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Re: CONCULTURE: dual planets

From:Caleb Hines <cph9fa@...>
Date:Wednesday, November 17, 2004, 17:26
The key points have already been touched on (osscilating tides, no rising
and setting planets, day = month, etc). There's a few more things I think
might be included. First of all, during every 'night' half of the month,
there would always be a 'full moon' (full planet?) in the sky. Since the
planets are fairly large, I expect this would be fairly bright. Also, with
elliptical orbits, the planet would change size as it got closer and
further, and the side facing the other planet would slightly change
(because the rotation rate is constant, but the orbital rate varies). One
more thing. I think there would be significant east/west 'wobble' as the
planet speeds up and slows down in its elliptical  orbit.

Here is what I think would happen. Let's assume that the planets are
orbiting in a counter-clockwise direction when viwed from above the north
pole. (if the direction is reversed, then switch east and west below) Then
throughout the course of a month/day, the other planet would be seen to do
the following:

It would start out relatively small on the western half of the sky, and
there would be relatively weak tides on the western side of the planet.
This corresponds to 'apogee', when the planets are furthest apart. As the
month progressed, the planet would grow a bit in size and head towards the
center of the sky. The tides would follow suit. For a short period of time
(because less time is spent at 'perigee', or closest aproach), the planet
would be seen at its largest (and brightest) in the eastern portion of the
sky, and you would have relatively large tides on that side of the planet.
These would decrease as the planet moved back towards the west. Note that
the amount of change in size, the amount of change in tides, and the amount
of motion back and forth all will depend on how ellictical the orbit is
(but will likely all be pretty small). If you want big bad tides, don't
have them be tidally locked.

In reference to the changing size and point-of-view, note that even our own
moon, between apogee and perigee, changes its size by about 12% and changes
slightly the side it points towards us (this is called liberation). For a
graphic demonstration of this, see: and The moon doesn't woble
back and forth in the sky because Earth isn't tidally locked to it. But I
would imagine the Earth wobbles a bit in the Moon's sky during the course
of a month.

The length of the month would be determined from the masses of the two
planets and their distance. The distance has to be at least the Roche
limit. Probably quite a bit more. I can get equations later if neccessary.