What is Moons: Definition and 98 Discussions

A natural satellite, or moon, is, in the most common usage, an astronomical body that orbits a planet or minor planet (or sometimes another small Solar System body).
In the Solar System, there are six planetary satellite systems containing 205 known natural satellites.
IAU-listed dwarf planets are also known to have natural satellites: Pluto, Haumea, Makemake, and Eris. As of September 2018, there are 334 other minor planets known to have moons.A planet usually has at least around 10000 times the mass of any natural satellites that orbit it, with a correspondingly much larger diameter. The Earth–Moon system is the unique exception in the Solar System; at 3,474 km (2,158 miles) across, the Moon is 0.273 times the diameter of Earth. The next largest ratios are Neptune at 0.055, Saturn at 0.044, Jupiter at 0.038, and Uranus at 0.031. For the category of planetoids, among the five that are known in the Solar System, Charon has the largest ratio, being 0.52 the diameter of Pluto.

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  1. johann1301

    Why Isn't the Moon's Shadow Line Perpendicular to the Sun-Moon Line?

    So I'm watching the sky at daytime, and i see the moon. The shadow on the surface of the moon goes in a straight line. I therefore conclude that the angle of MeMoonSun must be 90 degrees. But a peculiar thing shows; The shadow line on the moon is not perpendicular to the straight line between...
  2. B

    Calculation of the Distance to the Moons Apogee/Perigee ?

    I want to calculate whether the perigee / apogee of the Moon always takes place at the exact same distances (radius to Earth) as measured by the LLR (Lunar Laser Ranging). I trust that we know the size of the orbit of the moon very exactly, and also that the orbit is growing larger. But I...
  3. M

    Impact & Gravitational Effects of Asteroids/Comets on Moons Orbit

    How big would an asteroid or comet need to be to say, disturb the moons orbit, either by impact or gravitational attraction? And would this significantly alter the chances of "Death From Above"? :)
  4. Y

    Question about the Moons formation.

    I am not a scientist, just an observer and an enthusiast. I have read that the moon was formed by a huge collision between the Earth and another large body. I was wondering if this had any affect on the Earths position in the solar system. Could it have moved the planet at all? With the extra...
  5. J

    Moons Orbit Ellipse: Perilune, Apolune, Coordinates

    For an object in an elliptical orbit around the moon, the points in the orbit that are closest to and farthest from the center of the moon are called perilune and apolune, respectively. These are the vertices of the orbit. The center of the moon is at one focus of the orbit. A spacecraft was...
  6. FtlIsAwesome

    Jovian Moons: Is a Small Jovian a Moon of a Larger One?

    My guess is that a small jovian can be a moon of a much larger jovian. Is this likely, and how would we detect it? Just something I've never seen before. It seems that the assumption is that all moons of jovian planets are terrestrial. Also possible are jovian binary planets, but I'm talking...
  7. Loren Booda

    Composition of planets vs. moons

    In our Solar System, how does the ratio of gas planets to terrestrial planets compare with that of gas moons to terrestrial moons?
  8. Simfish

    Do moons always get more attention than the gas giant planets they orbit?

    It certainly seems like it. People are far more excited about Titan than they are about Saturn. And the same is true for several other Saturnian moons. Same with Jupiter and its 4 biggest moons. People seem to care more about its 4 moons than Jupiter itself. I'm no exception. But certainly, I...
  9. nukeman

    Need help converting values regarding Jupiter's mass, and moons.

    Here is my problem. I have charted out the data points of one of Jupiter's moons, and have concluded for this example, a value of: P(in days) of 7, and a (height of the peaks above zero) of 4 Now, here is what I am having trouble doing: "Convert P and a to years and AU. Then, convert a to...
  10. V

    Orbit of Moons & Shapes of Planets due to Warped spacetime

    First post, I am just a layman, so go easy. The way I understand it, the moon is following a straight line through space. However, the mass of the Earth is so great that it warps the fabric of spacetime and it appears that the moon is going around the earth. Same with any moon or planet, they...
  11. D

    Finding the Angular Distance Between Jupiter and its Moons (Galilean)

    I am having the hardest time figuring out the angular distance between Jupiter and Io, Europa, Ganymede and Callisto! I asked my teacher for help: "How would one find the angular distances between Jupiter and its moons?" and he said: "Do you know the field of view of your telescope? Once...
  12. H

    Telling time with Jupiters moons?

    I was watching the Nova show, "Lost at Sea; The Search for Longitude", which describes our quest to accuately determine our longitude as we travel across the sea. It said that watching the moons of Jupiter was an accurate way of keeping the time of your home port but wasn't practical to do on...
  13. G

    Slowing down the earths rotation and changing the moons orbit

    I have a friend who believes that if he spins in a circle he is changing the angular momentum of the Earth and therefore is slowing down the Earth's rotation. Can someone explain exactly why this is not true? To me it is common sense, but I am no physics major. Also, the same friend is...
  14. P

    What does this tell you about how the moons were created?

    Homework Statement Two identical spaceships are sent to explore a planet with two moons. Each spaceship orbits one of the moons. a)The orbital radii are observed such that the spaceship have the same period. What is the relative mass of the two moons if the two radii differ by a factor of...
  15. T

    Extrasolar Moons: Could There Be Earth-Sized Worlds Around Gas Giants?

    I know that they cannot currently be detected but according to the amount and variety of moons within our own Solar System I would imagine that other solar systems must have a huge number of them. Although I am not biologist or astronomer (or a scientist whatsoever) it seems fairly obvious to...
  16. E

    How Does Uranus' Tilt Influence the Orbits of Its Moons?

    One of the leading theories that explains Uranus' tilt is that it was hit early in its lifetime by a protoplanet, which disrupted its rotation axis and caused its unusual tilt. How does this hypothesis explain Uranus' moons as well? Most of them are aligned to the equator, and some are also...
  17. Pythagorean

    Hopsitals and Police busier on full moons

    Have you ever heard this? Apparently, officers and nurses agree. Hypotheses? mine: The bright assed light in the sky tinkers with your circadian rhythm and people are likely hitting a point in the evening where their brain is tired and they're less likely to inhibit their behavior.
  18. A

    Weight of 10 kg Mass on Moon: 60N

    19. The acceleration due to gravity on the Moon is 1/6 of that on Earth. The weight of a 10 kg mass on the Moon is A. 98 N B. 60 N C. 16 N D. 1.7 N (For this question I divided 9.81 by 6 and the multiplyed this by 10) Is this the correct way to do it ?
  19. B

    Energy required to the Moons orbit

    How much energy would it require (per second or per orbit) to keep the moon in orbit, if gravity did not exist? Pretend the gravity from Earth did not exist, and the moons still should orbit like it does. Does it exist a equation to calculate that?
  20. S

    What Would Happen if Earth Had Two Moons?

    Let's think about this for a second: Unlike earth, Mars has two moons. Let's switch the two. Instead of just one moon, what would happen if the Earth has two moons? I've been searching the Internet like crazy for answers on this, I haven't been able to find much. All I've found is stuff on...
  21. P

    Why do Jupiter's moons line up?

    So I took my telescope out last night and got to actually, really see Jupiter for the first time and it was amazing! While I was looking at it I could see it has 4 moons (though I later found out it has way more than that, but the ones I observed were the big Galilean moons). Another thing I...
  22. 8

    Can We Detect Moons Around Earth-Sized Exoplanets?

    how far are we from detecting a moon around an Earth sized planet in another star system? i read an article recently (http://www.newscientist.com/article/mg20126932.100-why-are-the-sun-and-moon-the-same-size-in-the-sky.html") about how the moon being so close to Earth wasn't, or doesn't...
  23. V

    Need help with a problem involving planets and moons.

    Homework Statement While a planet P rotates in a circle about its sun, a moon M rotates in a circle about the planet, and both motions are in a plane. Let's call the distance between M and P one { lunar unit}. Suppose the distance of P from the sun is 2100 lunar units; the planet makes one...
  24. L

    Exploring the Formation of Planets and Moons

    I have for a long time now wondered about how the planets and moons actually congealed into their current forms. How can loose mass and gas develop the gravity to pull themselves together? Could the cause of these actions been related from the "big bang" which threw out micro black holes of...
  25. S

    Inventing Moons and their Eclilpses-help

    Inventing Moons and their Eclilpses--help! I'm not mathmatically inclined, but I don't mind learning. I used the model of the typical eclipse seasons for Earth, and it works for Earth, but what about inventing a totally different planet with two moons? I hate guessing. I'm calculating an...
  26. S

    Synchronous Moon Stability: Conditions & Twin Planet Distance

    I have been toying with an SF scenario in which an Earth-like planet has a synchronous moon. Among the consequences are regular eclipses every spring and autumn, poor visibility of the stars on the side facing the moon except during eclipses, and no need for chronometers to determine your...
  27. S

    Need Help Creating Moons for Fiction Story

    I'm not a big astronomy whiz or anything, nor am I great at math, but I need help. For some reason or other, I can't seem to figure this out. I have 2 moons for a planet I'm writing about, and I need to accurately figure out their sidereal months. The planet has 722 days a year. Moon...
  28. S

    Change in Moon's Orbit: Impact of Earth's Mass Doubling on Circular Orbit

    What would happen to the orbit of the moon if the Earth's mass were suddenly (magically!) to double? **Assume the orbit is initially circular This is everything given in the question. So, basically what I have come up with is using F=GMM/d^2 ... I have found that when the Earth's mass...
  29. S

    How Would Doubling Earth's Mass Affect the Moon's Orbit?

    What would happen to the orbit of the moon if the Earth's mass were suddenly (magically!) to double? **Assume the orbit is initially circular This is everything given in the question. So, basically what I have come up with is using F=GMM/d^2 ... I have found that when the Earth's mass...
  30. H

    Two moons on August 27th, 2007

    First off, forgive me if this should have gone in the “Astronomy Events” sticky. Long time reader… but hardly ever post. I got this email today. It actually has been going on for some time now, since 2003 I believe. “Two moons on 27th August 2007* *27th August; the day the Whole...
  31. cepheid

    Seeing the Galilean Moons with Celestron 7x50 Binoculars

    I have a pair of Celestron 7x50 binoculars. I was looking at Jupiter a few nights ago. Sometimes, I imagined I could just barely make out a row of specks that might have been the Galilean moons, but it was really hard to hold the binoculars steady enough to be certain. Is it at least...
  32. L

    Orbital Radii of planets and moons of jupiter

    Homework Statement Finally, develop models for only the four innermost planets and the four Galilean moons by reducing all the orbital radii by a factor that makes the orbital radius of the first planet or moon equal to one. Give possible reasons for the similarities and/or differences in two...
  33. G

    Exploring Planets and Their Moons: Where Did They Come From?

    Being new to astronomy, I was wondering, do all planets have moons? What are they? how did they get there?
  34. Astronuc

    NASA NASA's Hubble Discovers New Rings and Moons Around Uranus

    http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/33/full/
  35. S

    Exploring the Composition of Mars' Moons: Phobos & Deimos

    Is there any definite data on the composition of Phobos and Deimos?
  36. N

    Components of the Moons Acceleration

    Hello, I need help on deriving equations for the moon's acceleration around the earth. I've found that the total acceleration is a=Gm/r^2 (G is gravitational constant, m is mass of moon and r is the Earth to moon distance) what i need to do is split this into its x and y components, and derive...
  37. Astronuc

    NASA Report of the NASA Science Definition Team for the Jupiter Icy Moons Orbiter

    Executive Summary more at - http://www.spaceref.com/news/viewsr.html?pid=12642 Report in pdf - http://ossim.hq.nasa.gov/jimo/JIMO_SDT_REPORT.pdf
  38. F

    Can You Name All the Moons in Outer Space?

    Could somebody list all the (known) moons for me in outer space, or just elaborate on that however they can for me thanks
  39. N

    Several moons in the Saturn system

    There are lots of such wrt Jupiter; several moons in the Saturn system (3? 4? more??), and one wrt Neptune. There is also at least one asteroid that is in some kind of odd relationship with one of the stable Earth Lagrange points. Is that it? Are there any good papers showing that the...
  40. J

    Moons with Moons: Solar System Possiblities

    Are there any moons in our solar system that have moons? Or is this not possible, due to instability?
  41. S

    Shepherd Moons on ringed worlds

    What exactly is the physics behind the shepherd moons? Take Pandora and Prometheus of Saturn for example, they keep the F-ring in line by their combined gravitational effects, I went though a lot of books but all of them are quite general hand-waving kind of explanation and nothing...
  42. W

    Mass of Jupiter using as Many moons as Possible

    To calculate the mass of a binary system, one can use keplers law. P^2= (4*(pi)^2*a^3)/(G*(m1+m2)) To get a more accurate mass I must take into account as many moons as possibe for jupiter. Is it possible to change kepler's law alittle to make it accommodate more than a binary system? ty
  43. tony873004

    Stargazing Picture of Jupiter's moons without a telescope

    I took a picture of Jupiter's moons without a telescope. This is from my 300 mm zoom lens on my Canon Digital Rebel: http://orbitsimulator.com/orbiter/jupiter.jpg
  44. B

    Are There "Moons of Moons" in the Solar System?

    Greetings all :smile: This seems like a good place to ask a question that's been rolling around my head for a while... 1. To my knowledge there are no "moons of moons" within the Solar Sytem - no natural satellites of any of the moons of Earth,Mars,Jupiter,Saturn,Uranus, Neptune or Pluto...
  45. S

    How many moons does Jupiter have and how were they discovered?

    21 new satellites of Jupiter have been found this year. Many are thought to be due to an ancient collision of a larger moon with a comet. Jupiter has four large moons and dozens of smaller ones (there are 60 satellites known so far). Galileo first discovered the four largest moons of...
  46. Ivan Seeking

    New Theory: Catastrophe Created Mars' Moons: Space.com

    "PASADENA, California -- The two moons of Mars -- Phobos and Deimos -- could be the byproducts of a breakup of a huge moon that once circled the red planet, according to a new theory." http://www.space.com/scienceastronomy/mars_moons_origin_030729.html
  47. S

    Exploring the Dynamics of Planetary Moons and Orbital Evolution

    Some questions, based on things I've heard, but don't know to be true: The rotation of the moon has slowed down to the point that one revolution and one rotation have the same period. A number of the moons of Jupiter and Saturn also have this property. Mercury does something similar to this -...
  48. P

    Jupiter is up to 58 moons (more likely)

    egad! http://www.cnn.com/2003/TECH/space/04/05/jupiter.moons.ap/index.html
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