Does the Earth's orbital speed change and why do planets not travel around the Sun at exactly the same speed?
The Earth's orbital speed does change. The planets travel at different orbital speeds because they are at different distances from the sun. The main reason is that all the planets move around the sun in elliptical, not circular, orbits, with the sun at one focus. This is the first of Kepler's 3 laws, later derived from Newton's laws, that explain the behavior. See https://science.nasa.gov/resource/orbits-and-keplers-laws/ for details.
Yes, the speed at which a planet orbits the Sun changes depending upon how far it is from the Sun. When a planet is closer to the Sun the Sun's gravitational pull is stronger, so the planet moves faster. The Earth, on average, revolves around the Sun at a speed of approximately 29.78 km/s (18.51 mi/s), 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). A planet's orbital speed changes, depending on how far it is from the Sun. The closer a planet is to the Sun, the stronger the Sun's gravitational pull on it, and the faster the planet moves. The farther it is from the Sun, the weaker the Sun's gravitational pull, and the slower it moves in its orbit. The closer a planet is to the Sun, the stronger the Sun's gravitational pull on it, and the faster the planet moves. The farther it is from the Sun, the weaker the Sun's gravitational pull, and the slower it moves in its orbit. There is no connection between a planet's distance from the Sun and its rate of rotation (spin on its own axis or Planetary Rotation Period). While it is true that, generally speaking, the gas giants have higher rotation rates than the earth like planets closer to the Sun, any correlation is only coincidental. A planet's orbital speed changes, depending on how far it is from the Sun. The closer a planet is to the Sun, the stronger the Sun's gravitational pull on it, and the faster the planet moves. The farther it is from the Sun, the weaker the Sun's gravitational pull, and the slower it moves in its orbit. Orbital speed is determined by the mass of the object you are orbiting and your distance from it. All of the planets are orbiting the sun, so the mass is the same, but the distance is different. The further away they are, the slower they go. But planets spin at different speeds, for two reasons: First, the material joining each growing planet was moving in different ways and at different speeds. Second, each planet ended up with a different mass. In most everyday applications on Earth, friction counteracts motion by slowing things down, so energy must be added to keep an object moving (or spinning). Our planet doesn't slow down much simply because there is almost nothing stopping it from continuing to spin.
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