Is the far side of the Moon dark?

This picture of the Earth and Moon in a single frame was taken by the Galileo spacecraft from about 3.9 million miles away. Antarctica is visible through clouds (bottom). The Moon's far side is seen.  Image credit: NASA

This picture of the Earth and Moon in a single frame was taken by the Galileo spacecraft from about 3.9 million miles away. Antarctica is visible through clouds (bottom). The Moon's far side is seen. Image credit: NASA

Only some of the time! With the exception of the times when the Moon wanders into the shadow of the Earth, the Moon spends its journey around the Earth with half its surface in sunlight, and half its surface in darkness. The far side is harder to watch directly, though, because all of us humans are on the surface of the Earth, which only ever sees the near side. We can observe the far side thanks to the technological advancements that come with sending spacecraft out beyond the Moon, but few human eyeballs have seen the far side of the Moon directly.

Even without going there, we can figure out what should be happening on the far side of the Moon by looking at what isn’t happening on the near side of the Moon. If half the sphere of the Moon is illuminated, and we here on Earth are looking at a full Moon, then the far side of the Moon must be dark. But a full Moon doesn’t last very long- the next night the Moon will begin to look less circular in the sky, until a few days later, you’ll definitely be able to tell that the surface of the Moon facing us is not entirely illuminated.

The fully illuminated far side of the Moon, as seen by the DSCOVR spacecraft's EPIC camera. From the Earth, this would be a New Moon. Image credit: NASA

The fully illuminated far side of the Moon, as seen by the DSCOVR spacecraft's EPIC camera. From the Earth, this would be a New Moon. Image credit: NASA

The rest of that sunlight isn’t missing; it’s illuminating the side of the Moon that’s not facing us. As the month progresses, more and more of the far side of the Moon will be in sunlight, and less and and less of that sunlight will be visible to us on Earth. When we on Earth see a thin crescent Moon, the far side of the Moon is almost totally illuminated.

There are some permanently dark places on the Moon, but the far side of the Moon isn’t where you find them. They’re near the poles of the Moon - craters that are so deep, and the sunlight that reaches them is at such a shallow angle, that the light from our Sun only ever skims the surfaces of them. These are interesting places because they are so dark and cold - they’re one of the places that water seems to exist on the surface of the Moon.

An animation of the phases of the Moon. Libration, the minor wobble of the Moon that lets us see slightly more than 50% of its surface is also apparent. Image credit: public domain

An animation of the phases of the Moon. Libration, the minor wobble of the Moon that lets us see slightly more than 50% of its surface is also apparent. Image credit: public domain

With the exception of these deeply shadowed craters, the rest of the surface of the Moon spends about half its time in the sun, and half in the shade. What’s fun is that these periods of sun and shade each last about two weeks.

This is easiest to think about with the near side of the Moon; imagine some point (you can pick your favorite) on the surface of the Moon. As an example, let’s pick the very center of the near side. When the Moon is dark from our perspective, so is our test point in the middle of the near side. As the Moon progresses through crescent phases, our point in the middle is still dark! That part of the Moon is still in its nighttime period. When half the Moon is illuminated, our point on the Moon is dealing with a sunrise, as it’s right on the boundaries of the daytime and nighttime. From there, the gibbous phase, the full Moon, and right onwards through to the next quarter (where the other half is lit), our central point of the Moon stays in sunlight. If we ever have a human outpost on the Moon, this two weeks of daylight followed by two weeks of night will be something to contend with - though I’m sure folks who have lived in the arctic or antarctic (where night can last several months in winter) can give our explorers some pointers.


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How long would it take to deflate the Earth's atmosphere out into space?

My roommate and I were in a heated debate that lead us to read your post about the ability to survive the end of Portal 2. However, our question is slightly different. Suppose the same kind of portal was created on Earth’s surface to the Moon’s, how long would it take for the Earth’s air supply to be released through the portal into space?
At higher and higher altitudes, the Earth's atmosphere becomes so thin that it essentially ceases to exist. Gradually, the atmospheric halo fades into the blackness of space. This astronaut photograph captured on July 20, 2006, shows a nearly translucent moon emerging from behind the halo. Image credit:  NASA

At higher and higher altitudes, the Earth's atmosphere becomes so thin that it essentially ceases to exist. Gradually, the atmospheric halo fades into the blackness of space. This astronaut photograph captured on July 20, 2006, shows a nearly translucent moon emerging from behind the halo. Image credit: NASA

If any of you haven’t seen the previous Portal 2 post, I’d recommend having a look at it here, because I’m going to pull some numbers from it. I’m also going to make some slightly unphysical assumptions, but the results of those assumptions is that we’re going to calculate a lower limit to the amount of time it would take to bleed the atmosphere dry. In a world where portals actually worked, it would almost definitely take longer, for reasons we’ll go over later.

Our scenario is thus: we have opened a portal between the surface of the Earth and the Moon, as in the end of Portal 2. Effectively, we’re opening a window between the surface of the Earth and a pretty hard vacuum. The dramatic pressure difference here produces a tremendous, faster than the speed of sound, wind, as we worked out in that previous post. Presumably, if you left that portal open for a long time, you would reduce the amount of atmosphere left on the Earth. In the game, this portal is only open for about 30 seconds, but what if we left it permanently open?

The first thing I’m going to assume is that the whole atmosphere of the Earth is entirely at the same pressure (which it is not). Down at the surface where we humans live, the atmosphere is pretty compressed, and so we have an ambient atmospheric pressure of 1 atmosphere. (Yep. That’s the unit.) 1 atmosphere is equivalent to about 14.7 pounds per square inch, or psi. However, the further up away from the surface you go, the more diffuse the atmosphere gets, and both the density of atoms and the atmospheric pressure drops. If the density of the atmosphere drops, the wind speed through our window will also drop, because it’s the difference in pressure on the two sides of our window that drives the wind speed. By assuming that I can compress down the upper layers of the atmosphere so that the air on Earth is at a constant 14.7 psi, then the wind speed will stay at its fastest, and bleed the atmosphere out into Moon space as fast as possible.

A setting, waning crescent moon amid the thin line of Earth's atmosphere. Image credit:  NASA

A setting, waning crescent moon amid the thin line of Earth's atmosphere. Image credit: NASA

If you compress the atmosphere down, it would fit in a sphere 1999 km across, which then has a volume of 4.19 x 10^18 cubic meters. This...is a big number. How fast can we drop it to zero?

I will have a reasonable guess that the portal itself is about five feet tall by three feet wide - it seems a bit shorter than Chell in game, and wide enough for her to fit through. If we assume that it’s rectangular instead of an oval, the math is nicer, so I’m going to square up the portal dimensions at about 1.5 meters high by 1 meter wide. This gives a portal area of 1.5 square meters. This is key, because with the area of the window, and the wind speed, we can figure out the volume of air lost every second. At 411 meters per second, our speed from the older post, that means that after one second, a bit of air will have traveled 411 meters.

Every second, we’re going to lose about 617 cubic meters of high pressure Earth atmosphere into the space surrounding the Moon. We know how much we have to lose, so from here we can sort out how many seconds it would take to get the total volume of the Earth’s atmosphere out through our portal. As you can probably guess by the 18 zeros following the total volume of the Earth’s atmosphere, it’s going to be a lot of seconds.  In fact, it’s so many seconds that seconds are not a useful unit even a little bit. Converting into years is a little better.

It would take 215 million years.

Most ISS images are nadir, in which the center point of the image is directly beneath the lens of the camera, but this one is not. This highly oblique image of northwestern African captures the curvature of the Earth and shows its atmosphere. Image credit:  NASA/JPL/UCSD/JSC

Most ISS images are nadir, in which the center point of the image is directly beneath the lens of the camera, but this one is not. This highly oblique image of northwestern African captures the curvature of the Earth and shows its atmosphere. Image credit: NASA/JPL/UCSD/JSC

And remember, this is assuming that the wind speed stays the same the whole time, which it would not in real life. The other thing we’re assuming is that none of this gas will hang around the moon and increase the atmospheric pressure around the Moon. That would also start to balance out the pressure difference, slowing the wind speed down and making this take even longer. The moon historically is not very good at holding onto an atmosphere, so this would likely be a temporary arrangement, but millions of years is not very long for astronomical things, and it’s possible the lunar atmosphere could hang around long enough to slow down our wind. The estimates for the atmosphere around the young moon is that it would have stuck around for 70 million years or so - shorter than our fueling time, but long enough that we could expect it to hang around for a while, before we’re able to finish emptying the Earth’s atmosphere into outer space.

In reality, there would likely be an equilibrium point reached, where both the Moon’s newfound atmosphere and the Earth’s freshly drained atmosphere would find themselves at the same pressure, and the wind, having gradually slowed, would come to a stop, with only the vaguest breeze from the Earthward side as the Sun gradually stripped the atmosphere from around the Moon.


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Where is the Moon's water?

If there is water on the Moon, will it be on the surface or will it be within the ground?
This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

This is a spectacular high-Sun view of the Mare Tranquillitatis pit crater revealing boulders on an otherwise smooth floor. This image from LRO's NAC is 400 meters (1,312 feet) wide, north is up. Image credit: NASA/Goddard/Arizona State University

There is water on the moon! As we’ve outlined before, it’s somewhat tricky to keep water on the surface of the moon, because the combination of heat and particles from the Sun, a lack of an atmosphere, and no magnetic field means that it’s pretty hard to keep water that’s exposed to sunlight from evaporating away into space.

What that means is that if you want to have water persist anywhere on the Moon, it has to be sheltered from the Sun somehow. An easy way for this to happen is at the poles, where some craters are deep enough that the Sun’s rays never reach into the bottom of the crater. Places like this are called “cold traps”, because it can trap material in a solid form that would otherwise escape if it weren’t so cold.

Near the south pole of the Moon in particular, we found frost in some of these deep dark places. This frost makes the surface more reflective than it would be if there were only rock sitting around in those craters - so the coldest places also wind up being more reflective if you’re bouncing light off of ice. This particular study is careful to note that we’re not seeing frozen pond-style pools of water, but more like the frost that builds on the outer edges of leaves in fall.

But craters aren’t the only places that water ice could hide - we have a sneaking suspicion that the Moon also has tunnels woven under its surface. The Moon had a surprisingly long era of volcanic activity in its younger years, and where there are lava flows, you can wind up with lava tunnels. We are pretty sure that the moon has these. We see them most easily as they collapse, because then you get a snake-like pattern of collapsed ground, twisting its way across the surface as a series of giant potholes.

These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

These images from NASA's LRO spacecraft show all of the known mare pits and highland pits. Each image is 222 meters (about 728 feet) wide. Image credits: NASA/GSFC/Arizona State University

Every so often, there’s a more isolated cave-in, giving us a glimpse into a sublunar cavern - a deep shadow cast into the depths catches the eye and the imagination. If water had accumulated in these hidden tunnels, they would also be relatively protected from evaporation. However, it’s one thing to have a plausible place for water, and another to find it for sure in those places. Lava tunnels are an extremely appealing place for water, though - because they’re also an appealing place to put a human base on the Moon. While we don’t have to worry about humans evaporating, any shelter from intense heat and cold helps us as well. So if there were also water down there, they’d be a great place to put an inhabited base.

You can definitely also wind up with watery molecules bound up in the rocks themselves. A recent study suggests that instead of having lots of water ice hanging around, the Moon may have a lot of hydroxyl, which is one hydrogen and one oxygen bound to each other, rather than the two hydrogens and one oxygen that make up your standard water molecule. Hydroxyl binds easily to other things, so it can wind up binding itself to minerals in the earth - you can extract it and create water, but it’s more energy intensive than just having water lying around.

So the true answer is that there’s going to be a mixture of places where water will be found - on the surface in sheltered places, possibly in underground tunnels, and some not-quite water bound up in minerals!


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What Would We See If The Moon Rotated Every 24 Hours?

If the Moon rotated as fast as the Earth, would we only see one side of the Moon?
Framed by the Earth's horizon and airglow, the full moon floats in the blackness of space in this photo from the Expedition 10 crew on board the International Space Station. Image credit: NASA

Framed by the Earth's horizon and airglow, the full moon floats in the blackness of space in this photo from the Expedition 10 crew on board the International Space Station. Image credit: NASA

Originally posted on Forbes!

The Earth rotates around its own axis once every twenty-four hours. The Moon, on the other hand, rotates once around its own axis every 28 days, and once around the Earth in that same 28 days. The end result of this combination is that the same side of the Moon is always facing the Earth. As the Moon moves to be directly above a different portion of the earth, its face also turns at exactly the same rate, so that only one hemisphere of the Moon is ever visible from our home here.

If the Moon turned at any other rate (either faster or slower), we would eventually see all sides of the Moon, and what is currently the lunar far side would be a much more familiar sight to us. If we spun up the Moon to one rotation every 24 hours, how dramatic would this be?

I’m assuming that we’re not changing the Moon’s orbit here - so that the Moon would still orbit the Earth once every 28 days. This means that the rising and setting of the moon would happen in the same way as they do now - slightly later every day, and the phases of the moon would remain the same, because the phases are simply the combination of the Moon’s location in its orbit around the Earth, and what fraction of the near side of the Moon is illuminated by the Sun. So we would still have a new moon and a full moon about once per month. What would certainly change is which portion of the moon is illuminated.

Speeding up the Moon’s rotation so that it spins once every 24 hours is a pretty dramatic change. That means the Moon has to rotate the full 360 degrees of a circle in 24 hours, which puts us at 15 degrees of rotation every hour. That may not sound like a lot, but over the course of an evening, which we’ll say is an average of 12 hours (half of our Earth’s 24), that means that the Moon has rotated by 180 degrees. A full moon could rise with the familiar near side facing us, and by the time it sets, 12 hours later, we’d be looking at the unfamiliar jagged territory of the lunar highlands - what is currently the lunar far side. In a six hour period, you’d expect the Moon to rotate by 90 degrees. If you were in a half-moon phase, where only half of the Moon’s face is illuminated, you would expect that illuminated portion to change completely, twice over, every time the Moon rose above the horizon.

However, if the Moon truly did rotate once every 24 hours, the two sides would probably look much more similar to each other than they do now. Part of the intense cratering of the far side of the Moon is because it is constantly facing “outwards” towards space, and it’s an easier target for interplanetary fragments of rock to hit, than the somewhat protected, Earth-facing side. If the Moon rotated faster, these meteoroids would have a pretty even chance of hitting any face of the moon, and the cratering would probably be more evenly distributed.

It’s fun to think about how this kind of situation might have influenced our calendars - since our months are roughly based on the lunar cycle, perhaps we would have used the appearance or disappearance of certain features of the moon as a smaller unit of time. But we certainly wouldn’t have grown attached to one side of the Moon - what we see now as the near side would be just as normal to us as the far side.

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Why Does The Moon Look Upside Down From Australia?

Why does the Moon look upside down from Australia?
The Moon seen from the southern hemisphere, taken on the 25th of November 2012, from Montevideo, Uruguay. Image credit:  Fernando da Rosa , CC BY A-SA 3.0

The Moon seen from the southern hemisphere, taken on the 25th of November 2012, from Montevideo, Uruguay. Image credit: Fernando da Rosa, CC BY A-SA 3.0

Originally posted on Forbes!

Those of us who live in the Northern Hemisphere of our planet are used to a very specific view of the Moon, and, if you never travel outside of the Northern Hemisphere, journeying only to Europe, North America, Asia or the Arctic expanses, that view of the Moon would never change by very much.

However, once you move to the Southern Hemisphere, visiting South America, Africa, Australia or New Zealand, something will indeed seem off about the Moon. It’s upside down in the sky, relative to what you’d be used to in the Northern Hemisphere. Likewise, if you’re used to a Southern Hemisphere sky, moving to the Northern Hemisphere will turn the Moon upside down relative to what you’re used to.

Many of the portraits of the Moon are oriented in the way you’d see them from the Northern Hemisphere. There’s nothing fundamental about this orientation relative to the Southern Hemisphere orientation, but we’ve designated North as “up” for long enough that that convention has expanded outwards to the whole solar system. With that convention, it makes sense to display the Moon “right-side up,” with the view from the Northern half of the planet.

Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Image credit:  Gregory H. Revera, CC BY A-SA 3.0

Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Image credit: Gregory H. Revera, CC BY A-SA 3.0

Why does the Moon look upside down from Australia? It’s because we’re on a spherical planet. If I stand at the North Pole, with my head “up,” and have a friend stand on the South Pole, with their head “up,” relative to the ground, our two heads are pointed in exactly opposite directions. If we both look at the Moon, then I see a Moon with dark Mare stretching along the “top” of the Moon, and a bright region at the bottom. At the South Pole, to a person whose head is pointed in the other direction, the Mare go along the bottom edge of the Moon, with the brighter region stretching across the top. If I were to move between the North and South poles, I would watch the Moon appear to rotate in the sky, as my perspective of “up” changes with the curvature of the Earth.  If I, on the North Pole, wanted to replicate my South Pole friend’s view onto the sky, I should do a perfect handstand, mimicking manually what the curve of the Earth has done more naturally. Obviously, this method of replicating the South Pole’s view isn’t perfect, because things that are directly overhead on the South Pole are blocked from my view at the North Pole by the bulk of the Earth.

In a less extreme case, someone living at 45 degrees North of the Equator (exactly halfway between the North Pole and the Equator) and someone living at 45 degrees south of the Equator, (halfway between the South Pole and the Equator) both standing on the ground, have their heads both pointed “up” but at 90 degrees relative to each other. Since their North/South separation is still an up/down change, then if the two moongazers could swap places, they’d say the Moon had rotated by about 90 degrees. It’s exactly the same kind of perspective shift on the Moon that my friend and I, at the North and South poles, have when looking outward.

The great hunter Orion hangs above ESO’s Very Large Telescope (VLT), in this stunning, previously unseen, image. As the VLT is in the Southern Hemisphere, Orion is seen here head down, as if plunging towards the Chilean Atacama Desert.  Image credit : ESO/ Y. Beletsky  

The great hunter Orion hangs above ESO’s Very Large Telescope (VLT), in this stunning, previously unseen, image. As the VLT is in the Southern Hemisphere, Orion is seen here head down, as if plunging towards the Chilean Atacama Desert. Image credit: ESO/Y. Beletsky 

The Moon is probably the most dramatic example of this in the night sky, simply because we know it so well, but it’s not the only object that may appear odd in the Southern sky if you’re used to the Northern view. Constellations do the exact same thing. Some Northern constellations are not visible in the Southern skies, but Orion, one of the brightest and easiest-to-spot constellations in the Northern winter sky, is visible from both hemispheres. And just like the Moon’s change, Orion appears upside down, his head towards the ground instead of the rest of the stars overhead.

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