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. J

    Creating a Habitable Planet with 3 Moons

    TL;DR Summary: I’m worldbuilding for a novel I’d like to write. I’d like my planet to have 3 moons, and I’m curious how to develop the planet and moons in a scientifically sound way. I apologize in advance if any of this doesn’t make sense or if terminology is way off or misused. This stuff is...
  2. Astronuc

    B Saturn's 145 Moons: The Latest Discoveries and What Sets Them Apart

    Saturn reclaims 'moon king' title with 62 newfound satellites, bringing total to 145 https://www.space.com/saturn-moon-king-62-newfound-satellites Astronomers have discovered 62 new moons orbiting the ringed planet Saturn, bringing to total to 145 Moons, with 121 irregular moons and 24 regular...
  3. Astronuc

    JUpiter ICy moons Explorer (JUICE) on it's way to Jupiter

    https://www.imperial.ac.uk/news/244571/imperial-led-jupiter-bound-instrument-successfully-deployed/ https://www.esa.int/Science_Exploration/Space_Science/Juice https://www.esa.int/Science_Exploration/Space_Science/Juice/How_to_follow_the_Juice_launch_live...
  4. E

    I Research paper and sources investigating Earth's moons

    Hi. I am looking for scientific papers, articles and/or even simulations that investigate how many moons it would be possible for earth to have and maybe modern theories about what these moons can be and look like (i.e what characteristics they have) if they were to exist. I have googled and...
  5. Amaterasu21

    I Why are Saturn's moons so bright?

    Hi all, In the Jovian system, Callisto is extremely dark (albedo 0.22) because its ancient surface is covered with meteoritic dust. Ganymede is brighter (albedo 0.43) as its surface has been resurfaced more recently and hasn't had as much time to accumulate dust; that's confirmed by the younger...
  6. L

    A satellite in orbit in a system with one planet and two moons

    1) Considering the forces on one of the moons, I have: ##\frac{GMm}{(10R)^2}+\frac{Gm^2}{(20R)^2}=m\frac{v^2}{10R}\Leftrightarrow v=\sqrt{\frac{G}{10R}(M+\frac{m}{4})}.## 2) Considering the initial situation in which the satellite is at rest on the surface of the planet...
  7. xpell

    I If Jupiter had a 5th large moon until "recently", would we know?

    Hi! I'm fascinated by the apparent "concentration" of the four large Galilean moons in a barely 1.5 million km "strip", and the vast, "empty" distance to the next moons (moonlets?) Out of a purely aesthetic sense of horror vacui, :wink: I've always wondered if there was at least another Jovian...
  8. S

    Reverse gravity-assist, using the moons of a planet

    Hello, We hear a lot about gravity-assist, which usually means using a planet like Jupiter to speed a spacecraft on its way to another destination, like Pluto. But what about using a moon of a planet as a reverse gravity-assist? For example, if you wanted to place a spacecraft in orbit...
  9. Clay Gillespie

    B Angular Meteorite Strikes on the Moons Surface

    Summary: An explanation into the mechanisms that make irregular shaped meteorites, including some up one tenth the size of the moon, seemingly vanish, leaving symmetrical craters with surprisingly shallow floors. We’re going to look at the initial conditions of the moons formation from the...
  10. A

    Stargazing Two moons, two different rotational paths around a planet?

    There is a fictional planet with two moons. 1) Is it possible for those two moons to orbit that planet on rotational axes that are different from the planet's rotational axis? 2) If so, can the moons themselves also have different rotational axes?
  11. Bystander

    Ghost Moons: Astronomers Confirm Unusual Find

    Might belong in Astro, but I'm going to start it here; https://mysteriousuniverse.org/2018/10/just-in-time-for-halloween-ghost-moons-confirmed-by-astronomers/
  12. diogenesNY

    B NYTimes: 79 Moons of Jupiter and Counting

    By Kenneth Chang July 18, 2018 On Tuesday, scientists led by Scott S. Sheppard of the Carnegie Institution for Science announced the discovery of a dozen moons around Jupiter, bringing the total number orbiting the solar system’s largest planet to 79. Next to the famous moons that Galileo...
  13. Ethan Singer

    Why aren't less massive objects also spherical like planets and moons?

    given that gravity pulls things together into spheres, how much mass is needed to do so? Smaller objects such as asteroids, meteors, and that bench in the park don't just turn into spheres, because they lack enough gravitational force to dominate the shape. So at approximately how massive do...
  14. C

    B Uranus's Moon Cressida is Doomed

    The first measurement of the mass of a small Uranian moon suggests it will be obliterated by smashing into another moon in a mere million years. By Ken Croswell Full story: New Scientist
  15. S

    I Ammonia in icy moons of gas giants

    Its generally a established that ammonia is present under the ice of many moons of gas giants. I guess many in the astronomy/astrobiologists community don't spend much time at home, as they would have noticed that ammonia is a household disinfectant. I understand we don't have any idea of the...
  16. M

    I Solar system simulation -- Significant deviation of some Jovian moons

    I am working on a simulation of the solar system, but I am having problems with the orbits with Europa and Io shown in the image below, the orbits of Jupiter, Ganymede and Callisto are consistent with NASA's simulations. For all bodies I have used identical parameters in generating the initial...
  17. A

    I Question about a magnetic dynamo on tidally locked moons

    All, Non-scientist amateur looking for an answer. I understand that planetary rotation aid is vital to maintain a magnetic dynamo. Venus' slow reverse rotation is often used as an explanation for its odd low magnetic field. Assume there is a moon or a double planet that is tidally locked...
  18. R

    The force LRO experiences due to moons gravity?

    Homework Statement The Lunar Reconnaissance Orbiter (LRO) with mass m= 1850, maps the surface of the moon from an orbital altitude of 50 km. what are the magnitude and direction of the force the LRO experiences due to the moons gravity? Homework Equations Fg = (GMm)/r^2 The Attempt at a...
  19. S

    I Can a planet have multiple large moons?

    I thought that I'd posted this here, but perhaps not. I've had it on the OtherWorlds and the Physics stackexchange with no results. I grew up with countless alien planet covers depicting 2-3 moons all considerably larger (telephoto lens...) than Luna's appearance from earth. Luna from Earth...
  20. C

    B Uranus May Have Two Dark Unseen Moons

    And astronomers will be examining Hubble data to try and find them. Full story at New Scientist
  21. Noisy Rhysling

    Would an Earth with two moons the size of Luna work?

    Meaning "would a planet the size of Earth with two moons the size of Luna be a stable system?" I'm thinking of system where the moons are inhabited and the mother planet has been devastated by an asteroid impact. Naturally, they get to fighting (humans, what can you do with them?)
  22. S

    B Miscellaneous Trivia: Planets and Moons

    Hey everyone. A little introduction before I ask my questions. I enjoy playing Dungeons and Dragons a lot. For those that don't know, it's a tabletop fantasy game for supreme nerds like myself to enjoy and act out adventures filled with epic magic and battles and more of the such. Lately I've...
  23. W

    Finding Jupiters Mass with one of its Moons

    Homework Statement One of Jupiters Moons runs on a circle with the radius r1= 420000km in T1=1,77d. How big is Jupiters Mass? Can you calculate the Mass of the Moon with this as well? γ = 6,672*10-11m3*kg-1*s-2 (gravitational constant) Homework Equations I used following equation...
  24. N

    Centripetal forces in our solar system

    i calculated the centripetal forces in our solar system but i can't seem to find a connection using the centripetal force formula: Fmpz=(m⋅v2)/r i found out that the mean centripetal force for the planets around the sun is about 10^39, the mean centripetal of the moons around Jupiter is about...
  25. C

    Did a Giant Impact Make the Martian Moons?

    by Ken Croswell The Martian moons look like asteroids—dark, crater-pocked, and potato-shaped—suggesting Mars had snatched them from the nearby asteroid belt. But now planetary scientists have conducted the first computer simulations that bolster a controversial alternative idea: The satellites...
  26. Glenstr

    A shot I got of Jupiter & moons last night

    I was trying to get a couple of quick handheld shots of the waxing crescent moon with Venus with my Nex-5N last night, when just for fun I thought I'd snap Jupiter and see if the moons showed up with the Sony 200mm zoom. Much to my surprise I was able to see a couple of the moons with just a...
  27. S

    Angular separation between moons

    Please help I am not sure of how to work this out The angular separation between Styx and Nix 28 days after they are in line with each other on the same side of Pluto. Mass of Nix = 5*10^16 Kg Orbital Radius of Nix = 48675 Km Orbital period of Styx = 20 Earth days
  28. T

    Two moons in tidal lock orbit: phases, tides, axial tilt?

    Consider the following; We have Earth with two moons in orbit (discounting the existence of our own moon for the sake of this hypothetical scenario). One moon is the size and mass of Pluto, orbiting around 70,000km from Earth. The other is the size and mass of Pluto's moon, Charon, orbiting...
  29. O

    Why do natural Moons revolve around planets?

    Why do Moons revolve around planets? If because of gravitational pull, why won't they clash with planets. Please help me get answer to this question!?
  30. Q

    Possibility of multiple moons in Earth-Moon system

    I am interested in theoretical (what if) possibility of our Earth having more than one moon, I wonder if they can be arranged at the same orbital resonance as the Galilean moons 1:2:4 If yes, would such system be stable? If yes, should our Moon be the first, second or third satellite out of the...
  31. scotian280

    Jupiter's Moons - A Night of Stargazing from Romania

    Hi all. I'm new here but excited to be mingling with so many intelligent people. I'm British but live in Romania (Near Brasov) and 2 nights ago I was delighted to see some of Jupiters moons with the naked eye. I tried to get some pictures but without the right equipment it was a bit of a...
  32. M

    Orbital period of two moons in the same orbit

    1. Homework Statement Two identical moons of mass m maintain opposite positions in the same circular orbit of radius R around a planet of mass M. Find T2 the square of the orbital period. 2. Homework Equations T2=(4*pi2*R3)/ ( G*M ) [b]3. The Attempt at a Solution Hi...
  33. anorlunda

    Perfect Full Moons" - Find Date & Place to See Rare Celestial Event

    I invented my own phrase for a sky gazing experience; "Perfect Full Moon" It is something that I think most people could observe perhaps 1 or 2 times in a lifetime. I define it as follows. Each lunar month, the time of full moon is an instant. At that instant, there will be one place...
  34. C

    Hypothetical Scenario: Planet with Two Moons

    Alright, so there's a RP website I'm on that has a fairly detailed description of their solar system. However, they have no details on their planet's moons. This is something I've been trying to work out, as I want to introduce a lunar calendar of sorts to a nation I RP there. I've managed to...
  35. caters

    Cycle of blue moons from year 0

    Year 0 in some systems exists and in year 0 there is a blue moon. There are also blue moons in year 3, year 8 etc. This 4,5 I have seen so far and this timeanddate.com makes me think that it has that 4,5,3,16,17,2,9, and then repeats at 16 cycle. Sites including wikipedia say that a blue moon...
  36. G

    Questions about moons of the solar system

    At first, i would have questions about Io. How should i imagine the electric flux between it and Jupiter? Does it make a series of lightnings, aurora borealis, jam nearby radio communications? In an Asimov book they needed some magitech to reach Io, is it really so deep in the gravity well...
  37. A

    Questions about inhabited moons (or at least one)

    Hi All, I've got an idea brewing in my head that involves a inhabited moon. My original idea was a place that had day, night, and Really Dark Night(tm). It's a fantasy setting, so there won't be people tottering around in space, but I'd really like it all to make at least some modicum of sense...
  38. J

    Moons vs Planets: A Deeper Look

    Hi all, I was just wondering whether there is a proper difference between a moon and a planet. The only answer I've been able to find is that moons orbit planets, and planets orbit stars. But in reality, planets and moons both orbit their common centre of mass. So to frame my question...
  39. B

    Why Is Measuring Jupiter's Mass Using Its Four Largest Moons More Accurate?

    Homework Statement I've attached the lab sheets for the lab I am currently working on, and I am wondering if someone could help me with it. Homework Equations The Attempt at a Solution I am having difficulty with part two, steps 5-8. I have the slope from my excel graph 0.6458...
  40. S

    Gravity and mass with Jupiter's moons

    Homework Statement Table 5-2 gives the mean distance, period, and mass for the four largest moons of Jupiter (Those discovered by Galileo in 1609). A) Determine the mass of Jupiter using the data for Io B) Determine the mass of Jupiter using data for each of the other three moons. Are the...
  41. C

    How do the tidal forces warming moons theories hold when []

    How do the “tidal forces warming moons” theories hold when [..] How do the “tidal forces warming moons” theories hold when apart from heating from expansion, there may be also cooling from contraction? I can understand a temporary heating, from the tital forces exerted on the moon but...
  42. Gliese123

    Photo of Jupiter's moons with cellphone

    Hi there! Well, it's not just a photo taken by a cellphone but a cellphone plus a very good binocular. And as surprised as I became by being able to see the moons of Jupiter, I took a photo of them through, yes through the binocular. The result is what you see on the picture. And I wanted to...
  43. R

    Exploring the Gravitational Effects of the Moon on Earth's Rotation and Orbit

    Hi all Looking for some assistance if possible. Astrophysics is not my field--so please be gentle with me, as I am totally ignorant in your discipline. I’m doing experiments with magnets and rotating devices and would like to include the effect of drag and pull of the moon (at max and...
  44. S

    How Is the Moon's Mass Calculated Using Satellite Orbit Data?

    Homework Statement A satellite is in circular orbit around the moon 250.0km above the moon's surface, and takes 2.000 hours and 14.00 minutes for one revolution of the moon. Calculate the moons mass. The radius of the moon is 1737.4km, so if you add the extra 250.0km the satellite is above...
  45. J

    Questions about a planet with two moons

    Hi, this is my first post - i need some help with my astrophysics. I'm writing a sci-fi story where an Earth like planet has 2 moons. One of them is habitable and has conditions like Earth - like an atmosphere and so on (almost like a twin planet). The other moon would be smaller and more...
  46. A

    Data of several moons of the planets Uranus (new)

    Copy the table and determine keepers third law constant ku for Uranus using the data for the first floor moons sorry for the mirror effect its the only way i could get the page so you could read it .. its on the right # 32 b) find the average of the Ku, values of your calculations...
  47. S

    Developing writeer needs two moons

    I have taken a look over these forums and various other sites to get answers to this question. However, I want to re-frame it to get information which may be more useful. So, here it goes... If you have an Earth size planet, where would the best placement for two moons be? In other words I...
  48. S

    Calculating orbits of moons, when both the planet and star are involved.

    Hi there! I'm developing a game that involves simulating a star system. I'm using the Unity engine and its physics engine. I've got gravity working perfect. I've also developed an algorithm that works out how much force you would need to apply to a body to start it orbiting around another...