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# Orbital Motion

## Explore weightlessness, orbit, and uniform circular motion

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Mercury's Orbit

### Mercury’s Orbit

Credit: Laura Guerin
Source: CK-12 Foundation

The smallest of all of the planets, Mercury has an orbital period of approximately 88 Earth days. Since Mercury has almost no atmosphere, the surface is exposed to the greatest temperature range of all the planets. Unlike all the other planets, Mercury deviates from the expected motion that would be described by Newtonian physics.

#### News You Can Use

• Newton’s laws of motion can clearly describe the motion of any two-body system in our solar system. The orbiting mass would trace out an elliptical orbit with a spherical mass as the focus. Unlike the other planets in our solar system though, Mercury doesn’t follow the precession that is predicted by Newtonian physics. Several theories were proposed to explain this deviation such as other masses close to the sun or even a possible misshapen sun, but not of them were proven correct.
• Credit: Ferdinand Schmutzer
Source: http://en.wikipedia.org/wiki/File:Einstein1921_by_F_Schmutzer_4.jpg

Einstein created General Relativity which corrected Newton's laws [Figure2]

• The deviations from Newton’s laws were finally corrected with the creation of General Relativity by Einstein. He showed that the missing value can be explained by gravitation being mediated by the curvature of spacetime. By proving that General Relativity could explain the small errors when using Newtonian physics, Einstein was able to prove the validity of his theory.

#### Show What You Know

Using the information provided above, answer the following questions.

1. Before General Relativity was invented, why was Newton’s laws not thrown out since they could not explain the orbit of Mercury accurately?
2. Why would another mass or planet that is between Mercury and the Sun help explain the incorrect measurement made my Newtonian physics?
3. Why was the correct value for the shift in Mercury important for the validation of Einstein’s General Theory of Relativity?

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