them as starting points for the seasons seemed wrong to me
These astronomical events essentially track the daily duration of sunlight
- Summer solstice is the longest day of the year
- Winter solstice is the shortest
- Equinoxes are an even split
Duration changes because the earth’s axis of rotation is titled
- In fact, this tilt of 23.5 degrees is the reason we have seasons in the first place
- The tilt is effectively fixed, at least on any timescale that matters to us
- That means during some parts of the year, one pole of the earth will be more directly facing the sun than the other
- As a surface leans more and more towards the sun, it receives more and more sunlight – i.e. length of the day increases
- As a surface leans further and further way from the sun, it receives less and less sunlight – i.e. length of the day decreases
As the earth orbits, the surface that most directly faces the sun changes
- Northern hemisphere leans most directly to the sun on the summer solstice – hence the longest day
- Northern hemisphere leans furtherst away from the sun on the winter solstice – hence the shortest day
- And this is what made no sense to me about starting seasons on the solstices as a kid
- The developing analytical and cynical part of my brain thought if summer is defined as the longest days of the year, and winter is defined as shortest days of the year, why aren’t we setting the starting points to do that?
We’ve collectively decided a season should last three months, or about 13 weeks or about 91 days
- Therefore, summer should be the 13 weeks containing the longest days
- Let’s call the summer solstice day N
- Following day, N+1 is a bit shorter, as is the next day N+2, and so on all the way to N+91
- But if you look at the day before the solstice, N-1, that amount of daylight is the same as the day after solstice N+1
- Same as two days before the solstice N-2 and two days after the solstice N+2
So if you’ve going to define summer as the 90 ish days with the most sunlight, it shouldn’t be N to N+91, it should be N-45 to N+45
- That makes the summer solstice the middle of summer
- Meaning summer would start around May 7 and end around August 6
- Similarly, winter would start around November 5 and end around Feb 4
Of course, I felt vindicated a handful of years later when I learned about Midsommar in Sweden and midwinter festivals welcoming back the sun
- And it’s cool to see those dates aligning with the dates of the ancient Roman seasons described earlier of February 7, May 9, August 11, and November 10
To complicate matters more, there is also an accepted concept in the scientific community of meteorological seasons
- This method bounds seasons by average surface tempertures
- Coldest months in the northern hemisphere are generally December, January, and February so winter starts on December 1 and ends on February 28
- Hottest months are generally June, July, August, so summer starts on June 1 and ends August 31
- To complete the picture, Spring is March 1 to May 31, and fall September 1 to November 30
So the meteorological seasons start about three weeks earlier than the astronomical seasons, which is not enough to be centered on the solstices
- And this is because the summer solstice is not the hottest day of the year, nor is the winter solstice the coldest
- The earth has a lot of mass, and that mass takes time to heat up
- Not to mention the oceans are excellent heatsinks, which do a lot to moderation surface temps
- Therefore, surface temps always trail changes in solar radiation
- This delay is called seasonal lag
Despite solar radiation peaking on the summer solstice, surface temps may not peak for up to seven weeks later
- Seasonal lag also works with decreasing solar radiation and cooling temps
- So the coldest surface temps may not occur until seven weeks after the winter solstice
Seasonal temperature change is greatly impacted by latitude
- Latitudes close to the equator see relatively little temperature change from summer to winter
- This is due to areas around the equator experience less change in solar radiation throughout the year, i.e. the earth’s tilt has less impact on the equator than it does on the poles
- These regions also experience less variance in daylight hours than mid and upper latitudes
Geography like proximity to oceans can also impact temperature variances, which is why we’re often told that southern California has no seasons
- Altitude, vegetation, also play roles locally
- And I would be remiss not to mention potential impacts from climate change
At face value, a season is a bounded period of the year known for characteristics that make it unique from the rest of the year
- Seasons can be defined technically, such as in terms of surface temperature or daylight or astronomical events, with definitive starting and ending dates
- They can also be defined based on personal, cultural, or regional experiences, using colorful colloquialisms
Seasons have real meanings, while at the same time being arbitrary
- And whether scientific or poetic, seasons are part of our shared experiences, and probably always will be
OUTRO AND CREDITS
Thank you for listening to this episode of The STEM Sessions Podcast – a podcast researched, written, and produced by Cody Colborn.
This episode’s music is titled “The Metal” and was composed by Ivan Selivanov and can be found on Pixabay.com.
The shownotes and transcript can be found at thestemsessions.com, which is also the best place to provide feedback, corrections, and value for value support.
Remember, doing your own critical research will always be to your benefit, and any pundit or scientist that tells you “just trust me, bro” or scolds you for fact checking and questioning isn’t offering true education.
So until the next one, keeping learning.
REFERENCES
https://www.seversondells.com/seasons
https://www.weather.gov/cle/seasons
https://www.livescience.com/25202-seasons.html
https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons