Day of Dread/Day Without Magic from 1000 AC
by Brad MitchellThis is a tidal height chart for an arbitrary point at the intersection of the equator and prime meridian, and you can read the chart forward or backward as you move south/north or west/east from there. For example, Specularum and Thyatis City are both around 25 degrees north latitude, so they are 6 hours "behind" the PM/Eq tide.
My open call for astrographic data remains active, and I'll update this model with any new information which comes in.
How Does This Help My Game?
The ordinary tides of earth are about 3', average, twice a day. If we reuse these tides for Mystara, then the complications on sea travel created by tides are only interesting when the DM goes out of their way to make them interesting. They could decide (or let the dice decide) that a certain town, village, or city is inaccessible because it's low tide. Or that the villain's offshore island fortress cannot be reached by your army because of the tide. That's a narrative complication, and more or less impossible to predict. But the 25' tides of this model, which come only twice a week, create instant benefits.
1. Predict the tides in advance. Maybe the players don't know this, and they have to consult a sage or a local fisherman, but *somebody* knows what the tides are likely to be. This lets the players plan in advance, which is important because ...
2. Dramatic tides create problems to be solved. The players want to sail from Specularum to Thyatis; on what ship? A ship large enough to offer first-class accomodations or extra cargo capacity will be larger and have a deeper draft. That means it needs more water to sail; a "Sailing Ship, Large" might be able to sail with as little as 15' of water in the harbor, but a "Sailing Ship, Huge" needs at least 20'. Those facts limit the days and times when the ship can enter or leave. It also tells you exactly _when_ you can assault the villain's offshore fortress, and how long your window of attack will be.
3. It makes smaller boats much more interesting and efficient, since they can come and go at almost any time. "Why would I ever take 3rd class travel, when I can afford 1st?" "Because 3rd class is on a small galley which needs only 8' of water under it, and you're in a hurry, and the next high tide is 21 hours way."
4. It changes how your players interact with coastal hexes. 25' tides mean that the last 1-2 miles of each coastal hex are either cliffs dropping down into the sea, or else getting flooded at high tide. If they're getting flooded, they're probably saltwater marshland (similar to Louisiana), or bare bedrock, or a long rocky tumble into the sea, or else a _very_ long and shallow sandy beach. At mid-tide, the coasts are what they are on the maps, but any long beaches are going to still be that 1 to 2 miles. At low tide, depending on undersea topography, the ocean might recede as little as half a mile, or as much as a full 8-mile hex (with attendant new restrictions on shipping, close approaches by boats, etc.). This also has complications for any nautical adventures - if the boat needs to get beached to make repairs ... where?! A cliff or rocky coast won't help! This adds tension to the situation.
5. It makes cites much more fantastical, especially to the modern mind. Cities built on the coast will have sea-walls. Harbors will have tidal gates to keep the harbor flooded at low tide, and any city with a tidal gated harbor will also have a "trader's row" of jetties and quays accessible even at low tide, to service the smaller craft. This distributes the naval district(s), and increases opportunity for criminal hijinks or schemes on the part of players or pirate/thief NPCs. Smaller cities, towns, etc will have long quays with very deep stairs to allow for that 25' variation. Even the smallest villages will be able to make use of tide-powered mills (water flows in, fills a basin; when the water flows out, it turns a wheel to do work). Maybe they even use the tide to uh ... flush ... the city (looking at you, Specularum with your open sewers in the middle of the streets).
I will explain my assumptions and methodology in the comments below. Happy Gaming!
Methodology:
1. Start with the assumed masses, diameters, and orbital heights of Matera and Patera. Matera is 2160mi in diameter, orbits at 238k miles, and has a mass of 7.35×10^22 kilograms.
Patera is 950 miles in diameter, orbits at 51,500 miles, and has a mass of 6.25×10^21 kilograms. Patera orbits around the poles, at a right angle to Matera's orbit and always in-line with the sun.
2. Calculate tidal forces.
Tidal force is easy to calculate; mass of the moon in kilograms divided by distance in meters, cubed. We did this three times, using the real world values for the moon, then values for Matera and Patera. Then we compared the values, to get an idea of how much "stronger" each moon pulls on Mystara. If Luna pulls on earth with force "1", then Matera pulls with force 1.01, and Patera pulls with force 8.48.
3. Determine tides
Earthly tides with 1.00 force are about 3'. 1.01 x 3' = 3.03 feet for Matera's influence, and 25.44' for Patera's influence. Patera is the dominant force, and Matera will apply "mitigating" influences, either raising or lowering the tides depending on it's relationship to Patera.
4. Determine Speed
This one was tricky. I used the /zodmoons.html page on Vaults of Pandius as a reference, but we know that Patera orbits every 3.5 days, Matera orbits every 28 days, and Mystara turns every 24 hours.
5. Modify Patera's tides
When Matera is NEW or FULL, it is directly in-line with Patera and pulls the tides extra high. When Matera is First or Last QUARTER, it is 90 degrees off-angle, and pulls the tides down low. These are called Spring and Neap tides.
6. Apply the sun's influence
The sun pulls the tides slightly during the day making them higher, and lower at night (water pulled to the day-side of the planet).
7. Run the numbers
Once we had the (extremely rough!) model, which is in no way a complete or scientific product if only because it fails to integrate the tidal forces through all phases of both moons, we started at Day 1 -- Hour 0 at the Prime Meridian / Equator, when Patera is directly overhead, and so is Matera, and both are NEW phase. Then we ran the numbers for each hour, accounting for Patera's movement up to the north pole, the back to the south pole, then back to the equator. We ran the numbers for 24 hours x day x 28 days and charted the outputs. Then graphed it!