How Much Closer To The Sun Does The Moon Travel?

Does the Moon get closer to the Sun than the Earth?
Photographed by an Expedition 28 crew member onboard the International Space Station, this image shows the moon at center, with the limb of Earth near the bottom transitioning into the orange-colored troposphere, the lowest and most dense portion of the Earth's atmosphere. Image credit: NASA

Photographed by an Expedition 28 crew member onboard the International Space Station, this image shows the moon at center, with the limb of Earth near the bottom transitioning into the orange-colored troposphere, the lowest and most dense portion of the Earth's atmosphere. Image credit: NASA

Originally posted on Forbes!

 

In order to tackle this question, we have to understand a bit about the geometry of the solar system, and how both the planets and the moons which orbit those planets behave.

Most of the planets in the solar system circle the Sun in a very thin plane, meaning that if you drew the orbits out on a sheet of paper, you wouldn’t be missing out on any hidden geometry of our solar system. With the exception of Pluto, there’s very little vertical motion in the solar system that would be obscured by drawing it out on flat paper.

The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.Image: NASA. Orbital lines: wikimedia user ZYjacklin. Public domain.

The orbit of Triton (red) is opposite in direction and tilted −23° compared to a typical moon's orbit (green) in the plane of Neptune's equator.Image: NASA. Orbital lines: wikimedia user ZYjacklin. Public domain.

Moons are under no particular obligation to follow this pattern, and it’s often thought that if a moon of a particular planet is doing something particularly odd with its orbit, we can use that information to guess that it might have arrived at that planet in an unusual way, rather than forming around that planet. Neptune’s moon Triton is a good example; not only is it angled quite sharply with regards to the plane of the solar system, but it also goes “backwards” - it orbits in the opposite direction of Neptune’s rotation. These have been taken as hints that Triton didn’t form around Neptune, but formed elsewhere, and got trapped around Neptune after being jostled too near to Neptune's gravitational well.

If the Moon happened to orbit in a circle the way a hula hoop rolled on its edge moves, forever tumbling along the direction of the Earth’s travel, then the Moon would never get any closer to the Sun at any point in its orbit. These sorts of orbits aren’t impossible, though in our solar system, they're non-standard.

Our Moon's orbit is, in fact, quite close to perfectly flat with respect to the direction that the Earth travels. It’s tilted by only five degrees relative to Earth’s orbit around the sun. If your arms, like mine, are about six feet from fingertip to fingertip, five degrees is about 3 inches away from horizontal. If you hold both arms out sideways, point one index finger up, and one index finger down, the tips of your fingers are about five degrees offset from the line drawn by your arms.

Earth–Moon system (schematic). Image credit: NASA, arrangement by wikimedia user brews_ohare. Public domain.

Earth–Moon system (schematic). Image credit: NASA, arrangement by wikimedia user brews_ohare. Public domain.

Five degrees of an offset means that the distance between the Moon and the Sun will vary almost exactly by the distance between the Earth and the Moon. Everything is moving in the same plane, so drawing it out on a sheet of paper won't be ignoring much geometry. The Moon’s orbit is also quite close to circular, which again helps with this - there’s no long, comet-like orbit for our Moon, which is why we see it as very nearly the same size in our skies. So with all that behind us, how close could the Moon get?

The Moon orbits the Earth at a distance of about 238,900 miles from our home planet. The Earth, in its turn, orbits the Sun once every year (by definition), at a distance of about 93 million miles from the Sun. Because we know the distance between the Sun and the Earth, and the distance to the Moon from the Earth, if we line everything up just right, then we can place the Moon directly in between the Earth and the Sun. We know this situation happens - this is how we get solar eclipses, when the Moon lines up exactly between the Earth and the Sun.

Annular eclipse. Taken from a 8" Reflector with a solar filter. Image credit: wikimedia user Smrgeog, CC BY SA 3.0

Annular eclipse. Taken from a 8" Reflector with a solar filter. Image credit: wikimedia user Smrgeog, CC BY SA 3.0

This configuration subtracts 238,800 miles off of the 93 million miles which separate the Earth from the Sun. So in the end, even though we have a pretty ideal setup, the Moon can’t ever get that much closer to the Sun. At best, the Moon manages to get a grand total of 0.25% closer to the Sun than the Earth.

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What Does The Sun Sound Like?

What sound does the Sun make, and would it be musical if we could hear it?
This montage of 366 images shows our Sun through the eyes of ESA’s Proba-2 satellite, as seen each day in 2016. The satellite’s SWAP camera works at extreme ultraviolet wavelengths to capture the hot turbulent atmosphere of the Sun, known as the solar corona. Each image was created from 30 separate images centred on 01:00 GMT each day, which were processed to enhance the features extending from the solar disc. Throughout 2016 the Sun’s 11 year activity cycle continued towards its minimum, a period when the number of sunspots, active regions, solar flares and eruptions diminish. Nonetheless, the most active region of last year can be seen in the 17 July image. The bright region close to the centre of the Sun produced eight of the 20 most powerful flares witnessed last year. Other prominent features are coronal holes – darker regions indicating lower levels of emission. However, coronal holes can produce streams of fast solar wind that can trigger geomagnetic storms on Earth. One of the largest holes observed last year can be seen towards the north of the Sun on 24 November, and was present for several solar rotations. Image credit: ESA/Royal Observatory of Belgium

This montage of 366 images shows our Sun through the eyes of ESA’s Proba-2 satellite, as seen each day in 2016. The satellite’s SWAP camera works at extreme ultraviolet wavelengths to capture the hot turbulent atmosphere of the Sun, known as the solar corona. Each image was created from 30 separate images centred on 01:00 GMT each day, which were processed to enhance the features extending from the solar disc. Throughout 2016 the Sun’s 11 year activity cycle continued towards its minimum, a period when the number of sunspots, active regions, solar flares and eruptions diminish. Nonetheless, the most active region of last year can be seen in the 17 July image. The bright region close to the centre of the Sun produced eight of the 20 most powerful flares witnessed last year. Other prominent features are coronal holes – darker regions indicating lower levels of emission. However, coronal holes can produce streams of fast solar wind that can trigger geomagnetic storms on Earth. One of the largest holes observed last year can be seen towards the north of the Sun on 24 November, and was present for several solar rotations. Image credit: ESA/Royal Observatory of Belgium

Originally posted on Forbes!

Sound is a tricky thing in space. Sound is a pressure wave, an oscillation in the density of air or water, which moves through the air or through water until it reaches something it can rattle. If that sound is reaching a human ear, and if the oscillation is within the range of frequencies we are sensitive to, it will be heard. Our Earth produces a number of these pressure waves, from the sound of a person next to you speaking, or the crash of a wave on a beach, or a sonic boom of an airplane above you. However, there are plenty of sounds produced which we are outside our range of hearing - with an instrument tuned to receive those pressure waves, we can prove their presence, but it would be impossible to play back and hear it without speeding up the recording.

In space, we have a major problem with recording sounds; there’s no atmosphere for sound waves to travel through, so any pressure waves an object may be producing will be instantly silenced without a medium to compress. However, if you’re clever about it, there are other ways of recording information which can be translated into a sound; the easiest one is vibrations. The ‘crunch’ of Philae, Rosetta’s lander on the comet 67P, hitting the surface of the comet made the rounds - but this noise is not, in fact, the result of a microphone on the landerThis noise is a translation of the vibrations of the feet of the lander at the moment when it hit the surface of the comet.

Rosetta’s lander Philae has been identified in OSIRIS narrow-angle camera images taken on 2 September 2016 from a distance of 2.7 km. The image scale is about 5 cm/pixel. Philae’s 1 m-wide body and two of its three legs can be seen extended from the body. The images also provide proof of Philae’s orientation. A Rosetta Navigation Camera image taken on 16 April 2015 is shown at top right for context, with the approximate location of Philae on the small lobe of Comet Churyumov-Gerasimenko marked. Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam, CC BY-SA IGO 3.0

Rosetta’s lander Philae has been identified in OSIRIS narrow-angle camera images taken on 2 September 2016 from a distance of 2.7 km. The image scale is about 5 cm/pixel. Philae’s 1 m-wide body and two of its three legs can be seen extended from the body. The images also provide proof of Philae’s orientation. A Rosetta Navigation Camera image taken on 16 April 2015 is shown at top right for context, with the approximate location of Philae on the small lobe of Comet Churyumov-Gerasimenko marked. Main image and lander inset: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; context: ESA/Rosetta/NavCam, CC BY-SA IGO 3.0

However, if you want to translate a data set into sound, you’re not limited to just dealing with vibrations. You can turn pretty much anything into a set of tones, if you’re creative enough. Sonification is a booming area of data manipulation -- it’s another face of the data visualization scene; instead of presenting the information visually, you can code it audibly, and listen to it over time. You simply have to decide what you want the pitch of the musical note to correspond to, what you want the timing between each note to correspond to, and what you want the volume to correspond to.

For data coming from a spacecraft which monitors the Sun, there is often a new image every hour and a half or so. In this case, the pacing between notes is easily given to the time between observations, which will form a regular cadence. However, extracting a volume and pitch out of the data will depend very much on exactly what part of the data you’re interested in reflecting.

For Rosetta’s “singing comet” sonification, there was a very low frequency oscillation in the magnetic field surrounding the comet, measured by Rosetta. The decision here was to use the frequency of that vibration in the magnetic field as the pitch, but sped up by a factor of 10,000, so that it could register in the human ear. The volume here is driven by how large the oscillations were, much as it would be for a sound wave on Earth.

Single frame enhanced NavCam image taken on 27 March 2016, when Rosetta was 329 km from the nucleus of Comet 67P/Churyumov-Gerasimenko. The scale is 28 m/pixel and the image measures 28.7 km across. Image credit: ESA/Rosetta/NavCam –  CC BY-SA IGO 3.0

Single frame enhanced NavCam image taken on 27 March 2016, when Rosetta was 329 km from the nucleus of Comet 67P/Churyumov-Gerasimenko. The scale is 28 m/pixel and the image measures 28.7 km across. Image credit: ESA/Rosetta/NavCam – CC BY-SA IGO 3.0

For the Sun, not only do we have to speed up the oscillation, but we also have to choose which observed oscillation we want to convert into a sound. The most common choice that I found was to examine the roiling surface of the Sun, which resembles nothing more than a pot of water at high boil. You could imagine examining how high the bubbles rise above the surface, and how quickly they do so. If we convert this amplitude and rapidity into a volume and a tone, we can get a musical note out for every bubble that rises to the surface. This is only one of many possible optionsfor sonifying the Sun, but it seems to be one of the more common choices.

What do we hear? Here’s a few examples.

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Do we really only have 1 billion years before the Sun kills us?

I heard that even though the sun won’t explode for 5 billion years, we’ve only got about a billion years before it kills us. Is that true?


It’s true that we won’t have to wait for the Sun to become a red giant so large that it engulfs our planet for the Earth to be uninhabitable. The exact timing of when the Earth becomes uninhabitable depends slightly on the model used to predict these things, but generally? About a billion years, give or take, is all we have before our oceans boil as the sun grows brighter.

The Sun is currently classified as a “main sequence” star; this means that it is in the most stable part of its life, converting the hydrogen present in its core into helium. For a star the size of ours, this phase lasts a little over 8 billion years. Our solar system is just over 4.5 billion years old, so the Sun is slightly more than halfway through its stable lifetime.

After the 8 billion years of happily burning hydrogen into helium are over, the Sun’s life gets a little more interesting. Things change because the Sun will have run out of hydrogen in its core - all that’s left is the helium, but it’s not hot or dense enough in the core to burn helium. Gravity and pressure are at constant odds with each other inside a star, with the outward pressure produced as part of fusing elements together. When the star has nothing left in the core to burn, however, gravity wins the fight. Eventually, gravity will compress the center of the star to such a degree that it can start burning hydrogen in a small shell around the dead core, still full of helium. As soon as the Sun begins to burn more hydrogen, it would be considered a red giant.

The process of compression in the center allows the outer regions of the star to expand outwards, and the burning of hydrogen in the shell around the core produces a lot more light than the Sun did earlier in life. Because the size of the star has expanded, the surface cools down, and goes from white-hot to “only” red-hot. Because the star is brighter, redder and physically larger than before, we dubbed these stars “red giants”.

It’s widely understood that the Earth as a planet will not survive the Sun’s expansion into a full-blown red giant star. The surface of the sun will probably reach the current orbit of Mars, and while the Earth’s orbit may also have expanded outwards slightly, it won’t be enough to save it from being dragged into the surface of the sun, whereupon our planet will rapidly disintegrate.

However, life on the planet will run into trouble well before the planet itself disintegrates. Even before the Sun finishes burning hydrogen, the Sun is evolving. The sun has been increasing its brightness by about 10% for every billion years it’s spent burning hydrogen on the main sequence. Increased brightness means an increase in the amount of heat our planet receives. As the planet heats up, the water on the surface of our planet will begin to evaporate.

An increase of the Sun’s luminosity by 10% over the current level doesn’t sound like a whole lot, but this small change in our star’s brightness will be pretty catastrophic for our planet. This change is a sufficient increase in energy to change the location of the habitable zone around our star. The habitable zone is defined as the range of distances away from any given star where liquid water can be stable on the surface of a planet. With a 10% increase of brightness from our star, the Earth will no longer be within that zone. This will mark the beginning of the evaporation of our oceans. By the time the sun stops burning hydrogen in its core, Mars will be in the habitable zone, and the earth will be much too hot to maintain water on its surface.

This 10% increase in the Sun’s brightness, triggering the evaporation of our oceans, will occur over the next billion years or so. Predictions of exactly how rapidly this process will unfold depend on who you talk to. Most models suggest that as the oceans evaporate, more and more water will be present in the atmosphere instead of on the surface. This will act as a greenhouse gas, trapping even more heat, and causing more and more of the oceans to evaporate, until the ground is mostly dry and the atmosphere holds the water, but at an extremely high temperature. As the atmosphere saturates with water, the water held in the highest parts of our atmosphere will be bombarded by high energy light from the Sun, which will split apart the molecules, and allowing water to escape as hydrogen and oxygen, eventually bleeding the Earth dry of water.

Where the models differ is on the speed with which the earth reaches this point of no return. Some suggest that the Earth will become inhospitable before the 1 billion year mark, since the interactions between the heating planet and the rocks, oceans, and plate tectonics will dry out the planet even faster. Others suggest that life may be able to hold on a little longer than 1 billion years, due to the different requirements of different life forms, and periodic releases of critical chemicals by plate tectonics. The Earth is a complex system, and quite hard to model, but it seems that as a rough guide, 1 billion years is a reasonably robust guess of how long life has left on our planet.

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Does the sun cast shadows on itself?

You can try this one at home. Get an incandescent light bulb, switch it on, and try to use the light from the bulb to cast a shadow on the bulb.

The problem you’ll run into is that every point on the surface of the light bulb is actively producing light, and in order to form a shadow, you need the surface to be dark, blocked from the light. It’s easy for a bulb or a star to cast shadows outwards; simply place a solid object at some distance from the light. A piece of paper or your hand will suffice for a light bulb; something on the opposite side of your hand will be deprived of the light from the bulb.

The shadow casters around stars tend to be planets and moons. Solar eclipses are the most dramatic proof of this. If you stick the Moon in the way of the Sun, some parts of our planet will be placed in the shadow of the moon. Lunar eclipses, show us the colour of the Moon when it travels through the shadow of the Earth. Like shading your eyes from the sun with your hand (yet another way of forming a shadow around our star), the Earth blocks the light travelling towards the Moon, plunging it into darkness.

Because every point on the Sun’s surface is glowing with light, there’s no way to deprive it of light by casting a shadow. Even if you put another source of light near the surface of the sun (for instance, a glowing filament as shown above), it doesn’t change the fact that the main surface of the sun is still glowing. Any attempt to put an object between the filament and the surface of the sun in order to cast a shadow would simply result in both sides of the object being illuminated! And very likely, said object would subsequent vaporize, given the 5500 degree Celsius temperatures near the surface of the sun.

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Is there a 'best time' to see the Northern Lights in Iceland?

Is there a ‘best time’ to see the northern lights in Iceland?

There are only a couple things you need to see the northern lights - dark skies, and an active sun.

The Sun, in periods of high activity, frequently spits out large volumes of very energetic particles. If these particles are aimed towards the earth (or more precisely, where the Earth will be in a couple of days) then our Earth’s magnetosphere - which is a magnetic shield against precisely this kind of thing - and these particles must collide with each other. Most of these particles will be redirected around the Earth - their paths will simply be bent by the magnetic field to move them away from the planet. However, particles that come close to the top of the planet can get caught. Near the magnetic poles of our planet, the magnetic lines that normally run parallel to the surface turn and sink into the surface. This creates a divot in the magnetic shield, and particles can get stuck in there, like bits of leaves in an eddy. The solar particles then run into bits of our atmosphere, and make it glow, lighting up the skies with the aurora.

So how do you optimize your chances of seeing the aurora? Since it’s a glow, it’s always easier to see when it’s quite dark. Now that the Northern Hemisphere is entering fall, the nights will begin to lengthen, which also lengthens the amount of time they might be visible.

The other thing that helps is being near to the magnetic poles - the closer you are, the weaker the aurora that will still be visible. Similarly, bright storms will look even brighter if you’re further north. It takes a very strong storm to be visible further south than the 45th parallel, but in Iceland you’re very close to the Arctic Circle, so your chances (geographically speaking) are pretty good.

The last ingredient is an active sun! We are currently in solar maximum. This means that the sun is as active as it gets right now. The sun goes through solar maximum every 11 years, after which point the magnetic field of the sun flips, resets, and the sun goes quiet for a while. The sun will gradually become more active again over the following 11 years.

So your best chance to see the aurora in iceland is right about now! If you keep an eye on a site like SpaceWeather, or if you see something in the news about a solar storm or a coronal mass ejection headed our way, that’s the best time to head outside and watch for the Northern Lights.

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