Why Are There Four Seasons?

Question posed by Omar.

Winter, spring, summer or autumn*, all you have to do is call...

Like many of the processes and cycles that we observe and take for granted here on our blue-green orb we like to call the Earth, the seasons are an effect of our steady and unstoppable** sweeping journey around the Sun. These things run in cycles because our orbit of the Sun is itself a cycle.

We take one year to orbit our Sun, spinning like a top. This top doesn't spin completely upright, though: it leans over at an angle of about 23 degrees, and this is what gives us our seasons.

Imagine hovering above the Sun, and looking down on our planet's orbit. From our point of view, the Earth's 23 degree tilt always points the same way as it makes its way around the Sun. This means that, at different points in the year, the North pole is tilted slightly towards the Sun and at others slightly away from it.

Here's a short video showing the idea. The Earth and Sun are quite considerably not to scale in it:

When the Earth's North pole is leaning towards the Sun, we in the Northern-hemisphere experience the season we like to call summer. Put simply, this is because more of the Sun's energy is hitting us head-on. At the exact opposite point in Earth's orbit, the North pole is leaning away from the Sun, so the energy we receive hits at a funny angle and is spread out more. This is winter.

The North pole leans in towards the Sun for approximately half of our orbit, and away from it for most of the other part, and the angle at which the Sun's rays hit each part of the Earth changes gradually.There are two points in our orbit at which the North pole doesn't lean either towards or away from the Sun, instead leaning in the direction of Earth's orbit, either looking forwards (at one point) or backwards (at the other). These two points are in the middle of the seasons that we call autumn and spring.

So, in summery***: summer happens in the Northern-hemisphere when the Earth's North pole is tilted towards the Sun. As the extent of this tilt changes, we cool down into autumn. As the tilt starts to point away from the Sun, we head into winter. When the tilt starts to change back the other way, we get a bit warmer and head through springtime until we warm up into summer, beginning the cycle again.

There are four seasons because there are two possibilities for the direction of the Earth's axial tilt (either towards or away from the Sun), and there are two periods of transition between these two possibilities.

In the Southern-hemisphere exactly the same thing happens, except when it's winter up North, it's summer down South (and vice-versa)!

You might like to check out this post from December 22nd, an important date when thinking about axial tilt!

* Yes, I know the lyric is fall rather than autumn, but I'm British.
** Barring major planetary collisions, of course. Which are, incidentally, unlikely to the point at which we can safely ignore them, regardless of what any 2012 end-of-the-world woo-peddlers might try to tell you.
*** This is a deliberate misspelling. It's a sort of joke.


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