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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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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How Do We Map The Earth’s Gravity?

Can Earth’s center of gravity be located? And if so, to what precision?
Satellite measurements offer scientists a new view of our planet. Warm colors (red, orange, yellow) represent areas with strong gravity. Cool colors (green, blue) represent areas with weak gravity. Image credit: NASA's Goddard Space Flight Center

Satellite measurements offer scientists a new view of our planet. Warm colors (red, orange, yellow) represent areas with strong gravity. Cool colors (green, blue) represent areas with weak gravity. Image credit: NASA's Goddard Space Flight Center

Originally posted on Forbes!

Earth’s center of gravity can be located! We talked a few months ago about measuring the force of gravity surrounding the Moon, and that the way we do this measurement is by having twin satellites, and calculating the difference in gravitational pull on each satellite. As the satellites go over high density regions, the one of them will feel an increased pull before the other, and the distance between the two satellites will change. These tiny changes in the distance between the two satellites allow us to map out the density of the ground below, but it's fundamentally a measure of the strength of the gravitational pull of the ground below the satellites.

We can do the exact same thing for pairs of satellites around the Earth, and we have! The Gravity Recovery and Climate Experiment (GRACE) is a NASA mission to do precisely this. It was a pair of satellites, launched in 2002, which bounced microwaves back and forth between them, very precisely measuring the distance between them, to a sensitivity of about a micron (many times smaller than the width of a human hair.) By additionally communicating with GPS satellites, the GRACE satellites were able to precisely communicate both their absolute positions in orbit around the Earth (to a precision of about a centimeter), and their motions relative to each other. Any deviations in their relative distances should be due to something down below, on Earth.

Artist's rendering of the twin satellites that will compose NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. Image credit: NASA/JPL-Caltech

Artist's rendering of the twin satellites that will compose NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. Image credit: NASA/JPL-Caltech

ESA has also had one of these earth-measuring satellites, called GOCE (Gravity field and steady-state Ocean Circulation Explorer) which operated between 2009 and 2013. Instead of having two independent satellites, it had two sets of accelerometers at opposite ends of one, long, tubelike satellite, which each measured gravity at their end of the satellite.

Both experiments were able to generate maps of the Earth’s gravitational field strength from their locations in orbit. In practice, these are often reported back as a “geoid”, which is a way of deforming the Earth’s sphere so that any point on its surface would have an equal gravitational strength. Anything built up into a lump outwards indicates that there’s extra mass there, and anything sunken inwards indicates that there’s less mass. GOCE managed to map the gravitational strength of the Earth beneath it to a precision of 10^–5 m/s2.  While we commonly quote the gravitational force as 9.81 m/s^2, this satellite was measuring it out to 0.00001.

ESA's GOCE mission has delivered the most accurate model of the 'geoid' ever produced, which will be used to further our understanding of how Earth works. The colours in the image represent deviations in height (–100 m to +100 m) from an ideal geoid. The blue shades represent low values and the reds/yellows represent high values. Image credit: ESA/HPF/DLR

ESA's GOCE mission has delivered the most accurate model of the 'geoid' ever produced, which will be used to further our understanding of how Earth works. The colours in the image represent deviations in height (–100 m to +100 m) from an ideal geoid. The blue shades represent low values and the reds/yellows represent high values. Image credit: ESA/HPF/DLR

You’ll notice that both of these experiments have another facet to their names - GOCE also says it’s monitoring the ocean circulations, and GRACE is also a climate experiment. That’s because these very precise gravitational measurements can also track the motion of water around our planet. Not just the locations of surface water, or the amount of water in the oceans versus at the poles, but underground water, in reservoirs. Water is a relatively dense material, and so its presence or absence in a certain location will alter the average density of the planet underneath either of these observatories.

GRACE has a follow-up mission, intended for launch this year - GRACE-FO. The FO stands for Follow-On, and is intended to increase the accuracy of the GRACE experiment dramatically, by using laser beams to check the distances between the satellites, instead of microwaves. GRACE-FO will also help us continually monitor our fragile world’s water supplies as the original GRACE satellites age. Not every satellite lasts 15 years, after all.

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Would You Float At The Core Of The Earth?

Thought experiment...if you built a bedroom sized room at the center of the Earth, and you are in that room, which way is down? Please explain to me why if you are surrounded by the same amount of mass in every direction, how does that NOT EQUAL NET ZERO? In other words, would that not be the exact same thing as weightlessness? So what would an illustration of the curvature of Space look like at the center of a massive body? Wouldn’t there be a vortex of some sort? It’s kind of important to me to understand where I’m going wrong?
A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's most recently launched Earth-observing satellite - Suomi NPP. This composite image uses a number of swaths of the Earth's surface taken on January 4, 2012. The NPP satellite was renamed 'Suomi NPP' on January 24, 2012 to honor the late Verner E. Suomi of the University of Wisconsin. Image Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring

A 'Blue Marble' image of the Earth taken from the VIIRS instrument aboard NASA's most recently launched Earth-observing satellite - Suomi NPP. This composite image uses a number of swaths of the Earth's surface taken on January 4, 2012. The NPP satellite was renamed 'Suomi NPP' on January 24, 2012 to honor the late Verner E. Suomi of the University of Wisconsin. Image Credit: NASA/NOAA/GSFC/Suomi NPP/VIIRS/Norman Kuring

Originally posted at Forbes!

You very nearly got there! Let’s run with your example of a small room at the center of the Earth, but for my sanity, I’m going to make your room a sphere instead of a square, because everything else involved in this example is going to be round, and it makes the example easier.

So; gravity pulls on any object with a force that’s related to the masses of the two objects involved, and inversely related to the distance between them. This tells us that the more massive the two objects, the greater the pull, and the greater the distance between them, the weaker the influence. For us, on the surface of the Earth, we can work out how strong gravity is. All you need is the mass of the Earth, the mass of a human, and the distance between the center of the Earth and the surface of the Earth.

The mass of the Earth in kilograms is 5.972 × 10^24. 10^24 is a septillion, which is a number so outrageously large that it might be more manageable to think about as a trillion trillion kilograms. (In SI units, which most physicists use, this is gives you the prefix yotta. 5972 yottagrams! It’s fun to say.)  The mass of a human, in comparison, is negligible. The radius of the Earth is 3,959 miles - 6,371 km. If you plug these numbers in, you pull out the gravitational acceleration at the surface of the Earth; 9.81 meters per second every second. This pulls you in towards the surface of the planet.

Photographed from a shuttle training aircraft, space shuttle Endeavour and its six-member STS-134 crew head toward Earth orbit and rendezvous with the International Space Station. Liftoff was at 8:56 a.m. (EDT) on May 16, 2011, from Launch Pad 39A at NASA's Kennedy Space Center. Onboard are NASA astronauts Mark Kelly, commander; Greg H. Johnson, pilot; Michael Fincke, Andrew Feustel, Greg Chamitoff and European Space Agency astronaut Roberto Vittori, all mission specialists. STS-134 will deliver the Alpha Magnetic Spectrometer-2 (AMS), Express Logistics Carrier-3, a high-pressure gas tank and additional spare parts for the Dextre robotic helper to the International Space Station. STS-134 is the final spaceflight for Endeavour. Image credit: NASA

Photographed from a shuttle training aircraft, space shuttle Endeavour and its six-member STS-134 crew head toward Earth orbit and rendezvous with the International Space Station. Liftoff was at 8:56 a.m. (EDT) on May 16, 2011, from Launch Pad 39A at NASA's Kennedy Space Center. Onboard are NASA astronauts Mark Kelly, commander; Greg H. Johnson, pilot; Michael Fincke, Andrew Feustel, Greg Chamitoff and European Space Agency astronaut Roberto Vittori, all mission specialists. STS-134 will deliver the Alpha Magnetic Spectrometer-2 (AMS), Express Logistics Carrier-3, a high-pressure gas tank and additional spare parts for the Dextre robotic helper to the International Space Station. STS-134 is the final spaceflight for Endeavour. Image credit: NASA

Now, we’ve done something sneaky here, which is to assume that we can place the entire mass of the Earth at the very center of the Earth, and consider ourselves simultaneously at the surface, and six thousand kilometers away from the Earth’s mass. We can do this because the Earth is a sphere, and that means that there's an awful lot of symmetry to work with. You can also do the math very carefully, considering the pull of the Earth’s mass to the left of you, which will pull you slightly to the left, and the pull of the Earth’s mass to the right of you, which pulls you equally strongly to the right. There’s no net force going sideways, because you’re standing on a symmetric planet, and all the left and right directions will cancel out. All that’s left is the 'downward' direction.

Again, if you do it carefully, you have to consider the gravitational pull from the ground directly beneath your feet (which is quite close) and the ground on the opposite side of the planet, which is 7918 miles away. There’s the same amount of planet closer to you and farther from you (relative to the center of our planet), so on average, the force is the same as if it came from a point at the center. Mathematically, our trick of assuming that the entire mass of the planet is contained at the core of the Earth is identical to doing it all very carefully, and it is much easier to do.

ESA’s Swarm satellites have led the discovery of a jet stream in the liquid iron part of Earth’s core 3000 km beneath the surface. In addition, Swarm satellite data show that this jet stream is speeding up. Launched in 2013, the Swarm trio is dedicated to identifying and measuring precisely the different magnetic signals that make up Earth’s magnetic field. Image credit: ESA CC BY-SA 3.0 IGO

ESA’s Swarm satellites have led the discovery of a jet stream in the liquid iron part of Earth’s core 3000 km beneath the surface. In addition, Swarm satellite data show that this jet stream is speeding up. Launched in 2013, the Swarm trio is dedicated to identifying and measuring precisely the different magnetic signals that make up Earth’s magnetic field. Image credit: ESA CC BY-SA 3.0 IGO

What does this mean for your spherical room at the core of the planet? Well, this principle of canceling out forces if they’re pulling on you in different directions still holds, and so you’re absolutely on the money to say that you should be weightless in there. You absolutely could float at the center of the planet; the entire mass of one half of the planet pulling you to the left would cancel the remaining mass of the planet pulling you to the right. And the same is true of being pulled upwards/downwards, or any direction that you care to slice the planet in half. There would be no 'down'.

What does a depiction of space time look like in the middle of the planet? Let’s remember that our depictions of space time usually depict divots surrounding massive objects, where gravity pulls you “down” into the gravitational well. With no gravitational force, and no 'down', your room in the core of the Earth would have a space-time curve that was very flat. No bending, no vortex. If there’s no net gravitational force, there can be no slant or directionality to space-time.

It’s flat because you’re at the very very bottom of the gravitational well. To leave your room at the center of the Earth, you’d have to climb your way all the way back up to the surface of the Earth, and as soon as you left, there would be a net force, pulling you back down. As you climb out, more and more of the Earth is left below you, and the downward dragging force you would feel would increase almost continuously until you reached the surface. The surface is a much more hospitable place, in any case; it’s got all my favorite things on it.

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How Long Until The Moon Slows The Earth To A 25 Hour Day?

At this rate of the Moon’s gravitational force slowing down Earth’s rotation, how long will it take to increase an hour to our day?
NASA's Lunar Reconnaissance Orbiter (LRO) recently captured a unique view of Earth from the spacecraft's vantage point in orbit around the moon. Image Credit: NASA/Goddard/Arizona State University

NASA's Lunar Reconnaissance Orbiter (LRO) recently captured a unique view of Earth from the spacecraft's vantage point in orbit around the moon. Image Credit: NASA/Goddard/Arizona State University

Originally posted on Forbes!

The Earth’s rotation is indeed being slowed down by the presence of the Moon - every year, the Moon gains a little energy from the Earth, and drifts a little farther away from us. This drift is imperceptible to the human eye, but measurable, with the aid of undertakings like the Lunar Laser Ranging Experiment, which regularly bounces a laser off of a retroreflector that Apollo astronauts placed there.

Both the drift of the Moon and the slowing of the rotation of the Earth are very very small effects- the slowing of the Earth’s rotation over the last 100 years is estimated to be about 1.4 milliseconds. That’s a slowing of 0.0014 seconds total, over 100 years. Another method of estimating the slowing of the Earth uses historical records of solar eclipses to figure out exactly how fast the Earth must have been rotating in the past, and comes up with an average slowing of 2.5 milliseconds each century. To extrapolate out into the future, I’m going to use the average of these two numbers, and guess that we’re dealing with a slowing of approximately 0.002 seconds every century.

As a point of reference, this rate of slowing means that it will take 25,000 years to add a half a second to the Earth’s day. A whole second will take 50,000 years.

The release of the first images from NOAA’s newest satellite, GOES-16, is the latest step in a new age of weather satellites. This composite color full-disk visible image is from 1:07 p.m. EDT on Jan. 15, 2017, and was created using several of the 16 spectral channels available on the GOES-16 Advanced Baseline Imager (ABI) instrument. The image shows North and South America and the surrounding oceans. GOES-16 observes Earth from an equatorial view approximately 22,300 miles high, creating full disk images like these, extending from the coast of West Africa, to Guam, and everything in between. Image Credit: NOAA/NASA

The release of the first images from NOAA’s newest satellite, GOES-16, is the latest step in a new age of weather satellites. This composite color full-disk visible image is from 1:07 p.m. EDT on Jan. 15, 2017, and was created using several of the 16 spectral channels available on the GOES-16 Advanced Baseline Imager (ABI) instrument. The image shows North and South America and the surrounding oceans. GOES-16 observes Earth from an equatorial view approximately 22,300 miles high, creating full disk images like these, extending from the coast of West Africa, to Guam, and everything in between. Image Credit: NOAA/NASA

To add an entire hour? Every hour contains 3,600 seconds - (60 minutes to an hour, and 60 seconds to a minute). And so, to wait long enough to gain 3,600 seconds, we’ll need to wait 50,000 years 3,600 times over - 180 million years.

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