Thread by @MaxFagin

3... 2... 1... 22:23 GMT! Happy #WinterSoltice Earth!

I'm going to let you in on a little trade secret: There is script astronomers are supposed to follow today when explaining what the solstice is, and that script goes something like this:

1) Construct a strawman who mistakenly thinks winter is because Earth has gotten further from the sun.

2) Ask the strawman "Ah! But if that *were* the cause of winter, then why is it summer in the southern hemisphere right now?"

3) Pivot to a discussion of the tilt of the Earth's axis (the real reason for the seasons).

4) Bonus Points: Use the words "obliquity", "eccentricity" and "precession" as much as possible to give your audience some impressive words to drop at their holiday parties.

But you know what? It's Friday, and I don't really feel like going through that script again. So this solstice, let's try something more fun. Let's talk about the solstice on another planet: Mars!

The notion that seasons are caused by Earth getting closer and farther from the sun isn't *entirely* wrong. Earth's distance from the sun *does* change by +/-1.5% over the course of the year, causing the amount of sunlight we receive to change by +/-3%.

Credit: @apod 2009 July 3

It's just that the amount of sunlight a mid-latitude region on Earth receives also changes by +/-40% due to the tip of the Earth's equator, so the effect of the sun's distance tends to get swamped out.

But that's *not* the case on Mars. Mars's orbital radius (in red) changes by a whopping +/-10%, causing a +/-20% change in sunlight. This is on top of the +/-40% change a mid-latitude region will receive due to the tip of Mars's equator. So the 2 effects are much more comparable.

And the extremes of these 2 cycles on Mars are very nearly aligned. Furthest approach from the sun (aphelion) occurs within a month of the northern summer solstice on Mars. Closest approach to the sun (perihelion) occurs within a month of the southern summer solstice.

This means that the northern seasons are smoothed out (temperate summer, mild winter) and the southern seasons are extreme (hot summer, cold winter). (Note: These are relative terms of course. Even a hot martian midsummer equatorial noon is still a cold day by Earth standards).

This exaggerated seasonal cycle is *the* defining feature in Mars' modern climate. It's why the atmosphere is ~20% thicker in northern summer than in southern summer (the south ice cap begins to sublimate in summer, and the extra CO2 gas thickens the atmosphere).

It's why Mars has such huge dust storms. As the CO2 sublimates, it's driven north by heat from the sun, picking up dust as it goes, eventually forming storms that can cover the planet. Here is the storm that killed the Opportunity rover earlier this year.

msss.com/msss_images/20…

Landing dates on Mars are set by orbital transfers, which don't care about Martian seasons. But all else being equal, the best time to land is in the late northern fall when the air is thickest (although the risk of dust storms is also highest at this time).

image: N.G. Barlow

So this solstice, be thankful that the reason for the season is only Earth's 23-degree obliquity and not a 0.06 orbital eccentricity. Because Earth would be a very different place with an orbit as eccentric as Mars!