Physics of SP4's Planet

[kelianmao]

Rotation Speed

So I just listened to the 17th Shard podcast on SP4, and one thing they kept talking about was the speed of the deadly sunlight moving across the surface. This inspired me to put my thoughts about this into a post.

They mentioned a value of ~1,600km/h (~1,000mph), which is the speed of the terminator (edge of sunrise) at Earth's equator, then said how the ships would have to be moving faster than the speed of sound (1,230km/h on Earth). Even if the planet was smaller. However, there is much more to consider. 

First of all, the speed of rotation of a planet is independent of its size, meaning you can get a terminator that moves at essentially any speed you want. Just look at our own solar system:
Earth: r=6,400km, T=24h, v=1,700km/h (Mach 1.4 on Earth)
Venus: r=6,100km, T=5,800h, v=6.6km/h (easily runnable, though obviously not forever)
Jupiter: r=72,000km, T=9.9h, v=46,000km/h (Mach 37)
(Source: nssdc.gsfc.nasa.gov /planetary/factsheet/. Speed roughly calculated as v=(2*pi*r)/T, ignoring sidereal days, etc.)

And to the other extreme, the terminator does not move at all on a tidally locked planet (think our Moon, which always has the same side facing the Earth), so you can get it to move as slowly as you want. Aux actually mentions this: 

Quote

Planet must have a slow rotation, the knight observed to his sometimes erratic squire.  Note how these ships can outrun it easily.

Thus, planetary physics does not limit the speed of the terminator on SP4's planet. Sigzil thinks that he would require a hoverbike to stay ahead of the daylight, suggesting it's moving quite quickly. We might also be able to get an estimate from the fact that dawn seems to take on the order of minutes:

Quote

He looked toward the ever increasing light as the plants—vibrant only minutes ago—browned and withered.

But without knowing the size of the planet (it does have rings, so is it big?), its atmosphere, and their location, this is still very unconstrained.

The deadly sunlight clearly moves at the rate Brandon wants it to for the best story. He talks about this as part of the worldbuilding, as a mechanism to reinforce the idea that Sigzil must keep moving.

Latitude

Beyond the rotation speed of the planet, the speed of the terminator also depends significantly on the latitude: if you are nearer the poles, the circle you have to move on the surface on the planet is smaller (by a factor of cos(theta)), so you can move much slower and stay ahead of the terminator. At the pole itself, you don't even have to move at all; however, you'd be permanently half in darkness and half in sunlight, so you'd die anyways. 

I assume the start of the story happens nearer the equator, where they can chain up these people and give them no hope of outrunning the day. Then, their flying ships could move further North (assuming they're in the Northern hemisphere), thus flying slower and saving fuel. The fast moving equator could even be a sort of soft-barrier, a la the open desert band on Arrakis that only the Fremen could consistently cross.

This would be further constrained by the locations of the continents on this planet. Or, I suppose with the intense and sporadic heat, the planet might have no oceans and be entirely land, with water mostly be in the air as clouds and rain and underground - I'm not a climate scientist so I have no idea how the weather on such a world would work.

Axial Tilt

And finally, if the planet has an axial tilt, like the Earth does, then there will be regions around the poles that are in permanent sunlight or permanent darkness depending on the season. The region of permanent darkness could be an oasis, providing relative stability for half the year. Narratively, perhaps Sigzil could have a chance to rest when/if they reach one of these oases, a moment of stillness before he must resume his endless flight away from the Night Brigade, the sunlight, and people trying to kill him.

They bring up in the podcast the impossibility of developing technology on a planet such as this (how can you invent and build flying ships when you have to keep moving just to survive), but this would be possible if there were oases at the poles. Or caves underground. Or they could have just arrived with the technology already...

 

All in all, I think this is a very interesting world that Brandon has created, with a constantly approaching threat, and I look forward to seeing what he does with it. I wonder how the unique physical (and Invested) nature of this planet has affected the people living on it.

(And an unrelated question: If what Sigzil needs to Skip is lots of Investiture, and Aux can absorb and convert different sources of Investiture, couldn't he just go stand out in the sunlight and super-charge himself?)

[Johnny Silverlight]

My theory is that the Threnodites already had all their technology with them when they arrived on this planet.

[+Oltux72]

On 18.4.2022 at 3:05 AM, kelianmao said:

Thus, planetary physics does not limit the speed of the terminator on SP4's planet. Sigzil thinks that he would require a hoverbike to stay ahead of the daylight, suggesting it's moving quite quickly. We might also be able to get an estimate from the fact that dawn seems to take on the order of minutes:

Inconclusive. He may just be a pessimist and assume that he would soon arrive at a shore and he certainly won't outswim a sunrise.

On 18.4.2022 at 3:05 AM, kelianmao said:

This would be further constrained by the locations of the continents on this planet. Or, I suppose with the intense and sporadic heat, the planet might have no oceans and be entirely land, with water mostly be in the air as clouds and rain and underground - I'm not a climate scientist so I have no idea how the weather on such a world would work.

It probably wouldn't. The storms would be devastating. And that really is the problem. He should experience hurricane force winds so near the terminator.

And that raises a question: Is the destruction really a purely physical effect, or do you get an overdose of Investiture and that is the deeper reason nobody started digging bunkers?

 

[Njvodin]

On 4/18/2022 at 9:05 AM, kelianmao said:

(And an unrelated question: If what Sigzil needs to Skip is lots of Investiture, and Aux can absorb and convert different sources of Investiture, couldn't he just go stand out in the sunlight and super-charge himself?)

There's a passage pretty early on, I don't exactly remember what was said, but it said that Aux would also be destroyed by the sunlight. However, from the reflected sunlight from the rings, Aux could potentially absorb a few BEUs an hour, but nothing substantial.

[Treamayne]

5 hours ago, Njvodin said:

There's a passage pretty early on, I don't exactly remember what was said, but it said that Aux would also be destroyed by the sunlight. However, from the reflected sunlight from the rings, Aux could potentially absorb a few BEUs an hour, but nothing substantial.

Please note that this post is two years old, which means it was theorized based on preview material - so the reference you add may just have been added in a later draft and was unavailable at the time this was written. 

 

[Njvodin]

On 4/15/2024 at 5:45 PM, Treamayne said:

Please note that this post is two years old, which means it was theorized based on preview material - so the reference you add may just have been added in a later draft and was unavailable at the time this was written. 

 

oh oops LOL thanks, I didn't even see the date