[Dragaera] Dragaera and Moons **Minor Vallista Spoiler, nothing plot related**

[Jon Lincicum]

----- Original Message -----

From: "Konrad Gaertner via Dragaera" <dragaera at lists.dragaera.info> 
To: dragaera at lists.dragaera.info 
Sent: Friday, November 3, 2017 5:04:29 AM 
Subject: Re: [Dragaera] Dragaera and Moons **Minor Vallista Spoiler, nothing plot related** 

On 11/2/2017 10:19 PM, Jon via Dragaera wrote: 
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[snip] 

>> to the rotational period of the planet itself. Effectively, it means 
>> this moon must orbit at a point close to (but not exactly at) the 
>> natural altitude for a fully geosynchronous orbit. It cannot diverge 
>> from this orbital period by more than about 2 hours per day (given 
>> Dragaera's 30-hour day), however, or else it would always take fewer 
>> than nine days to appear again in the sky at any particular point on the 
>> surface of the planet. 
> 
>Is this the only option using normal physics? I'm asking because in 
>another series I'm reading, there's a world with a moon that is similar 
>in size and phases to Earth's, but is only visible every eighth day. 

This is the only solution I can see that does not involve the observer being at some extremely specific location (such as near one of the poles, which does not seem to be the case given the shirtless condition of the character in the scene), or some other extremely-significant-yet-entirely-unmentioned factor, such as dust clouds obscuring the view of the moon, for example. This does not appear to be the case here, given the phrasing of the passage. 

Regarding how one could have a moon that was visible only once every eight days--well, that makes no sense to me using normal physics. There's no normal orbit that would cause a body the size of the moon to be visible for only one day, then completely missing the next 7. Even with a highly elliptical orbit with an 8 day period, an object the size and mass of the moon would still have to be pretty clearly visible as a moon on days 7 and 9 (and visible at least as a bright star the rest of the time) else, it would either have to be moving so fast that there's no way it would be able to maintain an orbit, or else it's orbital period would have to be much longer than 8 days. (And I hate to think what the impact of tidal forces of such an object in such an orbit would be like on the planet's surface. Yikes!) 

Certainly there could be exotic solutions involving other factors such as temporal displacement via wormholes, obfuscation by clouds, or the moon getting repeatedly kidnapped by space goblins, but none of these are a result of orbital mechanics alone. 

Majikjon