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# Re: CONCULTURE: dual planets

From: Michael Poxon Wednesday, November 17, 2004, 16:00
```It's highly unlikely that you'd ever get to the stage where two Earth-sized
planets could revolve around each other. They'd have to
be a fair distance apart for (as you say) orbital decay not to be a problem.
Tidal effects are really another manifestation of that.
When the planets were forming, tidal effects would be crucial (if they were
too close, one would simply gain mass at the expense of the other in a
positive feedback system) as the planets would not yet be properly solid.
Even if you reached the stage where you had two Earth-sized planets orbiting
their common centre of gravity, the system would be so sensitive to any
change in mass that the equilibrium would be easily upset, and in the early
stages of planet formation from the parent gas cloud, there would almost
certainly be a lot of this going on.
You can easily work out the apparent size any object would have as seen from
another. The formula is
S = 2 * arctan (R/d)
where S is the size you're actually after, R is the radius of the object,
and d its distance. Obviously R and d have to be in the same units.
You need a calculator though!
It looks daunting but isn't! Since R/d is in brackets, work that out first.
I'll use the Moon (as seen from the Earth) as an example.
Since the diameter of the Moon is about 2160 miles, its radius (R) is 1080
miles. Its average distance from us (d) is about 240,000 miles.
1080 / 240000 is 0.0045. We now want the angle whose tangent is 0.0045.
Press the arctan / inv tan / tan-1 button and you'll get (hopefully)
something like 0.2478 (make sure your calculator is in degree mode). Because
this is the radius, we need to double this result to get the diameter
(hence the 2* above) and so we arrive at about 0.5 degrees, which *is* the
angular diameter of the Moon as seen from Earth.
Mike
>
> Mike, if you're an astronomer, perhaps you could tell me if it's possibleto
> have two planets revolving around one another (roughly earth size), ifthis
> is astronomically possible, how far apart they would have to be, how big
> each planet would look in the sky to the other, whether there would be
> orbital decay that would threaten both worlds, and whether there would be
> disastrous tidal effects on both worlds.  I've been considering this kindof
> system for a world I'm designing.  I had the vivid image of the remains of
> dead cities stretching far out into the wet horizon, visible only a low
> tide, that would be flooded with water when the tide came crashing in;land
> masses that had cliffs, living cities moving further and further back asthe
> tide ate away at the coast line.  But I am decidedly NOT an astronomer, soI
> don't know my physics, although I do read quite a lot about astronomy, but
> as a rank amateur.  Would you be able to advise me?
>
> yry wemry eftoihs!  Sally
>
>
>
> > ----- Original Message -----
> > From: "Sylvia Sotomayor" <kelen@...>
> > To: <CONLANG@...>
> > Sent: Monday, November 15, 2004 5:39 PM
> > Subject: Re: CONCULTURE: First thoughts on Ayeri calendar system
> >
> >
> >> On Monday 15 November 2004 08:53, Roger Mills wrote:
> >> [lots of interesting stuff on moons & planets]
> >> > Well, make that "18 to 70" for Carsten/me, respectively...:-) Neither
> >> > of
> >> > us, alas, seems to be near a university. It appears we could both
> >> > profit
> >> > from some good help and advice-- if either of you gentlemen are so
> >> > inclined. Do you know of anything useful (intelligent-amateur level)
> > that's
> >> > on-line?
> >> >
> >> > Since this is basically Off-Topic, best if we discuss privately.
> >> >
> >> Not too off-topic! I've been following the discussion, since I'd liketo
> > have
> >> 4 moons for Terjemar. But beyond some basic calculations, I haven't
> >> progressed very far in determining size, distance, orbits, tides, etc.
> >>
> >> -S
> >> --
> >> Sylvia Sotomayor
> >> sylvia1@ix.netcom.com
> >> kelen@ix.netcom.com
> >>
> >> Kélen language info can be found at:
> >> http://home.netcom.com/~sylvia1/Kelen/kelen.html
> >>
> >> This post may contain the following:
> >> á (a-acute)  é (e-acute)  í (i-acute)
> >> ó (o-acute)  ú (u-acute)  ñ (n-tilde)
> >>
> >> áe ñarra anmárienne cí áe reharra anmárienne lá;
> >```