Fwd: The Moon Is *Not* A Planet

On 2/18/12 11:26 AM, Hägar wrote:
But any way you want to "word-smith" it, the fact remains that our
Moon orbits the Earth and it is irrelevant whether its orbit is concave,
convex, or isometric triangular or square, As long as the bary-
center well within the Earth's circumference, it will always be a
Planet/Moon system. The fact the both orbit the Sun is really
secondary.

An informal criterion for a double planet is that its barycenter must
be exterior to both bodies.

Interesting:

The opposing bulge has the opposite effect, but the closer bulge dominates due to its comparative closer distance to the Moon. As a result, some of the Earth's rotational momentum is gradually being transferred to the Moon's orbital momentum, and this causes the Moon to slowly recede from Earth at the rate of approximately 38 millimetres per year. In keeping with the conservation of angular momentum, the Earth's rotation is gradually slowing, and the Earth's day thus lengthens by about 23 microseconds every year (excluding glacial rebound). Both figures are valid only for the current configuration of the continents. Tidal rhythmites from 620 million years ago show that over hundreds of millions of years the Moon receded at an average rate of 22 millimetres per year and the day lengthened at an average rate of 12 microseconds per year, both about half of their current values. See tidal acceleration for a more detailed description and references.

The Moon is gradually receding from the Earth into a higher orbit, and calculations[9][10] suggest that this would continue for about fifty billion years. By that time, the Earth and Moon would become caught up in what is called a "spin–orbit resonance" in which the Moon will circle the Earth in about 47 days (currently 27 days), and both Moon and Earth would rotate around their axes in the same time, always facing each other with the same side. However, the slowdown of the Earth's rotation is not occurring fast enough for the rotation to lengthen to a month before other effects change the situation: about 2.1 billion years from now, the increase of the Sun's radiation will have caused the Earth's oceans to vaporize, removing the bulk of the tidal friction and acceleration.

Ref: http://en.wikipedia.org/wiki/Orbit_of_the_Moon

On Feb 18, 11:20*am, Sam Wormley wrote:

* *An informal criterion for a double planet is that its barycenter must
* *be exterior to both bodies.

Of course, if Titan orbited Saturn at a *sufficiently remote
distance*, the barycenter of the Saturn-Titan system could be external
to Saturn. So there is a problem with that definition - a barycenter
outside the primary doesn't necessarily imply the two bodies are even
approaching equality in size.

Also, back when Pluto was "a planet", or, more specifically, a major
planet instead of a dwarf planet, Charon still wasn't "a planet" even
though the Pluto-Charon system _was_ recognized as a double planet.

So even if the Earth-Moon system were a double planet, that wouldn't
make the Moon "a planet" - the nomenclature works in a rather more
complicated way than that.

John Savard