key term - Ellipse

Definition

An ellipse is a closed, two-dimensional shape that is the locus of all points where the sum of the distances from two fixed points, called the foci, is constant. Ellipses are a fundamental concept in Kepler's Laws of Planetary Motion, which describe the orbits of planets around the Sun.

5 Must Know Facts For Your Next Test

1. Kepler's First Law states that the orbit of every planet is an ellipse, with the Sun at one of the foci.
2. The eccentricity of an ellipse determines its shape, with a circle being a special case of an ellipse with an eccentricity of 0.
3. The semi-major axis of an ellipse corresponds to the average distance between a planet and the Sun, while the semi-minor axis represents the variation in that distance.
4. The area of an ellipse is proportional to the product of its semi-major and semi-minor axes, as given by the formula $\pi ab$, where $a$ and $b$ are the lengths of the semi-major and semi-minor axes, respectively.
5. Kepler's Second Law states that a line joining a planet and the Sun sweeps out equal areas during equal intervals of time.

Review Questions

• Explain how the concept of an ellipse is related to Kepler's First Law of Planetary Motion.
  • Kepler's First Law states that the orbit of every planet around the Sun is an ellipse, with the Sun located at one of the foci of the ellipse. This means that the path of a planet as it revolves around the Sun is an elliptical shape, where the sum of the distances from the planet to the two foci (the Sun and an empty focus) is constant. The eccentricity of the ellipse determines how elongated or circular the planet's orbit is, with a circular orbit being a special case of an ellipse with an eccentricity of 0.
• Describe how the semi-major and semi-minor axes of an ellipse relate to the motion of a planet around the Sun.
  • The semi-major axis of an ellipse corresponds to the average distance between a planet and the Sun, while the semi-minor axis represents the variation in that distance. Kepler's Second Law states that a line joining a planet and the Sun sweeps out equal areas during equal intervals of time. This means that the planet moves faster when it is closer to the Sun (at perihelion) and slower when it is farther away (at aphelion), with the area swept out by the imaginary line being constant. The semi-major and semi-minor axes of the ellipse determine the range of distances the planet can be from the Sun, and thus the variation in its orbital speed.
• Analyze how the eccentricity of an ellipse affects the motion of a planet around the Sun.
  • The eccentricity of an ellipse is a measure of how much the shape deviates from a perfect circle, with a value between 0 and 1. A higher eccentricity indicates a more elongated ellipse, which in the context of planetary motion means the planet's distance from the Sun varies more significantly over the course of its orbit. This affects the planet's orbital speed, as per Kepler's Second Law - the planet will move faster when closer to the Sun and slower when farther away. Highly eccentric orbits can also lead to greater variations in the planet's received solar radiation and temperature, potentially impacting its climate and habitability. Understanding the relationship between eccentricity and planetary motion is crucial for accurately modeling and predicting the behavior of celestial bodies.

Related terms

Foci:

The two fixed points from which the distances to any point on an ellipse sum to a constant value.

Eccentricity:

A measure of how much an ellipse deviates from a perfect circle, with a value between 0 and 1.

Semi-major and Semi-minor Axes: The lengths of the longest and shortest diameters of an ellipse, respectively.