brave_new_world Posted January 9, 2005 Share Posted January 9, 2005 Kepler's 1st law of planetary motion states: Planets move in ellipses with the Sun at one focus. This question had been bugging me. There are two foci in an ellipse. If this theory is true, what would be at the other focus point? Kepler's Laws Link to comment Share on other sites More sharing options...
Ophiolite Posted January 9, 2005 Share Posted January 9, 2005 Nothing. There doesn't need to be anything. Link to comment Share on other sites More sharing options...
Jordan14 Posted January 9, 2005 Share Posted January 9, 2005 There is no second foci this focus is just imaginary: http://csep10.phys.utk.edu/astr161/lect/history/kepler.html Link to comment Share on other sites More sharing options...
Paul Trow Posted January 9, 2005 Share Posted January 9, 2005 On the contrary, the ellipse does have a second focus - there's just nothing there, as Ophiolite pointed out. Link to comment Share on other sites More sharing options...
Jordan14 Posted January 9, 2005 Share Posted January 9, 2005 Sorry Paul that was me being badly word again what you said was what I meant see the link Link to comment Share on other sites More sharing options...
brave_new_world Posted January 10, 2005 Author Share Posted January 10, 2005 There has to be some sort of mass at the second foci in order to cause the planets to orbit in ellipses. I must be missing something, can someone please explain how there can be elliptical orbits without a second mass? Also, does the moon or in an elliptical or circular motion? Link to comment Share on other sites More sharing options...
Jacques Posted January 10, 2005 Share Posted January 10, 2005 I just thaught that the planet of the solar system have small eccentricity: maybe the second focis is in the sun. I found the orbital element of the planet and of the sun. JPL's solar system dynamics WWW Mean Orbital Elements The sun have a diameter of 1,390,000 km. Quick calculation: e:=eccentricity c:=half distance focis a:=semi-major axis e=c/a => 2c=2ea The less eccentric orbit is Venus and it gave a distance betwen the 2 focis of 1,450,000 km. Earth 5 millions km (eccentricity=0.016) So no the other focis is not in the sun. It's somewhere betwen the sun and the planet. Is it a special point ? Mathematically YES Physicaly ? I don't think A good thing about this forum is that it force me to use some little math something I didn't do since a long time. I always been interested in astronomy from a general point of view. I know many different concept without much math around it. Elementary math for many people, here in this forum. I am new here and happy to exchange with you! So much thing to learn and unlearn... Link to comment Share on other sites More sharing options...
swansont Posted January 10, 2005 Share Posted January 10, 2005 There has to be some sort of mass at the second foci in order to cause the planets to orbit in ellipses. I must be missing something' date=' can someone please explain how there can be elliptical orbits without a second mass? Also, does the moon or in an elliptical or circular motion?[/quote'] No, there doesn't. It's because of the 1/r2 force. Circular orbits are elliptical orbits, but the foci are co-located. However, the moon's eccentricity is 0.0549, so it's not circular, but it's close. (e=(aphelion - perihelion distance)/(2 x semi-major axis), so e=0 is circular) Link to comment Share on other sites More sharing options...
brave_new_world Posted January 10, 2005 Author Share Posted January 10, 2005 What is the 1/r squared force? Link to comment Share on other sites More sharing options...
swansont Posted January 10, 2005 Share Posted January 10, 2005 The gravitational force varies as the inverse-square of r. Link to comment Share on other sites More sharing options...
brave_new_world Posted January 10, 2005 Author Share Posted January 10, 2005 So are you saying that the orbits of the planets are slightly elliptical because the radius of the sun is not the same throughout? Also since the earth is not a perfect sphere it casues the moon to orbit slightly elliptical. Link to comment Share on other sites More sharing options...
DarthDooku Posted January 20, 2005 Share Posted January 20, 2005 wouldnt the second foci be Earth itself. Do the Earth and the Sun pull on each other like the Earth and moon. Link to comment Share on other sites More sharing options...
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