The Darkest Night on Athas - Lunar conjunctions on Athas

Many of us are familiar with the “eclipse” that occurs every 11 years of the two moons of Athas, Ral and Guthay. This eclipse is really a conjunction of the two moons when they are both at full. Think of it this way. The four celestial bodies are in a straight line in this order Sun, Athas, Ral, and Guthay. So from the surface of Athas it appears that Ral becomes centered on Guthay exactly at midnight. (This assumes you are on the side of the planet you can see the moon) This marks the start of a new Endlean Cycle that happens every 4125 days. This occurs every year of Ral in the Endlean cycle. (The first occurrence of this conjunction in the Free year is on FY9 Dominary 1 at midnight)

Exactly halfway through this 11 year cycle (5.5 years), during the year of the Dragon on Morrow the 3rd, at noon, these two moons are again in conjunction. However this time they are on the opposite side of Athas at a new phase. Think of it this way - the four bodies are now lined up in this order Sun, Guthay, Ral, and Athas. Both moons have their dark sides facing Athas. This also means that the night will be at its darkest prior to and after this moment. It also means that Ral will move towards the center of Guthay as Guthay moves to partially cover the sun over several days making the daylight darker than normal. (The first occurrence of this conjunction in the free year is FY3 Morrow 3 at noon)

The Merchant Calendar program laid out a good way to see the moons and their cycles in the sky. But what is also interesting to note, is that these conjunctions are not the only ones during those 11 years. They are the only ones where both are full or new. However, there are several others. By several, I mean 90 others.

Though not cannon, the program provides mathematically correct formulas to make the above conjunctions work. It begins with synodic period of 33 days for Ral and 125 for Guthay. (33 * 125 = 4125). Synodic meaning the cycle of phase of the moon. Contrasted with sidereal period which is the time it takes to orbit Athas, a topic for a different discussion.

The formula for calculating a conjunction is given by the (p2*p1)/(p2-p1). Where p2 is the greater period of the two. This works for either the synodic or sidereal periods.

(125*33)/(125-33) = 4125/92 = 44.84

Which means that about every 45 days the two moons will be in conjunction again. Now, the computer program does not consider an orbital incline of the two moons, it rather assumes they are both in the same plane on the ecliptic making it appear that Ral covers Guthay. Otherwise during these times you would get one moon above or below the other, depending on incline angles and where the lunar nodes are. But, like I said the program assumes there is no orbital incline.

These other lunar conjunctions occur about 8 to 9 times a year. Another thing to note, is that on every conjunction, both moons will have the exact same phase. Only about half of these other conjunctions are visible from the same location on Athas. Some will be occur on the opposite side of the planet, a few near the horizon, and the others in various locations in the day or night sky. For example, The first visible one in FY9 (after a dual full moon conjunction) occurs on Fortuary 30 at 4:10 PM when both moons are 43.5% full.
In FY1 the first visible one is on Fortuary 26 at 2:20PM when they are at 37% full. I have a table of all of these that may make it easier to see and understand. I still need to go thru each phase to determine which of the 92 are visible or not. It should be symmetrical so I have to only determine 46 of them but it’s still a lot.


If Ral has a 35 day cycle and Guthay has a 125 day cycle, then they only align full and new once a year each. Depending on the year this lasts anywhere from 2-5 days as they come into alignment and and exit again. The Merchant’s Calendar program seems to support this.

I’ve got a Draji Moon Priest in my weekly campaign that uses astronomy/astrology quite heavily, and I track the moons for light/darkness conditions in evening encounters.

This is very interesting. It’s something I’ve never used and you awaken something in my brain. I don’t know if someone is proficient in programming to “redo” this delightful OLD program, but if it’s possible, I will be the first to applause such feat (in html5 per example).

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I have had the same thought. I would love to see a similar version of that program done in HTLM5. I think I have most of the equations figured out and I even created a model in Excel that shows the motion of the moons around athas as it revolves around the sun. But alas, I don’t know HTML5. However, I have seriously considered learning it to make this happen. Perhaps there is a webpage that already has most of the info I need that I could just modify to make it work.

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Care to share your work? I’m curious.

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Maybe. If I ever clean it up that that is not so embarrassing that no one else can identify what is going on in it.