- 2927 - EARTH - Revolutions and orbits are how we tell time. We not only know that Earth’s orbit slightly changes over time, but we can quantify and confidently state exactly what those changes will be. What does this mean for the speed of Earth around the Sun? Are years getting longer or shorter?
------------- 2927 - EARTH - Revolutions and orbits are how we tell time.
- The planet Earth completes one revolution around the Sun while spinning on its axis once every day. On a year-to-year basis, our orbital changes are so minuscule that they’re practically imperceptible, as the duration of a single revolution (1 year) is tiny compared to how long the planet has been revolving around the Sun (4,500,000,000 years).
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- Each year, Earth migrates slightly away from the Sun, and also takes slightly longer to complete a full revolution. Years are getting longer and temperatures are getting colder, Right?
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- When we think about the Earth orbiting the Sun, we typically make a few simplifying assumptions:
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- We think about the Earth rotating on its axis and moving through space, with the Sun’s gravitation being the only force working on it.
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- We consider the Sun and the Earth as each having their own fixed, constant mass;
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- We think about the space that Earth moves through being empty;
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- We think about the Sun as remaining in the same place while the Earth orbits in an ellipse around it;
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- We neglect the effects of the Moon, the other planets, and the effects that are exclusive to General Relativity; and more?.
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- In reality, we not only know that all of these assumptions are false, but we can, if we’re willing to be precise enough, quantify these effects, and determine which ones are important, how important they are, and what changes they cause over the most simplistic approximation.
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- If all we had were the Earth and Sun and treated them as two unchanging point masses, the Earth would simply make a closed, unchanging ellipse in its orbit: exactly what Kepler’s equations predicted.
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- The first effect we have to consider is the fact that the Sun shines. In this Universe, there’s no such thing as free energy, and that even goes for something like the Sun, which emits a whopping 4 × 10^26 Watts of continuous power. That energy comes from the nuclear fusion of hydrogen nuclei (beginning with protons) into helium-4 (with two protons and two neutrons), which occurs in a chain reaction that releases energy we get from the Sun.
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- Every time that four protons fuse together, producing one helium-4 nucleus, a total of 28 MeV, (one MeV imillion electron-volts) of energy is released. If we convert that into mass, E = mc^2, the Sun loses a total of about 4 million tons of mass due to nuclear fusion with every second.
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- Over the lifetime of our Solar System, the Sun’s mass has decreased by about 95 Earth masses due to nuclear fusion, or approximately the same mass of Saturn.
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- Our Sun also emits particles: the “solar wind“. The particles at the very limb of the Sun are held very loosely at the edge of the photosphere. Particles like electrons, protons, and even heavier nuclei can gain enough kinetic energy to get ejected from the Sun completely, creating a stream of particles that we call the solar wind.
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- In addition, solar flares, coronal mass ejections, and other intense events periodically and irregularly occur, further contributing to the Sun’s mass loss.
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- These events spread particles throughout the Solar System and the overwhelming majority wind up in the interstellar medium, carrying away approximately 1.6 million tons of mass each second.
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- Over the lifetime of the Sun, this results in the loss of 30 Earth masses due to the solar wind. When we combine the solar wind loss with the nuclear fusion mass loss, we find out that today's Sun is about 10^27 kilograms lighter than the Sun was some 4.5 billion years ago, right after the birth of our Solar System.
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- A fraction of those particles leaving the Sun also smash into our planet, causing a variety of effects. These charged particles get funneled by Earth’s magnetic field down onto our poles, where they produce aurorae upon striking the atmosphere. Some of the particles that collide with our planet can kick atmospheric particles into space, causing them to escape from Earth entirely.
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- Solar winds are causing Earth’s orbit to change because we have these solar wind particles inelastically colliding with planet Earth, changing our motion, mass, and both our linear and angular momentum. A total of about 18,000 tons of material strikes our planet every year, taking approximately 3 days to travel from the Sun to the Earth. This also changes Earth’s orbit, very slightly, over time.
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- The planets move in the stable orbits that they do because angular momentum is conserved.
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- The effect of the solar wind slamming into Earth pushes us ever-so-slightly outward, but the enormous mass of the Earth compared to the tiny amount of solar wind that strikes us ensures this effect is small. Over every million years, it pushes Earth’s orbit outward by about the width of a proton, (1 Ångstrom), or about half a micron over the lifetime of our Solar System.
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- The two causes of the Sun’s mass loss, the 30 Earth masses from solar wind production and the 95 Earth masses from radiation are more significant. With each year that goes by, this mass loss means that Earth spirals outwards at a rate of approximately 1.5 centimeters (about 0.6 inches) every year.
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- Over the history of our Solar System, taking into account how our Sun has changed, we’re somewhere around 50,000 km farther from the Sun versus where we were 4.5 billion years ago.
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- The Earth, on average, revolves around the Sun at a speed of 18.51 miles / sec), or about 0.01% the speed of light. This actually varies slightly, since the Earth makes an elliptical orbit around the Sun: moving faster at perihelion (nearest the Sun) and slower at aphelion (farthest from the Sun).
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- The difference is small, but computable. At our fastest, we move through space at 18.83 miles / sec, while at our slowest, we move at 18.20 miles / sec.
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- With each year that goes by, the Earth slows down by approximately 3 nanometers-per-second over how quickly it was moving the prior year. Time to reset your watch. Over the 4.5 billion year history of the Solar System, our planet has slowed by approximately 10 meters-per-second, or about 22 miles-per-hour.
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- Looking over time, the gravitational effects of the planets tugging on one another will potentially cause our orbits to become chaotic. Although the inner planets are all safe for the next billion years, there’s about a 1% chance that one of the four, Mercury, Venus, Earth, or Mars, will become unstable in our Solar System orbits.
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- If that occurs, Earth's orbit could change significantly, possibly even hurling our planet into the Sun or ejecting it from the Solar System entirely. This is the most unpredictable component of our planetary orbit.
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- The Sun will evolve quickly towards the end of its life, ejecting large quantities of mass and swelling into a “red giant“. At this stage, Earth’s orbit will spiral outwards significantly, increasing by about 10-15% while our orbital speed decreases by about the same percentage.
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- Meanwhile, the Sun expands, where it’s predicted to engulf Mercury and Venus, and will become larger than Earth’s current orbit. The ultimate fate of Earth remains unknown.
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- Here are some random encounters that occur that we cannot predict very far into the future: the passage of rogue stars, brown dwarfs, and other masses through our Solar System. Any of them have the potential to eject Earth or perturb our orbit, but these changes are unpredictable.
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- Earth will radiate its orbital energy away in the form of gravitational radiation, causing our orbit to decay and Earth to spiral into whatever’s left of the Sun after another 10^26 years.
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- The Earth spirals away from the Sun at a rate of about 1.5 centimeters every year, causing its orbital speed to drop by about 3 nanometers-per-second over that timescale.
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- If you add up all the tiny changes that have occurred over our Solar System’s history, you’ll find that we’re now about 50,000 km farther away in our orbit than we were 4.5 billion years ago, and move at about 10 meters-per-second slower around the Sun than we did way back when.
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- As time goes on, we’ll continue to spiral away and slow down, as the Sun continues to lose mass due to nuclear fusion and the solar wind. This might seem counterintuitive, but it makes more sense if you think about the Earth orbiting the Sun the same way you might hold a ball on a string and spin it around.
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- If your string is short and the force you exert is large, the ball will spin very fast. If your string is long and the force is small, the ball spins more slowly. As we lengthen the proverbial string representing the Earth-Sun distance, the gravitational force gets a little bit weaker, and hence the Earth has no choice but to move more slowly.
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- The effect may be small on a year-to-year basis, but the Universe has infinite patience. Enjoy your most recent journey around the Sun, because we’ll never have one that goes by this fast again. I guess this means I am actually aging more slowly over time. Better have another glass of wine, it has aged enough.
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- December 5, 2020 EARTH - Revolutions and orbits 2927
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