When you picture the four seasons - assuming you live in a place where four seasons are
a thing - you probably think of summer and winter as opposites - after all, one is hot
and one is cold - and the rest of the year as this middle ground we travel back and forth
through on our way from one extreme to another...in other words, fall seems like spring in reverse.
But the reality, at least based on how we technically define seasons, is totally different.
Because the astronomical system, which we four-season folk use for most things, defines
seasons not via temperature, but via daylight.
Over the course of a year, the amount of daylight you experience on a given day oscillates in
what’s basically a sine wave.
This is how the hours of daylight change across the course of a year in the northern hemisphere.
The places where the wave crosses this line are “equinoxes” - days with nearly equal
hours of daylight and darkness - and the peaks are “solstices” - the days with the most
and least daylight; these features are how we technically divvy up the seasons.
And when you look at the seasons like this, you can see that it’s actually summer that’s
spring in reverse - not fall; during spring, we go from equal hours of day and darkness
to peak daylight, and over the course of summer, we go right back to equal hours of day and
darkness, where we had started at the beginning of spring.
The same pattern - but inverted - occurs later in the year; winter is actually fall happening
in reverse.
But as clear as that is on the graph, it doesn’t make any sense as far as how we actually experience
the seasons; winter doesn’t feel anything like fall happening in reverse.
And if spring and summer have the same amount of daylight overall, why is summer so much
hotter?
Well, just like it takes a while for a pot of water to boil on a stove, it takes a while
for changing daylight patterns to have an effect on temperature.
Daylight increases over the course of spring, but since Earth’s temperature rises slowly
in response, we generally don’t have our hottest days until the amount of daylight
has already started to decrease.
And since the falling temperature lags behind the decreasing daylight, the coldest days
tend to happen once daylight has already started increasing.
Looking at the temperature graph, spring and fall are reverse versions of the same middle
ground - and everything we experience about seasons makes sense.
So why don’t we just use temperature to divide up the four seasons?
Well, compared to temperature, which can vary wildly from year to year, daylight is simply
more consistent.
Daylight is so predictable that we’ve relied on it for thousands of years to mark the passage
of time and track annual events, like the flooding of rivers and the best time to plant
seeds.
That deep-seed-ed relationship with daylight is probably why it came to dictate the transitions
between our four modern seasons, even if these days, it’s temperature - rather than daylight
- that puts a spring in our step.
Given that - at least for those of us in the northern hemisphere - our days are getting
shorter and our temps are getting colder, you might be looking to learn about the world
in a cozy, indoor, kind of way.
It’s a great time to check out Nebula, a platform built by independent education-y
creators like us as a way to explore new ideas - like our own MinuteBody series; the newest
episode explains how pigeons use their throats to make milk for their babies.
Like all the other creative content on Nebula, MinuteBody is brought to you with the help
of CuriosityStream, a streaming service that brings all sorts of incredible documentaries
right to your device.
I just streamed Seasons of the Forest, which gives you a front-row seat to some amazing
seasonal transformations.
When you sign up at curiositystream.com/minuteearth, for less than fifteen bucks you’ll not only
get access to CuriosityStream for an entire year...you’ll also get to experience all
sorts of cool content on Nebula: stuff from the folks behind TierZoo, Polyphonic, Wendover
- and all-new episodes of MinuteBody.
Check it out!