
Earth actually has two moons. - white_raven
http://news.discovery.com/space/earth-has-two-moons-111222.html
======
tzs
On the other hand, the thing we call The Moon shouldn't really be considered a
moon. The Earth/Moon system should really be classified as two planets in
essentially the same orbit around the Sun.

As I think was discussed recently in another thread here, and was pointed out
by Isaac Asimov in one of his science essays a long time ago, if you were to
plot the paths of the Earth and Moon through space, you'd find they are both
approximately 12-sided convex polygons around the Sun, out of phase by pi/12.
The Moon's path does not look like something a kid would draw with a
Spirograph, contrary to popular opinion.

When you look at moons of other planets, their paths do look like a Spirograph
drawing.

If you are having trouble visualizing this, imagine two horses racing around a
standard horse racing track. On the first straightaway imagine horse 1 is in
the lead. At the first turn, horse 2 gets the inside track and pulls ahead. On
the back straightaway, horse 2 leads, but on the second turn, horse 1 gets the
inside track and takes the lead back in the turn.

It probably wouldn't even occur to you to think of horse 2 as having done an
orbit around horse 1, yet if there were a remote control camera mounted on
horse 1, and you were controlling it from the stands and you were trying to
keep horse 2 in view at all times, you'd find that you have had to rotate the
camera around a full circle. So, from horse 1's frame of reference, horse 2
indeed did orbit it once!

Now imagine the horses on a modified track that instead of two straightaways
and two half-circle turns has four straightaways and four quarter-circle
turns. Now horse 1 thinks horse 2 circled it twice.

That's essentially what the Earth and Moon are doing, but there are 24 turns
in the race course, and the straightaways are not there--as soon as you leave
one turn you are starting the next. So, from Earth's point of view it looks
like the Moon goes around us 12 times a year. But alien astronomers watching
would be like spectators at the horse race--they'd just see two planets
orbiting the Sun in nearly the same order, taking turns using the inside track
to pull ahead.

Another way to look at it is to consider force ratios. For moons such as those
of Mars, or Jupiter, or Saturn, and so on, if you look at the force on the
moon from the planet, and the force on it from the Sun, you find the ratio of
those two is greater than 1. The planet "pulls harder" than the Sun does.

For the Earth and Moon, the ratio is less than 1. The Sun is pulling harder on
the Moon than the Earth is!

~~~
ars
The Moon is a moon because the center of mass of the system is inside the
Earth. It's as simple as that.

~~~
tzs
That's a rather unsatisfactory way to define moon-hood, because it doesn't
depend on any essential features of the two bodies. Replace Earth with
something of the same mass but more dense, and the Moon gets promoted to
planet, even though its size, shape, and orbit don't change? Yuck!

~~~
joshAg
I believe the center of mass for the earth moon system is something like 3000
km from the center of the earth (in a moonward direction). Just for reference,
the radius of the earth is 6300 km.

The density of the earth is ~5.52g/cc, and halving the radius of the earth
will increase the density by 8 which gives a density of 44.16 g/cc or 44,160
kg/m^3, which is about twice the density of osmium and a third as dense as the
center of the sun.

In other words, the scenario you imagine is not practically possible in a
system where one body is much larger than the other (if they're both about the
same size, then it a dual planet system).

~~~
johno215
I don't think this destroys your argument, but the barycenter of the Earth-
Moon system is located, according to wikipedia [1], 4670 km from the center of
the Earth.

Interestingly, both the Pluto-Charon and the Sun-Jupiter systems have
barycenters above the surface of the primary body.

[1]
[http://en.wikipedia.org/wiki/Barycentric_coordinates_(astron...](http://en.wikipedia.org/wiki/Barycentric_coordinates_\(astronomy\))

~~~
joshAg
seems i forgot to multiply by 1.6. whoops.

pluto-charon (imo) is a dual dwarf-planet system not planet and moon.

Sun Jupiter is my argument for why a planet shouldn't be defined only by
barycenter location.

updated density: 13.55g/cc.

this is about the current density of the core of the earth now. It's about
halfway between rhodium and mercury at STP, so not as horribly unlikely, but
still pretty out there.

Note: all density comparision made based on info from:
[http://en.wikipedia.org/wiki/Density#Densities_of_various_ma...](http://en.wikipedia.org/wiki/Density#Densities_of_various_materials)

~~~
maaku
> Sun Jupiter is my argument for why a planet shouldn't be defined only by
> barycenter location.

But Jupiter doesn't sustain nuclear fusion so it can't be considered in the
same category as the sun. If it was a red/brown dwarf, then we would be in a
binary system and that'd be a different story entirely.

------
moocow01
This article should be called "Astronomers actually redefine the term moon"

If this is the definition, then lets also report that Saturn now has a
gazillion moons.

~~~
Someone
We do. See <http://en.wikipedia.org/wiki/Moons_of_Saturn>

~~~
maaku
I think he's referring to Saturn's rings, which is largly composed of objects
like the “moon” in this article.

~~~
white_raven
yea but saturn's rings are made of ice, moons and planets are made of rock
just like asteroids not ice like comets.

~~~
joshAg
what's the difference between rock and ice? is there a boundary where if the
melting point is below a certain value then the substance is ice, and if it's
above that value it's rock? it's not like rock can't melt in the right
circumstances.

~~~
waqf
Ice is made of small molecules like water, methane or carbon dioxide.

~~~
joshAg
is there a concrete cutoff or shades of grey?

~~~
maaku
I think what the GP is getting at is the chemical difference of ionic minerals
(rock) vs amorphous collections of molecular compounds (i.e. water or methane
ice). There is a categorical difference between the two chemically, although
as an aside their properties in micro-gravity and the cold vacuum of the outer
solar system are quite different from what you might have learned in your
chemistry class.

------
dekayed
The article states that these 'moons' typically stay for around 10 months. Do
they escape from the Earth's gravity or are they pulled closer to the Earth
and eventually break up in the atmosphere?

~~~
jeroen
From <http://en.wikipedia.org/wiki/2006_RH120> :

2006 RH120 is a tiny near-Earth asteroid with a diameter of about five metres,
which ordinarily orbits the Sun but makes close approaches to the Earth–Moon
system every twenty years or so. Occasionally the object temporarily enters
Earth orbit through temporary satellite capture (TSC).

------
Kittynana
Why must journalists exaggerate every science headline?

~~~
AndyKelley
The irony is that the real headline is more exciting, which is that there is a
specimen orbiting earth, just waiting to be brought back for inspection.

------
baddox
Of course, the size requirements of a "moon" are arbitrary. In this article,
they're calling an object 1 meter across or larger a "moon."

~~~
furyg3
Natural satellites of all size are commonly referred to as "moons", so I'm not
sure how arbitrary it is.

The only exception I can think of at the moment are natural satellites which
are part of a greater planetary ring...

------
carsonbaker
Relevant video from QI: How many moons does the earth have?
<http://www.youtube.com/watch?v=D1zuAQAhhMI>

~~~
CheapBastid
FYI Fry was incorrect as it is not a mooon, but instead orbits the sun:
<http://en.wikipedia.org/wiki/3753_Cruithne>

------
akkartik
Does this finding increase the odds of a Tunguska-size impact?

~~~
te_platt
No, It doesn't change the estimate of the number of asteroids near Earth. It
just points out that more get caught in orbit than previously thought.

~~~
akkartik
If more get caught in orbit than we thought, then are we gradually learning
that the cloud of asteroids around us may not be uniformly distributed
anymore?

As an extreme example, if the cloud of asteroids was _entirely_ concentrated
in the earth's orbit that would increase the odds of a collision. So wouldn't
discovering that there's always one increase the odds (by a lot less)?

The number of pieces on a chessboard matters very little. You could be winning
or losing based on their arrangement.

Shit, this could be alarming. I'm gonna research this more. (Anybody here see
Melancholia?)

------
brudgers
Link to referenced MIT Technology Review post:

<http://www.technologyreview.com/blog/arxiv/27425/>

------
manojlds
If moon = natural satellite and if the "moons" of other planets have names,
what is the name of earth's moon? Why do they have to use it so
interchangeably?

~~~
eCa
Most likely because the Moon was given its name long before we knew that other
planets had moons.

Its pretty much the same as naming our planet the Earth, a pretty generic name
for a planet.

------
manojlds
How many moons does the Sun have??

------
Eliezer
This is the same team that announced the discovery of 14 previously unknown
days in February, I take it.

------
rsanchez1
It doesn't feel right to call it a moon though. It's a natural satellite, but
it's not that big.

