A Theory

[johndarcy]

Bear with me on this.

The Moon is to the Earth as 1/81 of the mass, and 1/50 of the volume, and 27% of the diameter. It orbits at 384000km and performs one orbit in 27.3 days. In spite of its relatively small mass, it exerts a significant gravitational influence on the Earth as evidenced by the tides and by the Earth's monthly oscillation around the barycentre (the balance point between the two, 1/81 of the way from centre to centre of each body).

If the Moon were a twin of Earth, then it would exert 81 times as much tidal force on the Earth and, no doubt, both bodies would have become locked in the way they face each other many millennia ago. (As it happens, only the Moon has become locked facing the Earth the same way all month, whereas the Earth's daily rotation has slowed only slightly).

Gravitational force diminishes proportionally to the square of the increasing distance, so for a twin of Earth to act upon the Earth with similar force to the Moon it would need to be 9 times further away.

In consequence of being 9 times further away (3.5 million km), it would appear to be only 41% of the size of the Moon, and it would perform one orbit (around the midpoint between the two) much more slowly than one month - I estimate as much as 85 days instead of 27. On the other hand, it would be much brighter than the Moon, because the Moon's albedo is a mere 12% (blacker than coal) but the Earth is 36.7%, so a Full Earth at 3.5 million km should appear 25% brighter than a Full Moon.

Finally, both members of the two-planet system would experience a variation in distance from the Sun of 3.5 million km over the 85-day "month". This compares with the Earth's actual variation of 2.5 million km over the full year. Such an 85-day oscillation might have climatic effects, but this would be difficult to model and would probably not negate the normal seasonal climatic changes that are due to axial tilt.

So, if all of the above makes sense to you ... then draw your own conclusions.

[chrimigules]

Are you proposing that Micras be made a literal double planet?

[johndarcy]

No, that is not my proposal.

However, in the past it has been suggested that a second planet in the Atos system might be "colonised", and there have been threads concerning the habitable zone and so on. I merely wanted to articulate an idea which had not previously been suggested, that of a twin-planet arrangement, and to establish the characteristics of such a system.

If the MCS wants to develop the idea, then you have my blessing to use the data above.

--

John

[Ryan]

The second habitable planet, as proposed, would have been Momiji (Japanese, meaning maple). However, there seems to be strong opposition to instituting such a plan.

What I am not directly opposed to is mapping, charting and determining what the system is comprised of and applying some aesthetic names to the planets, asteroids and other celestial bodies in the immediate vicinity of Micras.

This information could be used for space expeditions and cultural aesthetics. But colonization wouldn't be possible at this point. Perhaps at some point in the future there will be a wide enough margin to change that, but until then we must wait.