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Earth actually has two moons.
- baddox 15y agoOf 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 15y agoNatural 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...
- tzs 15y agoOn 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!
- joshAg 15y agoIsn'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 15y agoFrom 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.
- joshAg 15y agothe orbital period of the satellite isn't as important as the orbital velocity of the planet around the sun. the earth goes around the sun at 29 785.8944 mps, so for a satellite to appear to stand still (from the sun's perspective) it needs that orbital speed (i'm assuming that everything orbits in the same plane to make things easier/cleaner). using the equation v^2 = GM/R gives an orbital radius of(G = 6.673 x 10^-11 N m^2/kg^2 and M = 5.98x10^24 kg, 1N = 1kg * 1m /1s) 449, 777 m or 449.7 km. Of course, the radius of the earth is 6.3 million meters (6300 km), so this isn't exactly possible. the only way for a satellite orbiting a planet to appear to move backwards with respect to the thing the planet orbits is for the satellite to have a faster orbital velocity than the planet's orbital velocity.
- keenerd 15y agoNo. 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.
- joshAg 15y ago
- dekayed 15y agoThe 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 15y agoFrom http://en.wikipedia.org/wiki/2006_RH120 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 15y agoRelevant video from QI: How many moons does the earth have? http://www.youtube.com/watch?v=D1zuAQAhhMI http://www.youtube.com/watch?v=D1zuAQAhhMI
- CheapBastid 15y agoFYI Fry was incorrect as it is not a mooon, but instead orbits the sun: http://en.wikipedia.org/wiki/3753_Cruithne http://en.wikipedia.org/wiki/3753_Cruithne
- Kittynana 15y agoWhy must journalists exaggerate every science headline?
- AndyKelley 15y agoThe 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.
- Kittynana 15y agoWhy must journalists exaggerate every science headline?
- moocow01 15y agoThis 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 15y agoWe do. See http://en.wikipedia.org/wiki/Moons_of_Saturn http://en.wikipedia.org/wiki/Moons_of_Saturn
- maaku 15y agoI think he's referring to Saturn's rings, which is largly composed of objects like the “moon” in this article.
- white_raven 15y agoyea but saturn's rings are made of ice, moons and planets are made of rock just like asteroids not ice like comets.
- joshAg 15y agowhat'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.
- Eliezer 15y agoThis is the same team that announced the discovery of 14 previously unknown days in February, I take it.
- rsanchez1 15y agoIt doesn't feel right to call it a moon though. It's a natural satellite, but it's not that big.
- brudgers 15y agoLink to referenced MIT Technology Review post: http://www.technologyreview.com/blog/arxiv/27425/ http://www.technologyreview.com/blog/arxiv/27425/
- manojlds 15y agoIf 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 15y agoMost 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 15y agoHow many moons does the Sun have??
- akkartik 15y agoDoes this finding increase the odds of a Tunguska-size impact?
- te_platt 15y agoNo, 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 15y agoIf 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?)