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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When we talk about the Universe's first second, what do we really mean?

In writings about the Big Bang, there are discussions of what happened in the first picosecond, billionth of a picosecond, etc., etc. My question is: what is the measure of time used by the writer? Our time as we experience here on Earth? The instantaneous time passage there, which would be influenced by the infinite concentration of mass and energy (a singularity?)? What is the time scale?
This illustration summarises the almost 14-billion-year long history of our Universe. Credit:  ESA – C. Carreau  

This illustration summarises the almost 14-billion-year long history of our Universe. Credit: ESA – C. Carreau 

This is a really fun question, because the answer is that these time points you’re seeing are for time as we experience it here on Earth, where we’re trying to use an objective ruler of time to describe how rapidly things were changing during those early moments of our Universe. All measurements of time are based on what we use here on Earth, where we humans first developed our timekeeping methods. The second is now a unit of measure used for all sorts of things, though pretty rarely in extragalactic astronomy (with a few exciting exceptions like events that trigger gravitational waves) because the distances involved often mean things happen on billion year timescales. 

But when we’re talking about the very beginning times our Universe went through, a lot of things did happen in the first second - the Universe underwent a lot of dramatic changes in that first second. It went from a soup of energy to filled with protons and neutrons in that time - a dramatic change! And when we say this, we really do mean the second that you could watch tick past on a watch. This second comes from taking the speed of our Earth’s rotation, and dividing it into twenty four hours, dividing each hour into sixty minutes, and each minute into sixty seconds. It’s that second, 1/86,400th of an Earth-spin, that we use to describe the initial changes of our Universe.

It’s fun to think that a fluke of angular momentum that gave us (approximately) a 24 hour day also gave us a useful metric for describing the early state of the Universe in precisely the units that we do. 

As time has wound on, we humans have sought to make our units of measure ever more precise. To do this, we often wind up redefining our units in terms of something more fundamental than where we had begun. The meter was redefined to be the distance that light travels in 1/299,792,458th of a second instead of “one ten-millionth of the distance from the equator to the North Pole”, and the kilogram was recently redefined to be a function of Planck’s constant, instead of a very specific, carefully guarded, lump of metal in a vault in Paris. The second has also undergone this transformation. 

Bell jar display of prototype kilogram replica,  public domain via National Institute of Standards and Technology  

Bell jar display of prototype kilogram replica, public domain via National Institute of Standards and Technology 

As we measured the Earth’s rotation to higher and higher precision, we encountered the need for leap seconds to account for the fact that our Earth’s rotation is intrinsically slowing by a tiny, but measurable amount.  Instead of using the Earth’s rotation speed, then, a more fundamental, reliably measurable feature of our Universe was adopted as the official definition of a second - the length of time it takes a cesium atom to vibrate between two hyperfine states 9,192,631,770 times. While this may seem like a much more complex unit of time, it’s actually a better definition in that anyone, anywhere in the universe, should be able to measure this unit of time consistently. 

During this redefinition of the second, the length of a second wasn’t changed, but now we have a more persistent method of measuring it. So that first nanosecond (10^-9) of the Universe is the same length of time it takes a cesium atom – in a vacuum, at absolute zero – to vibrate 9 times.


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Is there lightning on Mars?

Is there lightning on Mars? Would lightning strikes endanger astronauts on Mars? Would static electricity be a factor to consider on Mars?
An astronaut orbiting over Bolivia captured a close-up of a lightning flash beneath a thunderhead on January 9, 2011. Image credit:  NASA

An astronaut orbiting over Bolivia captured a close-up of a lightning flash beneath a thunderhead on January 9, 2011. Image credit: NASA

There is lightning on Mars! Or at least, something like lightning occurs on Mars. In 2009, the first detections of lightning strikes on Mars were recorded, confirming something that planetary scientists had suspected already - electricity should arc through the Martian skies.

We knew a fair amount about Mars’ weather patterns even before detecting lightning, from a combination of orbiting spacecraft and our landers on the surface. These outposts have painted a picture of a thin atmosphere frequently tumbled into large dust storms. Mars has huge annual storms which can envelop the entire planet, and other strong storms that pop up irregularly through the year. On top of that, the dust on Mars is extremely fine, so once you begin to swirl it around in a wind, it’s reasonable to guess that the dust particles will start to rub on each other, and as you do that, you’ll start to build up an electric charge.

This static charge does more than just gradually build towards lightning; it’s also part of why the Mars rovers get so dirty. The rovers are dealing with more than just a fine sifting of dust falling out of the atmosphere, which a light breeze might easily remove; that dust is stuck to them like packing peanuts stick to your hands. It takes a stronger breeze - a new storm, or a wandering dust devil - to remove some of that dust, and it’s something that the long-lived Spirit and Opportunity rovers were both able to make use of on a couple of occasions.

A self-portrait taken by NASA's Curiosity rover taken on Sol 2082 (June 15, 2018). A Martian dust storm has reduced sunlight and visibility at the rover's location in Gale Crater. Credit:  NASA/JPL-Caltech/MSSS

A self-portrait taken by NASA's Curiosity rover taken on Sol 2082 (June 15, 2018). A Martian dust storm has reduced sunlight and visibility at the rover's location in Gale Crater. Credit: NASA/JPL-Caltech/MSSS

However, as much as dust devils can help you out, they can also do the opposite, dumping more dust on top of your solar panels, which, for a solar powered craft, will limit the amount of energy you have available to do science with, and eventually drop the craft below the threshold of power it needs to operate. This is the current theory for what happened with both Spirit and Opportunity. The Curiosity rover is less affected by this particular issue since its power comes from radioactive decay, but Curiosity is still fully coated in the fine Martial soil. This dust is actually a concern for human exploration of Mars - it’s going to be hard to fully remove this dust from spacesuits, and breathing in a fine particulate is never good for your lungs.

The lightning itself is actually less likely to be a hazard to astronauts on the surface of Mars than the dust is; for one I would expect any humans on the surface of Mars to take shelter during these bigger storms. Unlike what was presented in The Martian, even the 60 mph winds that can occur during a dust storm wouldn’t feel as powerful as a similar wind on Earth, since the atmosphere is so much thinner. The air simply wouldn’t exert the same pressure against you in the same way. Even on Earth, the likelihood of being struck by lightning is very low, and on Mars the best guess is that the lightning would not really resemble the large bolts of lightning we see here on Earth.

More likely is that this lightning would resemble the arcing jolts of electricity you can create by shuffling along in socks on carpet and then touching a doorknob. In a dark room, you can see the filamentary discharge of electricity between your finger and the doorknob. On Mars, you might expect to see little flickers of electricity arcing between parts of the dust storm, faintly lighting up the night sky. To be a hazard to an astronaut or a rover, you’d have to be very, very unlucky.


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What would happen if the amount of light reaching the Earth from the Sun were cut in half?

What would happen if the amount of light reaching the earth from the sun were cut in half?
The position of the Habitable Zone, as a function of the mass of the star the planets orbit. Image credit:  Chester Harmon , CC A-SA 4.0

The position of the Habitable Zone, as a function of the mass of the star the planets orbit. Image credit: Chester Harmon, CC A-SA 4.0

We’ve tackled a very similar question to this before here at Astroquizzical; check out this post! In that post, we explored what would happen to the Earth if we could slice the Sun in half. And because cutting the Sun’s matter in half doesn’t translate to a slice in brightness of one half, it’s a pretty dramatic shift for our solar system.

However, if we don’t go quite as far with our solar slicing, but instead just drop the brightness of our sun by half, we’ve actually only removed 18% of the mass. This is still a relatively massive star, at 82% the mass of our Sun, but that’s enough to change the distance from the star where liquid water is stable.

As the mass and brightness of a star decreases, that zone of possible liquid water (usually known as the habitable zone) shrinks to a smaller and smaller shell around the star, but because we’re changing the star by a smaller amount this time compared to the earlier post, the habitable zone won’t shrink all the way down to Mercury’s orbit - it would sit closer to where Venus is now. The Earth’s orbit might still fall within the bounds of the habitable zone, but it’d be more in the position that Mars finds itself in now - much colder than Earth now, but able to sustain water under certain circumstances.

Our Sun won't be dropping in brightness anytime soon - on the contrast, as our Sun ages, it becomes slightly brighter, increasing in brightness by about 10 percent every billion years. As it does, the habitable zone around our star has been gradually expanding outwards, and at some point in the next billion years, the Earth will exit the habitable zone entirely.


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Is it possible to have a planet orbit two stars, like Tatooine?

How does that two sun thing work in Star Wars: A New Hope? Is that possible?
This artist's concept shows a hypothetical planet covered in water around the binary star system of Kepler-35A and B.  Image credit: NASA/JPL-Caltech

This artist's concept shows a hypothetical planet covered in water around the binary star system of Kepler-35A and B. Image credit: NASA/JPL-Caltech

It is possible, and we’ve actually found a number of planets orbiting double stars, like Luke’s homeworld in Star Wars does. However, outside of the Star Wars Universe, there are a lot of ways for this setup to go very wrong. So far, we haven’t found an enormous number of planets orbiting double stars, which seems to speak to how rare it is for a planet to survive in an environment like Tatooine’s.

At the beginning of Star Wars, Luke Skywalker lives on a planet with a double sunrise, on a planet which orbits two stars. We can presume that the two stars are orbiting each other, and that this planet then orbits around both stars. The technical term for two stars which orbit each other is a binary system, and the easiest way for the stars to find themselves in this situation is if they both form out of the same cloud of gas, at the same time. The remainders of that cloud of gas would hang around long enough to make planets to surround the pair of stars.

This artist's concept illustrates a tight pair of stars and a surrounding disk of dust -- most likely the shattered remains of planetary smashups. Image credit:  NASA/JPL-Caltech/Harvard-Smithsonian CfA

This artist's concept illustrates a tight pair of stars and a surrounding disk of dust -- most likely the shattered remains of planetary smashups. Image credit: NASA/JPL-Caltech/Harvard-Smithsonian CfA

If a planet were orbiting far enough away from the two stars, it wouldn’t really notice a difference between orbiting the double star set, and orbiting one star of their combined mass. However, by the time you get really far away from the stars, there’s not a tremendous amount of sunlight reaching the surface of your planet. If you want your world to be habitable (and a desert world still counts), you’ll have to be on a planet that’s a little closer to the stars, and this is where things start to get tricky.

If you are a planet, it’s nicest if the two stars orbit each other closely and circularly. This kind of setup for the stars means that you’re more or less always the same distance from the stars, which guarantees you a pretty consistent amount of light from the stars. If you’re trying to be a habitable world, this is important, because it keeps your surface temperature roughly consistent as well. You’d still have some variability, because the stars will still eclipse or partially eclipse each other periodically, which would lower the amount of light you’d get on the surface.

This artist's concept illustrates Kepler-47, the first transiting circumbinary system. Image credit:  NASA Ames/JPL-Caltech/T. Pyle

This artist's concept illustrates Kepler-47, the first transiting circumbinary system. Image credit: NASA Ames/JPL-Caltech/T. Pyle

However, if you are a star, close orbits are more complicated than wide ones. Wide orbits are easier to maintain, because the two stars have a weaker gravitational influence on each other. In a smaller orbit, the two stars will exert a reasonably strong tidal force on each other, and will change each other’s orbits over time. When the orbits of the stars begin to change around, the planets’ orbits also change, and you are in prime conditions for what’s called a three-body interaction.

The three body interaction happens when you have three objects orbiting each other in relatively close range. This could be three stars or two stars and a planet, and in either case, the lowest mass object can wind up getting flung suddenly out of the solar system entirely. The other outcome is for the planet to wind up crashing into one of the two stars - not a habitable outcome there, either. The three-body interaction is of particular concern for two stars and a planet, as this means that if your planet is close enough to the star to get caught up in one of these interactions, it won’t stay as a planet in the solar system for a particularly long time.  This might partially explain the relatively low number of circumbinary planets we’ve seen so far with Kepler - these planets are prone to either being ejected or consumed by their parent stars.

So it’s not impossible for a Tatooine-like planet to orbit a binary system, but given how rare they are in our solar system, everything has to be exactly so, or Tatooine will wind up on a one-way trip out of its solar system on a journey through its home galaxy.


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