Troubleshooting Kepler's Third Law with Halley's Comet

In summary: Everybody makes copying errors (when they're not making *sign* errors), so I know just how you feel...
  • #1
ehrenfest
2,020
1

Homework Statement


When I plug in all of the parameters for Halley's comet (from Wikipedia) into Kepler's third law a get a semimajor axis of 38.56 AU when it should be about 17? Can someone else try it and see if I am crazy?

Homework Equations


The Attempt at a Solution

 
Last edited:
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  • #2
What parameters are you trying to plug into what equation?
 
  • #3
mass of halley's comet = negligable
mass of the sun
G
T = 76 years
 
  • #4
I get that 76^2 is pretty close to 17.8^3. Perhaps you are crazy. :) Remember that the Earth semimajor axis is 1 AU and it's period is 1 year.
 
  • #5
OK here are the details:

38.5654 = (T^2/(4 pi^2) * G * (Ms))^(1/3)/(1.4*10^11)

where T is the period in seconds, Ms = 1.991*10^31 and G = 6.674 * 10^(-11)
what am I doing wrong?
 
  • #6
ehrenfest said:
OK here are the details:

38.5654 = (T^2/(4 pi^2) * G * (Ms))^(1/3)/(1.4*10^11)

where T is the period in seconds, Ms = 1.991*10^31 and G = 6.674 * 10^(-11)
what am I doing wrong?

The mass of the Sun is 1.99*10^30 kg... (Your result for a is off by very nearly the cube root of 10.)
 
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  • #7
You beat me! I just figured that out. But, ehrenfest, for solar orbits if you work in AU and years, the constant proportionality k in R^3=k*T^2, is one.
 
  • #8
Dick said:
You beat me! I just figured that out. But, ehrenfest, for solar orbits if you work in AU and years, the constant proportionality k in R^3=k*T^2, is one.

My training's largely in astrophysics, so I have the solar mass by heart. I would usually take the proportionality approach myself as well, though...
 
  • #9
Funny, my training is in cosmology, so I know it's like to ten the fifty some proton masses. And fifty plus what I forget. Good job.
 
  • #10
Ahh! 30 minutes of frustration because my short-term memory is not good enough to look at a computer screen and then write down a two-digit number without botching a digit!

Thanks guys.
 
  • #11
Dick said:
Funny, my training is in cosmology, so I know it's like to ten the fifty some proton masses. And fifty plus what I forget. Good job.

Close enough... ;-) When I was an undergraduate, cosmology was called "the science where you're happy when your order of magnitude is right to an order of magnitude". Nowadays we speak of "precision" cosmology -- what an age we live in...
 
  • #12
ehrenfest said:
30 minutes of frustration...

Everybody makes copying errors (when they're not making *sign* errors), so I know just how you feel...
 

1. What is Kepler's Third Law?

Kepler's Third Law, also known as the "law of harmonies," states that the square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit.

2. How is Halley's Comet related to Kepler's Third Law?

Halley's Comet is used as a case study to test the accuracy of Kepler's Third Law. This is because it has a highly elliptical orbit and can be observed over multiple orbits, making it a good candidate for studying orbital mechanics.

3. What is the significance of troubleshooting Kepler's Third Law with Halley's Comet?

By using Halley's Comet to test Kepler's Third Law, scientists can refine and improve our understanding of orbital mechanics and the laws governing the motion of celestial bodies.

4. How is Halley's Comet observed to troubleshoot Kepler's Third Law?

Astronomers use a combination of ground-based observations and spacecraft data to track the position and movement of Halley's Comet. They measure its orbital period and semi-major axis, and then compare these values to what is predicted by Kepler's Third Law.

5. What have scientists learned from troubleshooting Kepler's Third Law with Halley's Comet?

Scientists have been able to confirm the accuracy of Kepler's Third Law through their observations of Halley's Comet. They have also been able to use this information to make more precise calculations and predictions about the motion of other celestial bodies in our solar system.

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