# World builder's handbook - orbit#s and HZCO

#### Hapax12

##### Mongoose
Howdy, first time poster here.

I recently purchased the WBH and have been poring through it incessantly. However, maths was never a forte of mine.

I've read and reread the sections on orbit#s, fractional orbits, and habitable zones, but I just can't work it out for some reason. The numbers turn to mush and I feel like I'm missing something.

Has anybody used this new book to generate a few systems, and if so, would you mind walking through exactly how you used the orbit#s and determined habitable zones, planet placement, etc.?

It is quite complicated. The original version of system builder used the Titus-Bode law, which defined orbits to specific radius: So Orbit 1 is 0.4 AU (Mercury), Orbit 2 is 0.7 AU (Venus) etc. T-B is NOT a "law" it was a formula someone worked out to account for the known orbital positions of the planets in the solar system. The first exo-planets blew this dubious method away. BUT, Traveller has used it forever.

If you are going to maintain this "rule" for Traveller to match all previous data, then you need Fractional orbit numbers. That section of the book is about figuring out what the actual distance (in AU) would be for these fractions, since it changes between each orbit number. I will see if I can work up an example to post later.

It is quite complicated. The original version of system builder used the Titus-Bode law, which defined orbits to specific radius: So Orbit 1 is 0.4 AU (Mercury), Orbit 2 is 0.7 AU (Venus) etc. T-B is NOT a "law" it was a formula someone worked out to account for the known orbital positions of the planets in the solar system. The first exo-planets blew this dubious method away. BUT, Traveller has used it forever.

If you are going to maintain this "rule" for Traveller to match all previous data, then you need Fractional orbit numbers. That section of the book is about figuring out what the actual distance (in AU) would be for these fractions, since it changes between each orbit number. I will see if I can work up an example to post later.
Awesome, thanks for this clarification! I will also attempt to roll up a system myself and share here. Hopefully my confusion/mistakes will be more obvious that way.

So far I have rolled up:

Type: G2V
• 2D = 10 = G type
• 2D = 11 = 2 subtype
• V as all stars are main sequence unless otherwise instructed...?

Mass: 1.02
• 1.1 - [( (1.1-.9) / 5 ) * 2]
• Figuring out what the increments between G0 and G5 are, then working out where G2 would fall by adding/subtracting the appropriate amount

Temp: 5,840K
• same method as Mass

Diameter: 1.04
• same method as Mass

Luminosity: 1.152
• again, same method

Age: ~9.5B yrs
• 10/(1.02^2.5) * 1 Billion years

I rolled no additional stars in this system.

For system worlds, I began with Gas Giants.

2D = 6 > Gas Giants are present

For quantity, I add DM+1 because it is a single class V star.

So... 2D +1 = 9 > 4 Gas Giants

Planetoid belts has a DM+1 since there is more than 1 Gas Giant present

So... 2D+1 = 10 > 2 Planetoid Belts

For Terrestrial planets, I rolled:

2D-2 = 3 > Add D3-1 = 1-1 = + 0

So, Terrestrial Planets > 3 Planets

Total Worlds = 9

How does my math look up to this point? I feel pretty confident I've been able to interpret the tables and formulas thus far. Now, here's where the trouble begins.

How would I go about determining the Habitable Zone Center Orbit (HZCO)? And then assigning planets to their respective orbits, determining which planets are in the habitable zone, etc?

I have attempted the math and could share, but don't have the time right now. I'd be appreciative to see how others would take the numbers I've rolled so far and determine the next steps.

I haven't the WBH yet. I can just comment on the end result.
Your star is of the same class than our sun (G2V), a bit heavier and far older. Maybe around half a billion years before it starts to expand into a red giant. The number of planets/belts & gas giants is normal (at least as far as our understanding of astrophysics goes).
The result looks fine so far.
HZCO should be very close to our own (maybe around 1.05 AU, IIRC it is based on luminosity).
It will be interesting to see the end result. It might give a nice habitable world.

That sounds correct to me! HZCO should work out to 1.07 or something. Okay, so far so good!

I'm going to attempt to calculate orbit#s and placement today and post the results. Again, this is where I'm getting lost.

Sorry I don't have the bandwidth right now to comment in detail, but the age of 9.5 billion in the example would be the main sequence lifespan and the star's actual age should be 9.5 times the randomly determined interval of equal to or less than 1 (either with a clunky D6 formula or the simpler d100 formula, both in the middle of the first column of page 21).

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