Along with massaging the plot for Sorcerer, the third book in the Jakirian Cycle, I’ve been working on setting and background material, trying to soak in enough of the world to start writing. One of the unique things about Yos is that it has two suns and two moons. I like that symmetry – after all our own world is bizarrely symmetrical in the astronomical department. We have one sun and one moon – itself strange as planets go. Not only that, but our sun and moon are exactly the same size in the sky – how freaky is that?
When I had my first ideas about Yos, I wanted to have a world where the two suns regularly eclipsed. That took quite a bit of research to get right. I guess it’s really anything goes in a Fantasy world – after all metal is magical on Yos - but for this there is enough of the engineer in me to want to get it right. The first thing I discovered is that for a twin-sun system, life is extremely problematic for the planets of each sun if they orbit the suns individually. The gravitational pull of the other sun would tend to de-stabilise their orbits, and generally make life hell. The planets would also have to be relatively close to their suns and each star could only accumulate a modest number of planets – the sort of arrangement seen in our solar system would not be possible due to the pull of the other stars.
I needed a stable planetary orbit and a multi-planet system with some big Jupiter-like companions to sweep up comets and the like. Both needed for the development of life (along with a sweet-spot orbit of course – liquid water). The solution was that the planets would orbit not the individual suns – but the centre of mass of the twin-sun system. Once I had that, I could get on-line and use a few nifty calculators to work out the orbital periods (OK – I took this all way too far).
So, I managed to work out the orbital period of my planet, and the frequency of the eclipse of the small red sun Uros across the big yellow one Larus. This is the basis for Storm Season on Yos. The yellow sun is the primary solar powerhouse. When the red sun (about a quarter of the apparent diameter in the sky) eclipses the yellow there is a drop in solar radiation that causes abrupt cooling. After the eclipse it is the re-heating of the planet that causes the violent storms. This pattern has also driven the development of evolution on the planet, and the development of the Heat – a biological mechanism allowing some warm-blooded animals to accelerate their metabolism (others just hide underground – hence the split between the two main humanoid species – Human and Eathal).
Then there were the moons. Of course once more I have a big one like our moon and another smaller one to match the large and small suns. The large one Asic has a period a bit longer than our moon, and the smaller one Rea orbits a lot closer to Yos, racing past its companion time and again – hence its appellation Rea the Runner. I am assuming Asic is tidally locked to Yos like the Moon is to Earth (rotation matched to orbital period & always showing the same face), whereas Rea is a newcomer that is not tidally locked yet.
Here, once I had decided on some orbits that felt right I really had to track the days through the books and work out where the moons where on any given night. This was particularly important as Moon Essence drives one of the major branches of magic (Moon Druids excel in healing).
I found a really neat pictorial diagram that helped to work out when the moons appeared in the sky at various points in their cycle. Here it is:
Here is how you read it. Say your moon is in its first quarter (half full and getting fatter). You look at the diagram and you will see that this corresponds to 6pm on the little Earth below it. This means that at first quarter, the moon will be at its highest point in the sky (mid-phase or standard point of culmination) at 6pm. So afternoon and post-dusk are the best times to see it. So for a full moon you can see that it’s at its culmination at midnight, and will rise around 6pm and set at 6am – basically visible all night. This is approximate, but it’s a really neat way of getting a quick grasp of the movement of the moon. And for a new moon (hey technically you can’t see it, but let’s say the merest crescent) technically it’s rising at 6am, but you can’t see anything during the day. It is visible only just after sunset.
So have you had some fun with the astronomy of your worlds? Found any neat on-line calculators?