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35_keplers_laws - Lesson 35 Kepler's Three Laws of...

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Lesson 35: Kepler's Three Laws of Planetary Motion Kepler took the data that Brahe had spent his life collecting and used it (especially the information on Mars) to create three laws that apply to any object that is orbiting something else. Although Kepler’s math was essentially wrong, the three laws he came up with were correct! It would be like you writing a test, and even though you did all the work on a question wrong, you somehow get the correct final answer. Kepler’s Three Laws of Planetary Motion are still the basis for work done in the field of astronomy to this day. Kepler’s First Law Kepler’s First Law went against scientists' major assumption at that time about orbits… in fact it is probably against the image of orbits that you have! If I asked you to describe or draw a sketch of the Earth’s orbit around the sun, how would you draw it. Think about in your head. You’d probably stick the sun in the centre and draw a circle around it to show the path the Earth takes. In fact, this is totally wrong, as Kepler’s First Law states… “The path of any object in an orbit follows the shape of an ellipse, with the orbited body at one of the foci.” So what does all that mean? An ellipse is shaped like a circle that someone has sat on. It’s squished in the middle and bulges out on the ends. Foci (the plural form of the word focus ) are two points inside the ellipse. If you were to push stick pins into the foci and put a loop of string around them, you could draw an ellipse. This means we have a shape that looks like this… 7/19/2007 © studyphysics.ca Page 1 of 6 / Section 5.3 Illustration 1: An ellipse. foci ellipse
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Remember that the object being orbited sits at one focus, and the object in orbit follows the path of the ellipse. This means that sometimes the Earth is closer to the Sun, and sometimes further away. This is not the reason for summer and winter! The seasons on Earth are created by Earth’s tilt on its axis. The diagrams I have drawn here are exaggerated quite a bit to show the elliptical shape and focus clearly. The true orbits of the planets are not this stretched out. Most of the planets orbiting our sun have orbital eccentricities just barely over zero. This elliptical shape does not apply just to planets orbiting the sun. It works for any object orbiting any other object. If you measured the orbit of the Moon around the Earth, it would have an elliptical shape, and so
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35_keplers_laws - Lesson 35 Kepler's Three Laws of...

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