That got me thinking about solar power in general in this kind of system.
If the planet is tidal-locked to the star, then solar power would be a
reliable "all the time" thing, and especially in the "Noon Zone"
Dig a well, rig some mirrors and a boiler, and you could get an
impressive Stirling Engine running. I'm thinking Stirling because the
mental image I have of the "Noon Zone" is like the Arizona Badlands,
and hauling in the water for a steam engine would be a PITA. An
air-filled Stirling would be simpler.

All the smelting and forging is done in that part of the world too -
big mirrors means no need for coal or water power for the forges.
That'd lead to railways to run the stuff to the cities. At first,
these are solar steam or Stirling, used just to carry loads around the
industrial area, but as the rail lines expand and sun angle becomes an
issue, they go electric. Big lead-acid batteries in what we'd call a
"coal car" next to the locomotive. The Battery Cars are shunted off to
a siding at charging stops, where the solar driven generator charges
them up.
Battery cars become another line of trade at some point - Due to the
high winds at some parts of the year, stringing high tension lines
doesn't work well, and even if they did AC power isn't commonly used -
the idea of "cyclic" isn't as ingrained into the local thought process
without a day/night cycle, so they haven't really clicked on the idea
so there's locomotives that shuttle trains of them from the cities
back into the Noon Zone to recharge, then back to the city.

I'm picturing the major cities in the "Twilight Zone", where the
temperature is more moderate. The Slums are, of course, on the darker
side of town. The prime residential real estate is on the Noonward
side of town, with big picture windows with Venetian blinds to control
the light levels. Due to the prevalence of DC power and associated
charging losses, there's a rail siding every block or two with a
Battery Car, and the rail line is used for trolley service the rest of
the time.

Inward from the cities are the farms and farming communities, with the
crops changing as you go depending on which plant likes which level of
sunlight. The edge of the Noon zone is where the succulent farms are,
and and the area closest to the cities are where you'd find the local
equivalent of tomatoes. Noonist forge workers tend to subsist on a
diet of the tough, chewy cactus pulp, with the easier to eat and
tastier aloe-ish plants being shipped to the rich people in the
cities. The area between cities is ranch land, where there's enough
light for scrub to grow and cattle can feed. In the winter, the cattle
migrate sunward, and in the summer they move back toward and even in
some areas a little past the cities as they graze.

Past the cities is the Nightlands. The oceans circulate plankton,
which is the starting point for life here, but with very, very limited
plant life on the land, almost everything is a carnivore. Some,
wolf-like and sized things work in packs and hunt both wild and
domestic cattle, making life exciting for the ranchers/shepherds.
Deeper into the Night, things get more dangerous as each layer has
evolved to be more dangerous than the last. There are stories of a
Darkling civilization at the Night Pole. Most common people agree the
Darklings are a myth, something told to children to make them behave -
"or the Darklings will take you"

The outer planets cooled first, and had a head start on life popping
up and evolving. The development of optical science for solar power
leads to some pretty decent telescopes, so they've had a chance to
observe the inner planets development, and one of the major ...
religion's not the right word. I'm picturing a sort of Buddhist
monastery type place, where the collection and sharing of knowledge is
considered the path to enlightenment. Sort of like a Librarian
Religion. They've got several places in the Noon Zone, each with a
mirror array streaming all the books in their library. There's one
array, larger, easier to see, that sends a 64x64 bit image stream,
followed by the word/symbol for it, which is the dictionary for all
the other books. The idea is that it's the easiest to see, and thus
will be the first to be seen and decoded by the other side. This cult?
religion? group? has annual celebrations of the starting of the
Dictionary Transmission, and the dates they got a flash back from an
inner world.

The Darklings do exist - they live in a volcanic mountain range and
farm the thermolichen and thermofungus that grow there, and raise
their own meat animals as well as other useful  creatures. One is a
thing like a parrot, but evolved with a bio-luminescent gland under
it's mouth which is uses to hunt prey. Darklings keep them in cages,
feed them 'mice', and use them as lamps. The Darklings mine the
mountains, extracting various metals and refining them in geothermal
forges. The society is very  caste-driven, with limited living space
around the volcanoes and everybody eating the same thing there's not
much diversity in living conditions, so 'moving up' in the world isn't
a big house Sunward, it's getting more social respect and being able
to treat the people below you like crap and get away with it.

The highest caste of them are Mystics who climb to the high
non-volcanic peaks, and there in the extreme cold lay back and watch
the flashes of light from one of the other worlds, and fire magnesium
flares in response. Mystics come back from their long sojourns on the
cliffs with new ideas and new industrial techniques, which they use to
become rich and powerful.

There is very, very limited ocean travel - Sailing ships on the trade
winds that are pretty regular, but navigation is a bit freaky with no
good clocks and no noon-sun shoot. Travelling through the Dark by ship
is dangerous - there are whale sized things that strain plankton
through filters ahead of their gills, but they also have teeth and
prey on anything smaller than them in the water. Picture a cross
between a humpback whale and a shark, constantly moving and getting
the bare minimum needed to stay alive, always looking for something
more substantial to eat. The islands midway are deadly hell holes,
where everything tries to eat you when you go ashore. The idea of a
"common trade port" is tossed around, and has been tried a few times,
but the expense of securing it keeps the project from being
profitable.

Because of this, communication and trade between the Light and Dark is
very minimal. Sort of like the Marco Polo era, maybe 20 ships a year
head out in either direction, maybe 1 to 3 make it back. Add in the
overland journey from the only decent harbor in the Dark to the
Darkling City, and you're looking at a nightmare. Small, easy to
carry, high value items are traded, the foremost being books written
by Mystics traded for strong spices grown in the Noonlands.

On Thu, Mar 2, 2017 at 2:04 PM, C. Berry <xxxxxx@gmail.com> wrote:
> It would be trivially easy to rig a heliograph for communication from outer
> to inner planets at their closest passes. A field of a few hundred ~1m2
> mirrors would do the trick. You aim them using this trick or something
> similar You can send a Morse code equivalent by having your several hundred
> assistants manually cover and uncover the mirrors in rough unison using
> signals from a horn or bell.
>
> Inner-to-outer communication is a bit more challenging, as the geometry
> doesn't favor a heliograph during closest approach.
>
> On Thu, Mar 2, 2017 at 10:52 AM, Jeffrey Schwartz
> <xxxxxx@gmail.com> wrote:
>>
>> The thought crosses my mind....
>>
>> If one were to stand on the night side of a planet, and aimed a strong
>> light at one of the outer planets, I suspect it would be visible
>> through even a primitive telescope.
>>
>> If all (or even some of) the planets in the system independently
>> developed sentient tool users, you'd have interplanetary
>> communications at TL 4-ish?
>>
>>