Moon Definition and 63 Discussions

The Moon is Earth's only natural satellite. At about one-quarter the diameter of Earth (comparable to the width of Australia), it is the largest natural satellite in the Solar System relative to the size of its planet, the fifth largest satellite in the Solar System overall, and is larger than any dwarf planet. Orbiting Earth at an average distance of 384,400 km (238,900 mi), or about 30 times Earth's diameter, its gravitational influence slightly lengthens Earth's day and is the main driver of Earth's tides. The Moon is classified as a planetary-mass object and a differentiated rocky body, and lacks any significant atmosphere, hydrosphere, or magnetic field. Its surface gravity is about one-sixth of Earth's (0.1654 g); Jupiter's moon Io is the only satellite in the Solar System known to have a higher surface gravity and density.
The Moon's orbit around Earth has a sidereal period of 27.3 days. During each synodic period of 29.5 days, the amount of visible surface illuminated by the Sun varies from none up to 100%, resulting in lunar phases that form the basis for the months of a lunar calendar. The Moon is tidally locked to Earth, which means that the length of a full rotation of the Moon on its own axis causes its same side (the near side) to always face Earth, and the somewhat longer lunar day is the same as the synodic period. That said, 59% of the total lunar surface can be seen from Earth through shifts in perspective due to libration.The most widely accepted origin explanation posits that the Moon formed about 4.51 billion years ago, not long after Earth, out of the debris from a giant impact between the planet and a hypothesized Mars-sized body called Theia. It then receded to a wider orbit because of tidal interaction with the Earth. The near side of the Moon is marked by dark volcanic maria ("seas"), which fill the spaces between bright ancient crustal highlands and prominent impact craters. Most of the large impact basins and mare surfaces were in place by the end of the Imbrian period, some three billion years ago. The lunar surface is relatively non-reflective, with a reflectance just slightly brighter than that of worn asphalt. However, because it has a large angular diameter, the full moon is the brightest celestial object in the night sky. The Moon's apparent size is nearly the same as that of the Sun, allowing it to cover the Sun almost completely during a total solar eclipse.
Both the Moon's prominence in the earthly sky and its regular cycle of phases have provided cultural references and influences for human societies throughout history. Such influences can be found in language, calendar systems, art, and mythology.
The first artificial object to reach the Moon was the Soviet Union's Luna 2 uncrewed spacecraft in 1959; this was followed by the first successful soft landing by Luna 9 in 1966. The only human lunar missions to date have been those of the United States' Apollo program, which landed twelve men on the surface between 1969 and 1972. These and later uncrewed missions returned lunar rocks that have been used to develop a detailed geological understanding of the Moon's origins, internal structure, and subsequent history.

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  1. E

    How to calculate Earth speed of the Moon induced orbit?

    It didn't work and I don't know how to do it.
  2. Parziter

    Solar System Forces -- Simulating the planetary orbits for my project

    TL;DR Summary: Solar sytem forces on Unity Hello ! For my last year in my school, I've got a project to do, and I wanted to recreate the Solar system with forces on Unity. My forces are Velocity and Acceleration (I'm using the Frenet's formulas). I'm sorry I'm not a physicist and that's why...
  3. 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...
  4. DANIELWR1998

    How do I find way of comparing the density of the Earth and the Moon?

    Surface acceleration is proportional to density and radius of planet (as 2 powers of R cancel with the volume) g(moon)/g(earth) = density(moon)*radius(moon)/density (earth)*radius(earth) = (1/4)*density(moon)/density(earth)
  5. LadiSilverfox

    Questions about a habitable second moon

    Good afternoon, I am working on writing a story that is set on a habitable second Moon. I suppose I could easily say it's a mild planet that splits into three main seasons and make up some story about how the first Moon appears every so often for a month of phases and then vanishes. As a...
  6. O

    I Is the eccentricity of the lunar orbit constant?

    The wikipedia article on Lunar distance contains a confusing graph. The graph seems to suggest that the eccentricity of the lunar orbit is maximal in january and ~july, and minimal in april and ~october. I think the eccentricity should be constant. Is wikipedia right or wrong, or is there some...
  7. Lren Zvsm

    Sapient Life Indigenous to a Planet Orbiting a Red Dwarf

    I am considering writing a story that features aliens indigenous to a planet orbiting a red dwarf star. Even so, because of its proximity to the red dwarf, the planet's surface receives much more radiation than Earth's surface does. What is more, the planet has no moon to stabilize its...
  8. YoungPhysicist

    B A strange phenomenon occuring when viewing the moon through window screens

    I found an strange phenomenon while observing the moon. When the moon is seen through the window screens, strange light lines will appear: Half open screen: Closed screen: Why is this happening? I saw similar things on street lights, car headlights, etc through screens.
  9. S

    I Frozen orbits of the moon

    Quick question for the people to better understand orbital mechanics. Due to large mass concentrations on the moon's surface, there are only four orbital inclinations that a satellite can be at to maintain an indefinite orbit: 27°, 50°, 76°, and 86°. My question is this: If a satellite was...
  10. H

    I How would the DSG orbit the Moon?

    Assuming it happens and that the Deep Space Gateway happens as a base for Moon landing and further exploration. What sort of orbit would it have in relation to this use? I have I-searched but not found any detail. Thanks.
  11. S

    B How hard is it to track moon cycles

    I know this may be extremely basic to almost everyone on the forums, but I really can't tell if moon cycles are predictable or not ! like can we tell in advance when is the full moon ? I feel like the answer sould be a definite YES, but my confusion is based on what I see each year. I live in an...
  12. T

    Planetary collision that formed the Moon made life possible on Earth The claim is that was how the excess Carbon, Nitrogen, and Sulfur arrived in the Earths crust, without them being in the core. For instance the Carbon/Nitrogen ratio of 40/1 is twice the 'expected' value.
  13. Chatterton

    Writing: Input Wanted Lunar Orbital Rings: Math Help Needed

    I'm working on a story set on the moon post-industrialization. The moon has an orbital ring with a spinning exterior to simulate Earth gravity. People work on the surface in lunar grav, then go up to live on the ring under conditions more favorable for human bodies. Two questions I need to...
  14. W

    B Why don't man-made satellites drift away from the Earth?

    What is the difference between man-made satellites and the moon that causes the former to drift towards the Earth but the latter drift away?
  15. A

    I Would going to the Moon delay going to Mars?

    Former 'Mars czar' in a interview: Read more at: But would it? Surely the Moon is a perfect launchpad for Mars, a testbed, and a supply base? Going from Earth direct is a truly huge task. From an established Lunar...
  16. DennisN

    Stargazing The Moon through my El Crapo Telescope

    Hi, I just managed to take my best picture up to this time of the Moon through my crappy $35 scope. Professionals would have cried if they looked at my setup :biggrin:: a Chinese $35 telescope a really (REALLY) unstable mount a mobile phone camera held up by hand in front of the eyepiece, set...
  17. S

    I Does the barycenter of the Earth-Moon system change?

    Hi to all! My question is: Does the relative position of the barycenter of the Earth-Moon system change due to mass distribution changes in the system and would this affect the Earth-Moon distance? For example: Could the sea level rise change the Earth-Moon distance? The only formula I found...
  18. DLeuPel

    I How can General Relativity explain the Moon drifting apart from Earth

    According to various sources, the Moon is separating from Earth 4 cm every year. I’ve searched for the explanation and I’ve found the following: The friction the seas and oceans from the Earth make with it’s soil causes the Earth’s rotation to slow down. This causes the Moon to speed up...
  19. J

    Is it possible to pour concrete in a vacuum?

    One idea for a moon base would be reinforced concrete domes, but is pouring concrete in a total vacuum even possible? Wouldn't the water just evaporate, or is there another material we could use instead of water? Are the ingredients of concrete plentiful on the moon, or could we synthesize them...
  20. J

    B The Moon's inclination to Earth's equator

    Is there an on-line calculator or a table that would allow me to plug in a date (year) which would give me the moon's inclination to the plane of the equator on that date? Thanks for any help.
  21. John Mohr

    Would a Very Large Mass (e.g. the Moon) Fall with g?

    Today, I had a thought experiment where I began to puzzle over inertia of very large masses being dropped. Imagine a scenario where the Moon was suspended 100 m from the surface of the Earth alongside a marble at the same height. Now ignoring air resistance and the gravity exerted by the Moon...
  22. riveay

    Simulating Jules Verne Earth to the Moon trip?

    Has anybody attempted simulating the trip to the moon described by Jules Verne? I've done a bit of research and found this two posts:
  23. A

    I Moon Poles: Which is better?

    North or South? Which is better for prospecting, mining, a site for a Moonbase and solar power. This could be quite a crucial question. The first solar array in permanent (ish) sunlight will stake a claim, because the next one will threaten to put it into shadow at certain times in the Lunar day.
  24. mr1batman

    B Distance between the Earth and The Moon

    we know that the moon is far away the distance between us and the moon can fit all the other 7 planets and some small dwarf planets but how ? it would take us 3 days to reach the moon and the spacecraft were traveling in at least 240,000 miles it should take us more than a year to reach the...
  25. P

    Maintaining interest in the new 'private' space race

    It's said that expectations drive the economy. So, how do you maintain interest in something that has no immediate bearing/benefit/utility of a population? I would think the answer to that question is to manifest a 'goal', in this case being an interplanetary species or even just building a...
  26. C

    Stargazing Looking for a beginner's telescope....

    Hi there, I love physics and I'd like to begin a bit of astronomy. I want to invest in a beginners telescope but I haven't a clue about what to buy. Some that I'm looking at are linked here:
  27. Chatterton

    Living on the far side of a tide-locked moon

    You're the Galileo for a developing society on a remote archipelago on the far side of a tide-locked moon orbiting a gas giant. How do you figure out your place in that solar system? How do you convince others, who believe your world to be the center of the universe, of the truth? Will a road...
  28. ImMaybeBrian

    B Mining of He-3 on the Moon?

    There's been talk about extracting Helium-3 from the Moon as a source of nuclear fusion energy because of it's efficiency and nonradioactive-ness--but I haven't seen anyone talk about any kind of side effects it could have either on the Moon itself, to people, etc. I understand because of it's...
  29. Chracatoa

    How often does the moon orbit intersect with sunset?

    Bear with me, this is a very specific question for a scifi book I'm writing. If we had a ring in place of the moon, and you lived somewhere in the United States, how often would the ring intersect with the sun when it's setting (or rising, if it makes it easier). I know that it can take up to...
  30. A

    I Where on the Moon should the first Moonbase be?

    Should it be on the farside to get radio silence for radio telescopes and explore there, or should it be at the pole for the ice, or in a cave?
  31. A

    Buzz Aldrin, Fashion Icon
  32. C

    Universal Gravitation and/or Tidal Force

    Homework Statement ANY HELP IS WELCOMED[/B] The single moon of an Earth-like planet creates tides on the planet that are slowing the planet’s rotation. The planet’s rate of rotation is decreasing at a rate of 7.00 x 10-7 radians/sec/century. The mass of the planet is 6 x 1024 kg, and its...
  33. V

    I Why exactly does the ocean bulge on both sides of the Earth?

    Here's what my prof says: "Define F_{mean} to be the mean force, F_close to be the force on the side of the Earth closer to the moon, and F_far to be the force on the side of the Earth furthest away from the moon. On the closer side the net force is F_close - F_mean > 0 On the further side the...
  34. Jadaav

    I RA and declination of Sun & phase of Moon program

    Hello there, I want to write a fortran program which gives me the right ascension and declination of the sun and the phase of the moon when a date and time is given. I just want to know how to write it and if there are any references that can help me. Thanks, Jadaav.
  35. N

    A Would a larger Martian moon create a stronger magnetic field

    I have read a previous forum discussing the possibility of creating a stronger magnetic field on Mars using tidal forces by putting a larger moon in orbit around Mars. My question is how large of an object would it have to be compared to Mars and how distant would its orbit need to be from the...
  36. Charles Kottler

    B Is a lunar space elevator a realistic proposition?

    I recently came across this article (from 2005) outlining a proposal for a lunar space elevator: Their is nothing in the article which stands out to me as being unrealistic, although the cost estimate ($10B) might...
  37. A

    I How can inhabitants of moon guess of its revolution

    How can inhabitants of the far side of moon guess that the moon is revolving around the earth?
  38. P

    I How much does the mass of the earth effect moon gravity?

    As I understand it, the effect of gravity is greatly diminished over long distances. Even so, the mass of the moon is sufficient to exert an influence on earth, for example on tides. My question is, to what extent does the mass of the Earth influence gravity on the moon. Gravity on the moon...
  39. Mech_Engineer

    Happy Moon Day 2016!

    Happy Moon Day 2016! I was lax in my duties last year and [gasp] missed Moon Day 2015, but that makes Moon Day 2016 all that more important! For those that didn't realize, today marks 47 years since mankind first set foot on the moon. In celebration of this achievement, we are enjoying...
  40. Greg Bernhardt

    Insights The Moon Quiz - Comments

    Greg Bernhardt submitted a new PF Insights post The Moon Quiz Continue reading the Original PF Insights Post.
  41. A

    B Determining the greater force?

    Q: Fred is on Earth, and Jane is on the Moon. a) Each lifts a 2.0 kg rock. Who has to apply the larger force? b) Each throws the rock horizontally with initial speed of 6.0 metres per second. Who applies the larger force? For both questions, I wrote: Fred, because the gravity on Earth is...
  42. 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...
  43. Jessica Ann Yost

    Why does the moon look full in my photos,.... close to the Sun's position? Is this even possible? First two, taken around May 1st, 2015. Last one, taken in October, 2015
  44. Kid Ljungblom

    How would Earth be affected by an additional smaller moon?

    I have been looking for an answer for so long. Everywhere I look I find answers to the question how two lunar sized moons would affect the Earth's tides and so on. But I want to know, if we had our own moon, with its normal orbit, and an additional smaller moon, (or just further away from...
  45. M

    How often do satellites pass the moon?

    So, I have really no background in astrophysics (other than touching on some areas in general physics and calculus). I understand that asking questions such as the one in my title, that I would need to be more specific, and I will attempt to be as specific as I can. I understand that most...
  46. Stephanus

    Orbits in the universe

    Dear PF Forum, Lunar Satelite orbits the moon, The moon orbits the earth, The Earth orbits the sun, I know that some of you know about this picture You might want to tell me. "No Steven, the Sun also orbits the earth" But for all practical purpose, we'd say that the earth orbits the sun. So...
  47. sevenperforce

    Suborbital docking - Apollo Landings

    Suppose that one of the Apollo landings had developed a fuel leak in the lunar ascent module, such that even with a lightened load, the ascent module only had enough delta-v to reach about half of lunar orbital velocity. One possible rescue solution would be to lower the command module's orbit...
  48. T

    How fast to shoot a bullet for it to hit the moon?

    Ignoring the fact that the atmospheric friction would probably disintegrate it. I know that to achieve LEO takes around 10km/s delta v. But only 2km/s of that is to overcome the drag and gravity. So you could shoot at 2km/s, get to LEO altitude then it falls back down because it is not at...
  49. J

    Contributions to Earth's orbit

    If one writes down the Newton's equations of motion for the Earth, the forces that act on the Earth are the gravitational forces exerted by all other massive bodies, such as the Sun, the Moon, the planets etc. The largest of these forces is that of the Sun: keeping into account only the Sun's...
  50. Universh13

    Moon's angular momentum

    I know that the angular momentum of the moon with respect to the Earth can be calculated by L = Iw but if the L of the moon is k * Learth-sun the there will be way more eclipses, one of each every 28 days. So the momentum of the moon must change over time, how does it work?