What is Earth: Definition and 1000 Discussions

Earth is the third planet from the Sun and the only astronomical object known to harbor and support life. About 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is covered with water, mostly by oceans, seas, gulfs, and other salt-water bodies, but also by lakes, rivers, and other freshwater, which together constitute the hydrosphere. Much of Earth's polar regions are covered in ice. Earth's outer layer is divided into several rigid tectonic plates that migrate across the surface over many millions of years, while its interior remains active with a solid iron inner core, a liquid outer core that generates Earth's magnetic field, and a convective mantle that drives plate tectonics.
Earth's atmosphere consists mostly of nitrogen and oxygen. More solar energy is received by tropical regions than polar regions and is redistributed by atmospheric and ocean circulation. Greenhouse gases also play an important role in regulating the surface temperature. A region's climate is not only determined by latitude, but also by elevation and proximity to moderating oceans, among other factors. Severe weather, such as tropical cyclones, thunderstorms, and heatwaves, occurs in most areas and greatly impacts life.
Earth's gravity interacts with other objects in space, especially the Moon, which is Earth's only natural satellite. Earth orbits around the Sun in about 365.25 days. Earth's axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth. The gravitational interaction between Earth and the Moon causes tides, stabilizes Earth's orientation on its axis, and gradually slows its rotation. Earth is the densest planet in the Solar System and the largest and most massive of the four rocky planets.
According to radiometric dating estimation and other evidence, Earth formed over 4.5 billion years ago. Within the first billion years of Earth's history, life appeared in the oceans and began to affect Earth's atmosphere and surface, leading to the proliferation of anaerobic and, later, aerobic organisms. Some geological evidence indicates that life may have arisen as early as 4.1 billion years ago. Since then, the combination of Earth's distance from the Sun, physical properties, and geological history have allowed life to evolve and thrive. In the history of life on Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinctions. Over 99% of all species that ever lived on Earth are extinct. Almost 8 billion humans live on Earth and depend on its biosphere and natural resources for their survival. Humans increasingly impact Earth's surface, hydrology, atmospheric processes, and other life.

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  1. felizgu

    How Long for a Beam of Light to Reach Earth?

    TL;DR Summary: This is an astronomy application. Mentor note: Moved from a technical forum section, so the homework template is missing. How long does it take a beam of light to reach Earth from the Sun when the Sun is 93,000,000 miles from Earth? Express your answer in seconds, using...
  2. H

    B Quasi-Moons

    A quasi-moon of Earth is a rock to that orbits both the Earth and Sun with the same solar orbital period as Earth.
  3. applied-physics

    I Sun shade on Earth ground, is it a straight line or a curved line?

    How did you find PF?: via Google search Hi, I wonder if the shade from the sun on the ground, on a sunny day, is it a straight line or a curved line?? and if it is curved, how curvature can be calculated ?? Let´s say I have a pole or building, and I can follow up the shade of its top on the...
  4. adhd_wonderer

    B Can Earth or any spherical object in space act as a particle collider?

    This is most probably a dumb idea as I'm far from deep physics knowledge but I was thinking. What if Earth is hot inside not because of the pressure and the radioactivity but because it's mass attracts particles (similarly to gravitational lensing) and they collide right in the Earth's center?
  5. marc873a

    How did Cavendish find the density of the Earth?

    I'm trying to find how Cavendish got the density of the earth to 5,48 times the density of water. In all of the YouTube videos and webpages I have seen, they mention different formulas where the mass of the earth or the gravitational constant, G, is included. But as far as I understand, these...
  6. Trazyn

    Is the Pastafarian Greeting Rooted in Earthly Origins?

    I stretch my tendrils to say hello.
  7. graphking

    I A strict proof of "why the Earth is a ball"

    "bubbles are ball" is called isoperimetric problem in serious mathematic. In this topic, many essay were written. Here's my serious essay about "why earth ball", which has been rejected by arxiv and my mentors...... I would want to know if physicists are interest? I really think that is...
  8. H

    B Consequences of a 1 Tesla magnetic field coming into the proximity of Earth?

    What would be the consequence if a powerful (1 Tesla) magnetic field came into proximity of Earth? Options open concerning size of generating body and distance. Thank You.
  9. BillTre

    How many living cells on Earth?

    A NY Times article "Exactly How Much Life is on Earth?" (research article here, behind a paywall) says that there are about 1030 cells currently on earth and the total numbers of cells that will be on earth through its future lifespan is about 1041 cells. That is: It was implied in the...
  10. LuisBabboni

    I Kepler´s 2nd law -- Do any two planets sweep out equal area in equal time?

    I mean. For example, Earth in one month sweep the same area* than Jupiter in one month? *The line joining the Earth with the Sun than the line joining Jupiter with the Sun. I think yes, but is not what 2nd law says. I think in the fact that the aceleration just depends on the distance to the...
  11. B

    How do we derive the Earth's yearly mean temperature?

    I'm new to this. I would like to know how we derive the earth's yearly mean temp. I assume there are thermometers all over...
  12. P

    B Supernova Ejection: How Large Are the Chunks?

    when a star goes supernova, does it eject out earth, mercury and mars sized chunks. Surely the asteroids that are even the size of Texas won't have enough gravity to compact hard enough to become one chunk. What is the current belief on how large of chunks are ejected from a supernova. Thanks
  13. S

    B Is the Earth's gravitational pull constant?

    In the first part, I'm asking about acceleration of freefalling objects with different masses. In the second part I'm asking about acceleration of one object with decreasing distance. Please explain where am I getting it wrong. Thank You!
  14. earthlingjeff

    Would Earth not be seen flashing from space like a lighthouse?

    Can anyone tell me if this would be correct? Would Earth not be seen flashing from space like a lighthouse to a distant planet when it reflects oceans and then DMS as the sons reflection per perspective travels across land then back to Ocean then back to land? I'm sure it probably says that...
  15. Lotto

    B What is the total kinetic energy of the Earth relative to the Sun?

    Because it rotates around its axis, then it has a rotational kinetic energy. But when it orbits the Sun, then it has an another rotational energy too. Would it be ##E_\mathrm{rot} = \frac 12 (\frac 25 MR^2+ Md^2) {\omega}^2 ##, where ##d## is the distance between the Sun and the Earth. I...
  16. H

    A 4D HyperEarth: 1000x More Space for Real Estate

    Let’s dubiously assume that these 4D atoms would have the same radius as 3D atoms. Let’s further dubiously assume that a 4D HyperEarth has the same number of atoms as 3D Earth. Then the diameter of HyperEarth would be less than a kilometer. Four dimensional Earth is much more compact. Wouldn’t...
  17. S

    B Does the inverse square law equalize gravity at different spots?

    Inverse square law would reduce the gravity from the parts of Earth that are farthest from our feet. It'll also reduce the gravity from Earth's center by a lesser amount, but would that be lesser enough so the gravity 20 kilometers under our feet is stronger than the core's gravity or even the...
  18. Asem

    Angular Velocity of a Large Pendulum on Earth as seen from the stars

    I don't understand the question. how am I supposed to find the magnitudes and directions of the velocity from the figure?
  19. Lotto

    B What is meant by "upper limit of work done on Earth"?

    I think that the work is meant to be work done for instance in power stations. Or is it similar to work I do on a body when I lift it for example? But how can we then do that work on our Earth? I just need to understand the task, otherwise I want to solve it myself. The problem involves...
  20. T

    Does anyone have a table of Earth mean temperatures?

    I would like a table of Earth mean temperatures going back as far as available. Where can I find that?
  21. N

    Earth ground meter Vs Multimeter

    Dear forum please help me with the following understanding: There is (are) such equipment Earth ground testers (https://www.fluke.com/en/products/electrical-testi...) - it is used to measure grounded electrical systems or to measure. Such equipment is having multiple poles that are attached to...
  22. EclogiteFacies

    I How meaningful is the notion of now here on Earth?

    So, not looking for anything over complicated. Just been picking up some special relativity. How meaningful is the notion of now here on Earth? Is special relativity worth even considering for day to day interactions with others?
  23. DANIELWR1998

    How do I find way of comparing the density of the Earth and the Moon?

    Surface acceleration is proportional to density and radius of planet (as 2 powers of R cancel with the volume) g(moon)/g(earth) = density(moon)*radius(moon)/density (earth)*radius(earth) = (1/4)*density(moon)/density(earth)
  24. James1238765

    I Modeling the Earth and Sun (2 body orbits) using general relativity?

    Modeling the time evolution of the sun and earth orbiting each other using ##F = \frac{GMm}{r^2}## is straightforward. However, it appears that modeling the time evolution of the same 2 body system using general relativity seems to be a hard/intractable problem? There was in depth discussion by...
  25. S

    B Calc Excess Rad on Earth in Curved 3D Space | Feynman Lectures 6-2

    I'm reading the Feynman lectures chapter on "Curved Space", section 6-2. Say we're trying to figure out a way to measure average curvature on Earth. We know that 3d space is curved if Euclidean geometry rules don't work - e.g. the ratio of circumference and radius of a circle isn't ##2\pi##, or...
  26. Astronuc

    Coldest Place on Earth - in Antarctica, of course

    What’s the coldest spot on Earth? NASA has pinpointed it — and the nights are deadly https://www.yahoo.com/news/coldest-spot-earth-nasa-pinpointed-125903009.html
  27. G

    B How did Earth get so far away from other galaxies?

    If the Big Bang happened 13.6 billion years ago how can the James Webb telescope now see galaxies 13.3 billion light years away? 13.3 billion years ago the universe was 0.3 billion years old. How large was the universe at that time? If physical mater traveled in all directions from the point...
  28. Z

    Angular deceleration of the Earth

    365 rotations - 365 days 365 days - 31536000 s apart from that I do not know how to continue the question
  29. DaveC426913

    B How much time passes on Earth?

    The recent threads about relativity and the (ersatz) twin paradox got me flummoxed (again). I referenced a story (A World Out of Time) where the man character came back to an Earth that was 3 million years older than when he left. The reason is that he journeyed to the galactic centre and back...
  30. Lotto

    B Time dilation - what is the time on the Earth?

    If I understand it well, 10 s did really passed in the rocket, it is according to the observer on the Earth, but if the man in the rocket measured the time, he would measure 10 s. But when we say that the man in the rocket is in an inertial frame of reference as well, he can claim that because...
  31. babaliaris

    How does Earth Ground "absorb" electric current?

    This is a topic that almost no one can actually explain from what I found on the internet. This answer https://physics.stackexchange.com/questions/74625/does-alternating-current-ac-require-a-complete-circuit/74999#74999 is the closest I found so far. The idea is how the grounded neutral...
  32. A

    B Color of a mirror on the Earth

    If I place a mirror on the ground facing upward, what color will those who observe it from space see?
  33. G

    B Exploring General Relativity: Altitudes Where Oblate Earth Approximation Applies

    This is actually two questions. They are In which courses or field work is general relativity taught as method and not philosophy? At which altitudes does the oblate Earth approximation matter? At stealth bomber altitudes? Somewhere between Earth and Moon (in perhaps inertial gyroscope...
  34. Sciencemaster

    I Database of binary star data info within 10 PC of Earth

    I'm looking for a database of binary stars within 10 PC of Earth, including information such as eccentricity of orbits, their distance from one another, etc. I'm hoping to find a list with this information, or just a collection of pages with this information. I've tried Simbad but I can't find...
  35. V

    How does the electroscope lose electrons to the Earth?

    I get step 1, in which due to electrostatic induction the top part of electroscope gets positively charged while the leaves of electroscope become negatively charged. Now if we Earth the positively charged end of electroscope as shown in step 2, then electrons must flow from Earth to...
  36. yohananregal

    How's it possible that 70% of Earth receives sunlight simultaneously

    How did you find PF?: Google Hello, I am new here. I am wondering, how is it possible for nearly 70% of the spherical Earth to be illuminated by the sun every July 8th? How can a sphere be illuminated beyond 50% with a single light source. How do we explain this?
  37. Astronuc

    Measuring Earth and its Mountains

    Earlier this year PBS NOVA broadcast a documentary on measuring mountains. It covered Everest (Nepali: सगरमाथा, romanized: Sagarmāthā; Tibetan: Chomolungma ཇོ་མོ་གླང་མ) and K2. The history behind measuring mountains and the elevations of Earth's surface is fascinating. The old way involved...
  38. Graham87

    Comp Sci Orbit of the Earth - numerical methods leapfrog

    I am attempting this homework exercise part b). I've modified my code but I get error overflow message. My goal is to modify my code so it returns kinetic and potential energy of Earth's orbit. I made a new f(z,t) and modified the rows 99 and 100 with dz[2]=-G*M*m/r, and dz[3]=0.5*m*y**2 but...
  39. Graham87

    Comp Sci Orbit of Earth: Plotting Trajectory

    I am attempting this homework exercise but my plot does not show the whole trajectory. I don't know if it is something wrong with my equations or if it is a plotting matter. Cheers!This is my code:import matplotlib.pyplot as plt import numpy as np import scipy.integrate as spi G =...
  40. qnt200

    I The gravitational center of the Earth

    We know that gravitational forces are nullified near the center of the Earth, so the gravitational field's influence is not felt. Is it because of the Moon's gravitational field that the area of zero gravity has shifted away from the center of the Earth? If this is the case, this eccentric area...
  41. malawi_glenn

    (Introductory SR) Light sent from a spaceship received by Earth

    I am taking a summer course on special relativity and I stumbled across this problem and solution which I tought look neat. However, I think the solution provided for a) there is wrong. I will here present two of my solutions for a) and one solution for b) and ask if you think mine are okay? :)...
  42. MiSTSiM

    Given two Earth satellites, A and B, find the potential energy/kinetic energy

    = -3.7298538168*10^13, -2.0594767123*10^13 Ive tried this equation with both masses and my homework keeps coming back with the wrong answer. I've tried checking my arithmetic but I cannot find anything wrong with it please help haven't gotten the chance to try B but I am pretty sure it will be...
  43. newjerseyrunner

    Search for Intelligence on Earth

    I was discussing the Fermi Paradox with a coworker at lunch and he made a very good point that I hadn’t considered and I don’t really hear mentioned on the topic: distribution. The assumption that if ET was everywhere, we should see it everywhere. The point was that that should be true for...
  44. Astronuc

    Art How the Earth Was Made (or Evolved)

    The History Channel has developed a series on "How the Earth was made". My wife has been listening to it in the evening and I've caught some of it. Mostly good, but the narration mentions some unnecessary commentary, and in the first program, it gets the description of carbon-14 uptake wrong...
  45. Field physics

    Is the Sun Playing Hard to Get?

    Just something dumb but funny to think about. Redshift indicates the object is moving away. The Earth orbits the sun The sun appears to be red. So in conclusion, the sun is running away and the Earth is chasing the sun. Lol hopefully that lightens your evening.
  46. J

    I Momentum, impact force and Earth

    Suppose two objects, A and B, with large lengths LA and LB, and masses MA and MB, collide at time t0. Both objects before collision are vertical and aligned concentrically, being object B positioned initially at a higher z coordinate than object A. The bottom end of object A is rigidly...
  47. Jarvis323

    I First Interstellar Object to Hit Earth

    I was hoping to hear some discussion about this eventually. It is one of the biggest events in all of human history if it is true isn't it? https://www.cnn.com/2022/04/13/world/interstellar-meteor-discovery-scn/index.html
  48. J

    I Gravity on the Moon vs. on the spinning Earth

    "If the Earth were not spinning, you would be heavier as you would feel the full force of gravity."so how does this apply to the moon? the moon does not rotate and therefore you would experience the full force of gravity as stated above,yes,,yet the appolo missions film footage showed...
  49. B

    B Distance between two points on Earth doesn't increase with altitude?

    Consider the following example: Point A has coordinates 45 lat, 0 long. Point B has coordinates 45 lat, 2 long. Both points are 5000 ft above sea level. The distance between them is X. Point C has coordinates 45 lat, 100 long. Point D has coordinates 45 lat, 102 long. Both points are at sea...