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Eccentricity. Lesson Goals. Understand the concept of eccentricity Calculate the eccentricity of a celestial object Understand how gravity and inertia keep celestial objects bound to the Sun
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Lesson Goals • Understand the concept of eccentricity • Calculate the eccentricity of a celestial object • Understand how gravity and inertia keep celestial objects bound to the Sun • Understand why the distance between the earth and Sun varies, and how this causes the earth’s speed to change in its orbit.
What is eccentricity? • Eccentricity describes how elliptical a planet’s path is around the Sun. • Ellipses are circles that have been stretched out: Perfect circle Slightly elliptical Highly elliptical
Parts of an ellipse Major axis Minor axis Focus
Formula for eccentricityEccentricity = Distance between foci/length of major axisALL MEASUREMENTS ARE MADE IN CM’S! When E = 0 Perfect circle When E = 0.5 Oval (ellipse) When E = 1 Line
Forces that control orbits:gravity and inertia Inertia Gravity Planet Sun at one focus Planet’s motion
Why do planets change speed during their orbits? The planet is close to the Sun, earth moves faster The planet is far from the Sun, earth moves slower
Conclusions • The shape of earth’s orbit around the sun is an ellipse. • Eccentricity measures the degree of ‘ovalness’ of an ellipse. • The elliptical shape of earth’s orbit causes the earth’s distance to the Sun to change. • Gravity and inertia are two forces that keep celestial objects bound to the Sun.