38 things you might not know about the Moon

(Unless, of course, you are extremely keen on the Moon, in which case you probably do. Anyway . . .)

1) Our Moon is the largest moon relative to the size of its planet in our Solar System. Some moons are larger - Titan and Ganymede, for instance - but are hundreds of times smaller relative to Jupiter and Saturn. This of course leaves out Pluto and Charon, which besides no longer belonging to the "planet" class are more of a several-body system. On that topic, our Moon is more than five times the mass of Pluto!

2) It's as bright as a sunspot and as dark as coal. A sunspot is a darker point on the Sun's surface. If you could isolate it, it would shine as bright as a full Moon - which looks extremely bright in the sky, especially when full. But its rocks are about the colour of coal. Its apparent whiteness is because it's nearby, and we see it contrasted against a darker space. Its albedo is roughly 0.1, which means it only reflects 10% of the light that hits it. (Ice is about 0.9, meaning it reflects 90% of the light, and charcoal about 0.04, meaning it reflects about 4%.) The Earth's albedo is about 0.3, though of course it varies from place to place. Mercury's is similar to the Moon's.

3) In pictures, the Moon is almost always drawn much bigger than it really is - because it is so bright and captures our imaginations. Its angular size in the sky is between 29.43 to 33.5 arcminutes, which is actually very small. The Andromeda galaxy appears six times larger than the Moon from Earth - but then, of course, we can't usually see that.

4) The size of the Moon appears to vary because of its elliptical orbit - and not because of where it is relative to the horizon. The famous "Moon illusion", the fact that the Moon appears huge when it's on the horizon, has been documented at least since Aristotle. Various theories have been put forward to explain it: one from Ancient Greece was that the Earth's own atmosphere had a magnifying effect. In fact, it doesn't - and you can check this for yourself by holding an object of fixed size next to the moon, at a fixed distance from your eyes. The cause is probably from the way we see the sky: we imagine it as fairly flat, or at most, a gently curving dome, making objects near the horizon seem further away than objects immediately overhead. There are also more landscape features like trees and buildings to compare the moon to on the horizon.

5) Actually, it's not quite simple. The Earth's atmosphere does have a lensing effect on the Moon which can turn it into funny shapes - but you need to be lucky to see it! It can be due to layers of air with different temperatures, such as here:

From the International Space Station, astronauts have seen a "squishy" Moon as a result of the Earth's atmosphere diffracting sunlight!

7) The Moon is 1/81th of the mass of the Earth, but its gravity is 1/6th. Why? Because of Newton's laws of gravity. (To be pedantic, the inverse square law specifically, which was not Newton's alone.) Gravity gets stronger the closer you are to the centre of something. If you have two bodies of exactly the same mass, but one is smaller than the other, the smaller one will have a greater gravity, though over a smaller area. This is why, in a binary star system where one star has died, the white dwarf, neutron star or black hole will often start accumulating matter from the star. As the Moon has a much smaller radius than the Earth, an astronaut standing on the Moon is much closer to the centre of the Moon than she would be to the Earth's standing on the surface of the Earth. Surprisingly, this effect is more significant than the mass of the body - double the mass and you double the gravity, but halve the radius and you multiply the gravity by four.

8) There is no atmosphere or liquid water on the Moon, meaning there is no weather. The escape velocity of the Moon is 2.38 kilometres per second (so it's much easier to fire rockets off the Moon than off the Earth). At Earthly and Moonly temperatures, this is easily low enough for all gases to escape immediately. (At a very, very cold temperature, such as Pluto and Charon, gases move far slower, so it's easier to hold onto them.) This has many implications . . .

9) . . . for example: Moon dust is dangerous to astronauts and their spacesuits! It's extremely abrasive - because no water has rubbed it and rounded it, the way rivers make pebbles smooth on Earth. It's very hard, because meteorite impacts give it a melted, glassy coating. This hardness and abrasiveness means it pierces spacesuits easily. It's very fine, and having no air or water to drive this fine dust around or turn it into soil, it stays there - static and clingy, getting into the Apollo craft and into the astronauts' lungs. The static comes from UV light which knocks electrons off the atoms and molecules. All this will need to be taken into account if we want to live on the Moon.

10) It has often been thought that there is water on the Moon - the Tintin characters, going there in the 1950s, see stalactites and stalagmites - and that hypothesis was correct: there is. It's ice, of course, and trapped within the rock - no rivers or lakes exist, and any water that seeped to the surface would immediately boil or be rapidly photodissociated. But absorption spectra by NASA's Moon Minerology Mapper show the presence of a tiny amount of water within the rock. This has wannabe lunar colonisers very excited indeed. Recently, researchers at the University of Michigan did a study on moon rocks which suggested that the water seems to have been there from the time the Moon formed. This is odd, because the Moon is thought to have been very hot when it formed, which would have boiled off any water. (The same, I suppose, goes for the Earth. Our water may have been brought via comets from the Late Heavy Bombardment.)

11) A "blue moon" is actually not a blue coloured moon at all, but simply the second full moon of any given month. Since the Moon's orbit is 27 days 7 hours 43 minutes, and your average month is 30 days and 10 hours, this doesn't happen often - every two or three years. Occasionally some Facebook page will tell you that a blue moon means something incredibly significant and spiritual. It doesn't: it's simply an inevitable lining up of human generated unequal series of numbers. Calendar months are entirely human choices. Sorry!

12) The Moon is red during a lunar eclipse because the light that reaches it is filtered through sunrises and sunsets. To visualise what is happening, look at this beautiful picture of Saturn:

Saturn's air itself is carrying the Sun's light around the planet (as the air does on a cloudy day on Earth). Earth's atmosphere does the same thing: the atmosphere around the edge of the planet carries the light on and diffuses it into Earth's own shadow. It's red light, for the same reason as sunsets are red: blue light is scattered and all goes off at an angle, leaving red light's path comparatively clear. There are also plenty of particles in the Earth's atmosphere, and particles tend to turn light redder. Light filtering through a sunrise and sunset has the most atmosphere to travel though. That's why people say there's "no protection" when you get sunburnt at noon: the Sun's light goes straight through the thinnest layer of air.

I made a silly little diagram to illustrate this for my March Galactic Orchids talk:

and incidentally, exactly the same thing happens when looking at spiral galaxies face-on versus edge-on:

(This lovely pair are NGC 4126 and NGC 3814. From the Sloan Digital Sky Survey telescope.)

13) We see perfect solar eclipses because of a wonderful cosmic coincidence: the Sun and the Moon appear exactly the same size. The Sun is 400 times larger than the Moon, but also 400 times further away. Actually, this varies - if the Moon is near apogee (furthest point away) due to its elliptical orbit during a solar eclipse, it won't quite block out the Sun, and we get the "ring of fire".

The shadow of the Moon on the Earth is actually surprisingly small. From the Planetary Habitability Laboratory:

(Eclipses are very emotional events. If you want to cheer yourself up, I strongly recommend watching this Sky at Night episode about Chris Lintott's trip to Turkey to see one!)

There is almost certainly no other planet on the Solar System where we could see such perfect solar eclipses - and this is in time, too, as well as space, because . . .

14) . . . the Moon used to be closer to Earth than it is now - and is moving away from us at the rate of 3.8 cm per year. We check this constantly by firing laser beams at retroreflectors placed on the Moon by the Apollo astronauts. As we know the speed of light, we can time how long the reflection takes to get back to us and get the Moon's distance to an accuracy of millimetres. In my lifetime, so far, it's moved away about 1.16 metres.

On average, that is. Its elliptical orbit varies by a huge amount more than that. But there's a slow progression. And the unavoidable conclusion is that the Moon used to be a lot closer to Earth than it is now. Tides would have been more dramatic; the Earth's own crust would have been under more strain, as would the Moon's. The friction this caused is why the Moon's face is always pointed towards us - and why we are heading the same way . . .

15) Earth's own orbit is slowing down, so one day we will always be showing the same face to the Moon, too. Our faraway descendants will never see a moonrise . . . But that will not be for a very long time.  At the moment, we only need to add a "leap second" less than once a year.

As Phil Plait put it: "I hope you liked 2008. Because you're going to get an extra 0.0000031689% of it today." (2008 was possibly the worst year of my life so I was not pleased, but the extra 0.0000031689% passed quickly!) As he explains, the Moon isn't the only influence - there's also the Sun, the fact that the Earth's structure is part solid and part liquid and generally uneven, earthquakes and tsunamis, and even the weather. So it's slow. But it seems that one day, a lunar orbit and an Earth day will be the same length - 47 of our present Earth days.

We know from fossil records and even rocks that the Earth's day was once 21 hours when life was very young, and 23 hours in the time of the dinosaurs. Coral is particularly good at showing this - they grow leaving marks like tree rings, marking days and years (or rather, periods of light and dark, and seasons, respectively). The older the records, the more days there seem to be in a year - indicating that days were shorter.

16) The Moon is speeding up! This is why it is receding from us, and how it will eventually only see one side of the Earth, as we do it. Essentially, it's taking energy from the Earth's rotation around its axis and putting it into its own orbit - like grabbing the hand of a spinning ice-skater. However, even though the Moon speeds up, it takes longer to complete its orbit, since it's further out.

17) The Moon may be responsible for the seasons. Some planets are very tilted (some ridiculously so, like Uranus); some are very sensibly aligned, with their equators on the same plane as the Solar System. Earth's is pretty tilted, and it keeps the tilt consistent as it goes round the Sun - hence, of course, the seasons. If the Moon was created in a giant impact (see later), this would have knocked us over; Uranus is thought to be tilted for the same reason. However, Earth also has a "wobble" (a very steady one; it's not going to fall over like a spinning top) which is shown in Milankovitch cycles. However, all these are steady and fairly minor, unlike Mars, which wobbles all over the place as its two titchy moons fail to exert any stability against the massive objects such as Jupiter pushing it around in the Solar System. The Moon has been shown to have a stabilising effect on the Earth's orbit - though this, much like the eclipses, will cease as it gets further away from us.

18) It may seem obvious, but art does not always capture it: the appearance of the Moon indicates where the Sun is, like an arrow. You will never see a crescent Moon looking like an open parachute: if the crescent is on its side, the "horns" will point upwards, indicating that the Sun is"below" the Earth's horizon. A full Moon indicates that the Sun is "behind" the Earth. My favourite ever scientist, Cecilia Payne-Gaposchkin, wrote in a 1954 astronomy textbook, agreed at the time to be the best in the world: "It is an amusing pastime to note the 'impossible moons' portrayed by some artists: a new moon high in the northern sky, for instance; a full moon near sunset in the west; or a crescent with horns pointed downward." (That is probably the American version of this book - I wonder if I have bought the only 1954 copy on the market?)

19) The Moon has a molten core and magnetic field. Both are very small - but it's enough to perturb the solar wind from the Sun. The molten core was found with the help of seismometers left by the Apollo astronauts. This is pretty interesting, since not all planets have a molten core or magnetic field - Mars's, for instance, has pretty much shrivelled up, and Martians have no protection from the solar wind.

20) Tides are caused by gravity. Nothing else. Contrary to what you may have heard, the Moon does not preferentially pull on water. It does not affect us "because we are mostly water". True, the Moon does have some cultural effects, hence the werewolf, the word "lunatic", and the Sussex police claiming that there's a rise in crime around the full moon. I've heard people tell me in all seriousness that they "feel different when it's a full moon", that they "felt really angry when the Moon was red during an eclipse", but the Moon isn't picking some of the molecules in your body and dragging them around whilst ignoring the others. I'm not going to pretend I know why these mood alterations seem to happen, but I suspect a lot of it's simply that we expect them. Moonlight is a wonderful, shivery thing, after all - I will never forget taking a walk in a moonlit wooded area around the lake on my university campus. The water was black and silver and the moonlight reflected astonishingly off the silver birch trees. The thrill was slightly marred by the fact that it was so muddy that one of my companions had wrapped his shoes in plastic bags . . .

The Moon's gravity - not to mention the Sun's - pulls on everything, including the Earth's crust. CERN had to take this into account building the Large Hadron Collider, if I recall correctly what was said on my visit there.  There are tides because water moves around easily. As for the Sussex police, I also recall Chris Lintott's comment on a podcast called "Living Space": "Anything to do with the Moon in Sussex has got to be Patrick's fault."

21) The much-missed Sir Patrick Moore wrote his first paper about the Moon when he was only 14. He was invited by a local astronomer, W.S. Franks, to come and use the Brockhurst Observatory which was very near where the young Patrick lived. Mr Franks was suddenly killed by a car knocking him on his bicycle, and Patrick was asked to take over the observatory. He presented a paper to the British Astronomical Association named "Small Craters in the Mare Crisium". You'll find the Mare Crisium on the far right here. "I sent it in, and was notified by the Association's Council that it had been accepted, but I felt bound to explain that I was not exactly elderly. I still have the reply, signed by the then secretary, F.J. Sellers: 'I note that you are only fourteen. I don't see that this is relevant'." (This, by the way, is exactly what you should be saying to young people interested in science.) You can read more in Sir Patrick's autobiography. (You can also leave a tribute for him here - and yes, he did play the xylophone.)

22) The darker areas of the Moon, "the man in the Moon", are called maria, meaning seas. Tell that to someone you know named Maria, if she'd be interested? The maria are of course not seas - they are in fact old lava flows, quite possibly made as the result of impact craters that filled with lava. There still seems to be debate whether the impacts caused an upwelling of lava or whether it was volcanism. They are dark because they are more iron-rich than the rest of the Moon's surface, indicating that they come from closer to the core. (Just like the Earth, the Moon has more iron towards its centre, because iron is heaviest, and it started off molten so heavy things sank to the bottom.) In this picture by Alan Friedman you can see a large lava basin which then received a later impact:

23) There isn't a "dark side of the Moon". There is a "far side" of the Moon that never points our way. But, as you can see for yourself, every side of the Moon experiences day and night with its rotation just as the Earth does. We can see the far side of the Moon, however, using spacecraft such as the Lunar Reconnaissance Orbiter:

24) The Moon does not, strictly speaking, orbit the Earth. Rather, the Earth and the Moon orbit a common centre of mass. This centre of mass, or barycentre, is just under 2000km below the surface of the Earth. That means that Earth goes round in a little circle, and the Moon goes round in a big one. (If they were the same size and mass, the centre of mass would be exactly halfway between them.)

The Earth and Moon feel equal and opposite forces: gravity balances a "centrifugal force" (I say that in inverted commas, because there's actually no such thing as centrifugal force, and the idle use of the term drives some physicists crazy. It's actually the law of inertia: the urge of the body to keep going in a straight line. If either body started going in a straight line, it would be going away from the other one. It doesn't, of course, because of gravity. When you whizz lettuce in a salad whizzer, it is flung against the wall of the bowl because it "wants" to go in a straight line, but the container prevents it from doing so). That's why we have two tides, not one. Water nearer the Moon feels the Moon's gravity more, and wells towards it. Water on the far side of the Moon feels the "centrifugal force" more, and moves towards outer space.

25) There are more maria and larger craters on the "near" side of the Moon that points our way, and the core is about 2km closer to Earth than the actual centre. It is tempting to conclude that all this is because the Earth's gravity pulled the heavy parts of the Moon towards us. But the Moon feels exactly the same effect as in the item above this: it has an equal and opposite "centrifugal force" pulling it away from the Earth as towards. It could be due to a massive impact (or several) coming from roughly Earth's direction; but we don't know. Perhaps one day we'll find out?

(It is not shameful, by the way, to answer a scientific question with "we do not yet know". It doesn't mean "scientists are stupid or lazy", but that the Universe is too big for us to have explored the whole thing yet. Otherwise we wouldn't need any scientists! Indeed, it's often when we think we're close to tidying up and claiming to understand the Universe that the greatest surprises of all occur - relativity and quantum theory, for example, or the accelerating expansion of the Universe. OK, rant over.)

26) The Moon is the second densest moon in the Solar System. The densest is Jupiter's volcanic moon Io.

27) Because of the Moon's elliptical orbit, we do get to see occasional corners of "the far side". This is called libration. Here, have a pic and get nice and dizzy.

There are other reasons for the libration: the moon's own axis is not quite at right angles to the plane of its orbit, so we see a "nodding" movement; and the fact that we're over 6000km from the Earth's centre, so we ourselves see the Moon from a very slightly different angle at the beginning and end of a night.

28) Phil Plait has a huge list of debunkations for conspiracy theorists who claim that the Moon landings were faked. You can read it here. I'll just go through one: the idea that upon launching, the Moon's dust "should have been blown around more".

Do an experiment. Next time you get out of the shower and the bathroom's full of steam, watch that steam for a while. Then blow on some steam quite far away. Or a balloon, if you desire; or a boiling pot of water or the kettle or a candle half a room away - whatever takes your fancy. You will notice that there is a pause, and then it will start swirling about. What did that?

Air, of course. The air from your lungs? Well, partly. But also all the air that's in between you and the steam. The air from your lungs knocked into air molecules, which knocked into more air molecules, which knocked into . . . well, on the Moon, this doesn't happen. Blown air, or blown anything, meets a vaccuum. Only that which is right next to the blowing gets touched. On the Moon, air will quickly dissipate into space.

For more lighthearted stuff, the Clangers are always willing to help.

29) For Moon observers, the best time to look at details is not at full moon, but when it's a crescent so there are lots of shadows. (This is only something I've been told. I'm a rubbish observer; I've used my telescope twice and then broken it!) Shadows show detail. Thank you Graham Bowes for this amazing image:

30) We've known for many centuries that there can't be air on the Moon, because its edges are sharp. Look at the Earth's own horizon, and it'll be misty, blue, and blurred. That's air getting in the way. You don't see that on the Moon. William Herschel, however, speculated that there might just be air in the craters - at the lowest points on the Moon's surface - and that aliens might live in these. He pointed out that at craters are on average 50% lit and may be lit from any angle, so alien buildings capturing the warmth of the Sun would probably be circular. Incidentally, Herschel has craters named after him on our Moon and on Saturn's moon Mimas.

31) Very occasionally, we see abrupt changes on the Moon, such as a spot turning brighter or darker or changing colour. These are transient lunar phenomena, thought to be caused by impacts, outgassing etc. You can hear a lot about the first ever recorded instance, in 1178, in Carl Sagan's "Cosmos". Some monks at a monastery in Canterbury saw "a splitting" of the very new crescent moon, and reported fire, smoke, darkening, and that the Moon "writhed" and "throbbed", "like a wounded snake". This was probably a large impact, and has been suggested to be the formation of the Giordano Bruno crater. Other odd phenomena include that static, UV-light-blasted moon dust, from number 9: the Apollo astronauts saw "twilight rays" towards the horizon, which was probably dust in a continually rising-and-falling state from the surface.

32) Going to the Moon made us much more aware of our own Earth. If you've ever spent a long time in a country other than your own, you'll know that you learn a huge amount about your own country, too. The first ever complete photograph of the Earth from space, "The Blue Marble", taken from Apollo 17 in 1972, and "Earthrise", taken by Bill Anders as Apollo 8 orbited the Moon in 1968, had a great effect on people's environmental awareness.

33) We're now pretty sure the Moon was formed by a giant impact on Earth, probably by an object the size of Mars. We can tell this from the fact that the Moon is made of fairly similar materials to the crust of the Earth, but not the core (it's less dense). Other theories of formation, historically, have included that it was captured, that the Earth and Moon formed together, or that the Earth was spinning so fast a piece of it bulged out and broke off!

It would take extreme luck to capture a passing body that was going at precisely the right speed to start orbiting the Earth, rather than colliding or simply escaping. If they had formed together, the Moon's iron core would probably be larger. This is also the case with the spinning theory. The giant impact hypothesis is supported by the fact that the Moon's composition is fairly similar to the Earth's crust. A more recent hypothesis is that there was a three-body collision, out of which the Earth and Moon formed together.

34) The temperature variation on the Moon is huge. At "night" - and a night lasts 2 weeks, since the Moon rotates at the same rate as it orbits the Earth - the temperature falls to -173 ºC, while by daytime at the equator, it rises to 127ºC.

35) There is a place near the Moon's south pole which is the coldest known place in the Solar System! There are very deep craters where sunlight never reaches the bottom. These have been measured to be -240ºC, 33ºC above "absolute zero" and 10ºC colder than Pluto. Of course, there might be similar permanently shadowed and even colder places in the Solar System just waiting for us to find them . . .

36) The Apollo astronauts brought back 2,415 separate samples of lunar rock, weighing a total of 380kg. You can go and see a piece of it at the Science Museum in London.

37) Nobody owns any land on the Moon. This has been decided in a treaty which specifically prohibits any country's sovereignity or its use for military purposes. So if someone tries to sell you a plot of land on the Moon (or to name a star after your loved one, or what have you), they are defrauding you - no matter how fancy the certificate you receive.

38) You can help explore the far side of the Moon and classify its craters with citizen science. Go to www.moonzoo.org.uk.

Thank you Graham Bowes for this ghostly galleon too.