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

[Jerry Friedman]

To: Konrad Gaertner <kgaertner at tx.rr.com> 


 Sent: Saturday, November 4, 2017 2:41 PM
 Subject: Re: [Dragaera] Dragaera and Moons **Minor Vallista Spoiler, nothing plot related**
   


  From: Konrad Gaertner via Dragaera <dragaera at lists.dragaera.info>To: dragaera at lists.dragaera.info Sent: Friday, November 3, 2017 6:04 AM
 Subject: Re: [Dragaera] Dragaera and Moons **Minor Vallista Spoiler, nothing plot related**
   
On 11/2/2017 10:19 PM, Jon via Dragaera wrote:
> Spoiler Space
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> Littlemoon, however, has the characteristic of not rising and falling 
> every day--indeed, we are told it will be at least nine days before it 
> will rise again.
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> A basic understanding of orbital mechanics reveals a few interesting 
> things about this circumstance. Firstly, this means that the orbital 
> period of this moon above the planet must be in fairly close proximity 
> 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?
That might depend on what "rise" means.  Another option is that the moon istoo lined up with the sun to be seen for at least nine days.
Our dark of the Moon lasts about a day.  If Earth had a satellite in a circularorbit that was dark for nine days, its synodic period (full to full) could be ninetimes the Moon's.  Then its sidereal period would be about 154 days(using the formula at Lunar Sidereal vs. Synodic)

  
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Lunar Sidereal vs. Synodic
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 and its distance from the Earth would be about 1.2 million km, ifI'm doing this right.  It probably would look little.
If the moon had an elliptical orbit, it would spend a different amount oftime "dark" during each month, depending on how the peridragaerahappened to line up with the sun.  Dolivar might happen to know,maybe because an astronomer in his "tribe" told him, that that monthhad an unusually long dark of Littlemoon and know how much timewas left in the dark phase.
(I don't think a satellite in a very elliptical orbit gets "locked" so itsmajor axis always points toward the sun.  Unless I'm wrong.)

> 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.

> (The series is Andrea K. Host's Touchstone, which isn't much like
> Brust's work except for being first-person and having some similar
> worldbuilding.)

As John Lincicum said, a moon that's visible only every eighth day would bevery weird.
Jerry Friedman