Suppose a planet is discovered orbiting a star in a highly elliptical orbit. While the planet is close to the star it moves _______, but while it is far away it moves _______. a. faster; slower b. slower; faster c. retrograde; prograde d. prograde; retrograde

An elliptical orbit is one of the key features of planetary motion as outlined by Johannes Kepler. Unlike a perfect circle, which has a single radius, an ellipse has two key points called foci. One of the foci is the star (like our Sun), and the planet orbits around it.
The shape of the orbit means the distance between the planet and the star changes over time.
When the planet is closest to the star, it is at a point called perihelion. When it's farthest away, it's at a point called aphelion.
Elliptical orbits are described by their eccentricity, a measure of how much the ellipse deviates from being a circle.

• Eccentricity close to 0 means the orbit is almost circular.
• Eccentricity close to 1 means the orbit is very elongated.

Understanding elliptical orbits is crucial for predicting a planet's position and behavior over time.

The speed at which a planet travels in its orbit varies based on its distance from the star. This variation in speed is described by Kepler's second law of planetary motion.
According to Kepler's second law, a line connecting a planet to the star sweeps out equal areas in equal times. This implies:

• When the planet is near the star (perihelion), it moves faster.
• When the planet is far from the star (aphelion), it moves slower.

The changing speed ensures that the areas swept out over consistent time intervals remain equal. This relationship can be understood by thinking about conservation of angular momentum.
Simply put, a planet's orbital speed increases as it gets closer to the star to conserve angular momentum, and it decreases as it moves farther away.

Planetary motion refers to the movement of planets within their orbits around stars. This motion is governed by three laws proposed by Johannes Kepler in the early 17th century.
Kepler's laws are:

• First Law (Law of Ellipses): Planets orbit the Sun in elliptical paths with the Sun at one focus.
• Second Law (Law of Equal Areas): A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.
• Third Law (Law of Harmonies): The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit.

Understanding these laws helps us to predict planetary positions and velocities. For instance, Kepler's second law helps us pinpoint why a planet moves faster at perihelion and slower at aphelion, as seen in the exercise. These principles are essential for modern astronomy and space exploration.