Tidally locked campaign world

[Glade Riven]

So I'm still tinkering with a homebrew campaign setting, and a major feature I am implimenting is that the planet Phaetos is tidally locked. That means one side of the planet is eternally day while the other side is eternally night.

The dark side of Phatos is covered in a super-glacier, warmed only in spots by volcanic activity. The continent that is closest to the sun is a parched desert. In between the desert continent and the icy dark side is a middle-world of oceans, continents, jungles, forests, etc ranging from tropical to sub-arctic dusklands.

Many of the "dark" or "underdark" races (drow, duergar, etc) are from the dark side.

Figuring out all the effects from this is rather tricky...night time staples such as vampires and werewolves need to either be adapted or relegated to the dark side. I'm placing another planet that eclipses the sun every 24 hours as a natural time-piece (although it would be unavalable for the dark side..maybe a wain in the aurora during the eclipse would work). Rest and sleep are still a biological requirement. Typical night-time activities would have to be done in broad daylight, too.

All sorts of interesting complications. Thoughts?

 

[The Shaman]

There was a discussion of this not too long ago, but my Google-fu is weak and I can't locate the thread.

 

[Aexalon]

I'm placing another planet that eclipses the sun every 24 hours as a natural time-piece

Any planet near enough to its star to orbit it in 24h would have to be very large to ever be visible to even the trained eye from your tidally-locked world. Additionally, it would be incapable of causing an eclipse, on the count of being too far away from your home world, and too close to the star. Interplanetary eclipses (or rather, transits) are typically difficult to observe without mechanical aid (filter glasses or solar projectors).

A moon with a period of 24h is somewhat more feasible, but still tough to explain away. E.g. to have the Earth's moon orbit in that period, you'd have to move it almost 10 times closer than it does now. it'd shine 80 times brighter in the sky, and (probably most importantly) have a smaller eclipse path than our moon does (moving the moon closer increases its umbra, but shrinks its penumbra). Finally, a moon that close would interfere with the tidal locking between the planet and the sun, which was your initial premise. So, imho, a moon is out too.

My best guess for an easily-witnessable astronomical phenomenon with a period of 24h, would be the orbit of a self-luminous stellar companion around the primary star. This companion could be of similar size (reduce the size of the primary as to not cook your planet, but you already did that so your planet could be close enough to tidally lock in the first place, right?), a failed star, or it could be a stellar remnant, e.g. a white dwarf. Brown or white dwarves suffer a bit from the same problem as your initial planet idea (still a bit difficult to observe), so my final recommendation has to go to placing your tidally locked star in orbit of a pair of red dwarf stars (which would very likely be tidally locked with each other as well).

 

[Glade Riven]

Actually, I was thinking something akin to a "Hot Jupiter" - a big ol' gas giant that has a rapid orbit fairly close to the star.

 

[Theo R Cwithin]

There was a discussion of this not too long ago, but my Google-fu is weak and I can't locate the thread.

You're probably referring to this one from a couple months ago about discussing a tidally-locked D&D world, inspired by the announcement of the discovery of Gliese 581g. At ten pages, it's got quite a bit of stuff in it.

 

[Aexalon]

I just did some calculations for a star like Gliese 581, considering 2 planets: a terrestrial one at the same solar intensity as Earth is (at 17 million kilometers, sidereal year = sidereal day = 24.5 earth days), and a puffy gas giant like COROT-1b with a sidereal year of 24h (~ 2 million kilometers; I know, should have aimed for a synodic year of 24h, but for now, this is close enough), and behold, the hot jupiter is (and looks) half the size (0.793 degrees) of the star (1.35 degrees)! Did not expect that. The Sun and Moon both measure about 0.5 degrees, for reference.

Further back-of-napkin calculations would indicate that the antumbra of the hot jupiter at the distance of the habitable planet would be 1.3 million km wide (in this zone, the gas giant would at some point appear wholly on the disk of the star), and the penumbra a staggering 4.7 million km wide (in this zone, the gas giant would obscure at least part of the star)!

That would definately result in quite noticeable transits, presuming the 2 planets' orbit planes don't differ by more than 15.45 degrees (penumbra) or 4.37 degrees (antumbra) — quite reasonable; e.g. Earth's and Jupiter's orbital planes are inclined at only 1.3 degrees to eachother.

So, geometrically, it's possible. Whether such a large planet that close to its star in combination with a terrestial planet in the star's habitable zone is physically possible (and/or likely), I'm not 100% sure. But it probably is.

And I had this whole rant prepared about how hard it is to see Mercury with the naked eye, and how this hot jupiter'd be 29 times closer to it's star... Good thing I tried the math first

 

[The Shaman]

You're probably referring to this one from a couple months ago about discussing a tidally-locked D&D world, inspired by the announcement of the discovery of Gliese 581g. At ten pages, it's got quite a bit of stuff in it.

That's the one.

Could someone please xp-bomb t_o_w for me?

 

[Glade Riven]

RE: Previous Thread:
An interesting read...and still working my way through it.

I was thinking about having seasons through the power of an eliptical orbit, but may be an extra complication that I don't want to deal with.

It is techtonically active, partially from the strain of tidal forces...maybe the core still rotates while the surface is tidally lock. That could make for some interesting (and cataclysmic) plate tectonics.

RE: Aexalon:
Well, orbits of Hot Jupiters have been noted at distances of .1 AU (14.9 million km), and the solar mass of this particular sun needs to be less than 1.25 of our own for a stable orbit that close to the star.

Now, my math is going to get a little fuzzy here, but with a few basic ratios and a thrown together model in 3DSMax, a sun of our size and a planet like Kepler-7b would be visible from the Earth passing between us and the sun. The Chinese report sunspots as early as 300+ BC.

So while it's not the full eclipse (such as when the moon passes between us and the sun), it would work as a basis for the development of measuring time (for the light side).

Or, y'know...fantasy ad hoc the whole thing . Really, though, the whole thing was a reason to add the concept of a "day" to a planet without a day/night cycle, so it doesn't have to be like a typical solar eclipse.

Addendum: Actually, a Kepler-7b like object crossing between Phaetos and it's sun (assuming an Earth size planet and same size sun) should be quite noticible in the permanent dusk/dawn - since one could look at the sun without burning out one's retinas. It won't dim anything overall, but the black spot transversing it would be noticible.

 

[Ariosto]

As to fantasy-genre inspiration, you might check out:

Jack of Shadows (1971) short novel by Roger Zelazny (inspiration, methinks, for the original D&D Thief)

White Dwarf (1995) TV movie

 

[DragonLancer]

What about something of an artificial nature? Maybe created and placed by one or more gods for some purpose?