Earth actually has two moons.

http://news.discovery.com/space/earth-has-two-moons-111222.html

by white_raven • 15 years ago

79 66 15 years ago

news.discovery.com

baddox• 15 years ago

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• 15 years ago

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...

tzs3• 15 years ago

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!

joshAg12• 15 years ago

Isn't that true for all moons, though, simply because the radius of the orbit around the sun is much larger than the radius of the moon around the planet?

also, isn't the force ratio issue more an issue of the inverse square law than anything else?

kijin• 15 years ago

From the point of view of the Sun, some satellites with short orbital periods might appear to move back and forth as they run around their planet. Earth's Moon, on the other hand, never moves backward from the point of view of the Sun.

That still wouldn't make the Moon a planet though.

keenerd5• 15 years ago

No. If the Earth's mass were to vanish, the Moon would continue to orbit the Sun. It would be a bit more wobbly, but the Earth is not an essential part of the system.

If the same happened to Jupiter, all of those moons would fly out of of the solar system. (Or at least go cometary.) None of the orbits would survive.

redthrowaway• 15 years ago

Isn't the criteria for planethood having sufficient mass to be largely spherical? I thought the moon was oblong due to tidal locking with the Earth. I realize that the Earth isn't a sphere, but in that case it's due to the centripetal force of its rotation on its own mass, which I think gives it a pass.

waitwhat• 15 years ago

Being "largely spherical" is (post-Pluto furore) a necessary but not sufficient condition for being deemed a planet. http://en.wikipedia.org/wiki/Definition_of_planet

tzs3• 15 years ago

The Moon is closer to spherical than the Earth is. The Moon's flattening is something like 0.001, as opposed to 0.003 for Earth.

ars2• 15 years ago

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

tzs3• 15 years ago

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!

digitalsushi• 15 years ago

I am so confused now. The center of mass of the solar system is often not inside the Sun.

dekayed• 15 years ago

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• 15 years ago

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).

carsonbaker• 15 years ago

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

CheapBastid• 15 years ago

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

Kittynana• 15 years ago

Why must journalists exaggerate every science headline?

AndyKelley• 15 years ago

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.

moocow01• 15 years ago

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• 15 years ago

We do. See http://en.wikipedia.org/wiki/Moons_of_Saturn

maaku4• 15 years ago

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

Eliezer• 15 years ago

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

rsanchez1• 15 years ago

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

brudgers• 15 years ago

Link to referenced MIT Technology Review post:

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

manojlds2• 15 years ago

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• 15 years ago

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.

manojlds2• 15 years ago

How many moons does the Sun have??

akkartik2• 15 years ago

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

te_platt• 15 years ago

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.

akkartik2• 15 years ago

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?)

Earth actually has two moons.

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