

The moon is not the Earth’s only natural satellite - jgrahamc
http://blogs.discovermagazine.com/crux/2015/03/02/earth-second-moon/

======
RBerenguel
Was also posted yesterday with a few comments:
[https://news.ycombinator.com/item?id=9133147](https://news.ycombinator.com/item?id=9133147)

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quarterto
As pointed out by the other commenters, Cruithne doesn't orbit the Earth, but
orbits the Sun with the in 1:1 resonance with the Earth. Its orbit is highly
elliptical: at perihelion it's closer than Mercury, and aphelion further than
Mars. Much of the time it's actually on the other side of the Sun to us.
Here's a couple of gifs of its orbit:

[http://upload.wikimedia.org/wikipedia/commons/2/23/Orbits_of...](http://upload.wikimedia.org/wikipedia/commons/2/23/Orbits_of_Cruithne_and_Earth.gif)

[http://upload.wikimedia.org/wikipedia/commons/c/c7/Horseshoe...](http://upload.wikimedia.org/wikipedia/commons/c/c7/Horseshoe_orbit_of_Cruithne_from_the_perspective_of_Earth.gif)

Note that its "orbit" around Earth actually takes _770 years_!

~~~
philh
More on wikipedia:
[https://en.wikipedia.org/wiki/3753_Cruithne](https://en.wikipedia.org/wiki/3753_Cruithne)

The yellow kidney bean orbit depicted only takes a year, but it doesn't
actually form a loop, and the kidney bean moves gradually away from the Earth
around the blue circle. When it goes almost all the way around, it's going to
start going back in the opposite direction. 770 years is the time it takes for
the yellow orbit to traverse the blue orbit once in each direction.

Not only does it not orbit the earth in any physically meaningful sense - it
doesn't even trace a shape that goes around the earth, which is annoying
because I'm pretty sure I've told people that it did.

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valarauca1
Earth actually has a few _better_ quasi-satelites. (164207) 2004 GU9, (277810)
2006 FV35, 2014 OL339 [1][2][3] these are very small bodies typically ~100-300
meters by 50-200 meters.

That also orbit in 1:1 resonance with the earth, their orbits are less
eccentric (not sun diving), meaning they can be reached with relatively very
low D-V requirements on the order of 11-13km/s, also incredibly weak gravity
of their own. (This doesn't count trojan bodies that orbit Earth/Sun L4/L5
points, which have been observed [5]).

Most quasi-satellites are typically temporary, Neptune currently has a large
one ~250km long, Venus even has one that'll only orbit for another 500 years
[4].

[1]
[https://en.wikipedia.org/wiki/%28164207%29_2004_GU9](https://en.wikipedia.org/wiki/%28164207%29_2004_GU9)

[2]
[https://en.wikipedia.org/wiki/%28277810%29_2006_FV35](https://en.wikipedia.org/wiki/%28277810%29_2006_FV35)

[3]
[https://en.wikipedia.org/wiki/2014_OL339](https://en.wikipedia.org/wiki/2014_OL339)

[4]
[https://en.wikipedia.org/wiki/2002_VE68](https://en.wikipedia.org/wiki/2002_VE68)

[5]
[https://en.wikipedia.org/wiki/2010_TK7](https://en.wikipedia.org/wiki/2010_TK7)

Orbital mechanics and near earth objects are really cool!

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grabcocque
It's not a satellite of the Earth, it orbits the sun in a resonant orbit with
the Earth's.

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leoedin
Surely we do give the moon a specific name: "moon". The concept of calling
other large rocky natural satellites "moons" almost certainly came later.

</nitpick>

~~~
dingaling
Even more confusingly, the Greek name for the body was Io, later allocated to
a satellite of Jupiter.

Moon came into Emglish from Danish for 'month'.

Luna was the Roman goddess represented by the body.

~~~
thaumasiotes
> Moon came into E[n]glish from Danish for 'month'

This is flat-out wrong. Check etymonline (
[http://etymonline.com/index.php?term=moon](http://etymonline.com/index.php?term=moon)
):

    
    
        moon (n.)
            Old English mona,
            from Proto-Germanic *menon-,
            from PIE *me(n)ses- "moon, month"
    

That's a straight line of descent from proto-Indo-European to modern English.
No borrowing at any point.

Interestingly, etymonline also points out that the english word "moon" is
cognate with the classical Greek word for the moon "mene". This leads nicely
into your more interesting, and less wrong, claim:

> the Greek name for the body was Io

This is a little unconventional. I'm not really trained in classical Greek,
but I also thought the word was selene.

Doing a simple search of greek dictionary entries through perseus (
[http://www.perseus.tufts.edu/hopper/definitionlookup?type=be...](http://www.perseus.tufts.edu/hopper/definitionlookup?type=begin&q=moon&lang=greek)
) quickly tells us that there are many, many, many greek words relating to the
moon that use a form of selene or mene (aselenos, moonless; selenion,
moonlight; hyposelenios, under the moon; triselenos, of three moons; numenia,
the new moon; dichomenia, the full moon; skotomene, moonless night; etc. etc.
etc.) Further inspection reveals that the term selene never appears in Homer,
while mene does, so mene is probably older. (Knowing that mene comes from a
PIE root also supports the idea that it's older.)

Io also appears, with a short gloss of "the moon". But the related dictionary
entries don't really deal with that sense; they prefer to define Io as the
mythological daughter of Inachus. But there is a citation to "Eust.ad D.P.92"
in which Io is the "name of the moon at Argos". Wikipedia's entry on Io (the
mythological figure) also says, "The ancients connected Io with the Moon",
with a citation to "Eustathius of Thessalonica commentary on Dionysius
Periegetes, 92", which looks like the same thing as the LSJ cite perseus
gives. Wikipedia also cites this to a 10th-century Byzantine encylcopedia
which Eustathius is known to have been familiar with, and to a 5th-century
Greek philologist. None of those cites go to web resources, and I would be
unlikely to get much out of them even if they did, but it does appear that
there was a minor strain of referring to the moon as Io in some places at some
point.

But selene and mene are clearly better candidates for "the word for the moon",
since only they are used to form moon-related words. (Really, there are two
words for the moon -- both terms appear as goddesses, but obviously have an
ordinary, non-divine sense.) As far as I can see, Io is a mythological
allusion, not "the word for the moon". It's even capitalized in the dictionary
entry with the moon gloss.

How'd you learn that the Greek name for the moon was supposedly Io? Do you
remember?

Finally,

> Luna was the Roman goddess represented by the body.

Sure, that's true, but luna is also just the ordinary Latin word for the moon.
You might notice that the same thing happens with both selene and mene.
Personification deities are dirt-common in classical literature. Take the
goddess who started the Trojan war, Eris. She's referred to (by us) as the
goddess of discord. And Eris isn't really a name -- it's just the word for
quarreling and strife.

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LanceH
If we don't get this debris cleared, we'll be demoted from planet status.

~~~
Roboprog
And then, there were 7 :-)

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claudius
QI did a series of questions on this subject:
[https://www.youtube.com/watch?v=kXzi89b3LHw](https://www.youtube.com/watch?v=kXzi89b3LHw)

~~~
ibmthrowaway271
QI is riddled with inaccuracies:-

[http://www.theregister.co.uk/2011/01/18/stephen_frytard/](http://www.theregister.co.uk/2011/01/18/stephen_frytard/)

~~~
quarterto
According to the Register, that bastion of quality journalism...

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arkydon
The one above is a Quasi-sattelite [http://earthsky.org/space/does-earth-have-
a-second-moon](http://earthsky.org/space/does-earth-have-a-second-moon)

An example of a "minimoon" : 2006 RH120 orbitted the earth in 2006 for less
than a year before resuming it's orbit round the sun
[http://sservi.nasa.gov/articles/simulations-show-mini-
moons-...](http://sservi.nasa.gov/articles/simulations-show-mini-moons-
orbiting-earth/)

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swamp40
Was that video supposed to clear things up?

I kept trying to figure out how both the Earth and Sun were standing still.

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dnautics
So the earth has not cleared its orbital neighborhood, and is a dwarf planet.

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masmullin
Thats no moon... its a space station!

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bibabo
It is.

