Which statement best describes Kepler's First Law?

Which statement best describes Kepler's First Law?

• A. The orbit of a planet is a circle with the Sun at the center.
• B. The orbit of a planet is an ellipse with the Sun at one focus.
• C. The planet's speed is constant along its orbit.
• D. The orbit is a hyperbola around the Sun.

Answer: (B)

Kepler's First Law states that planets travel in ellipses with the Sun at one focus. This means the distance between a planet and the Sun changes as it moves, and the Sun is not at the center of the orbit. A circular orbit would place the Sun at the center, which is not the general description Kepler gave (a circle is a special, degenerate ellipse with equal axes, not the typical case for planetary orbits). The planet’s speed isn’t constant because gravity speeds it up when it’s closer to the Sun and slows it down when it’s farther away. A hyperbola describes an unbound path, not a bound planetary orbit. So the statement that best describes Kepler’s First Law is that the orbit is an ellipse with the Sun at one focus.

Kepler's First Law states that planets travel in ellipses with the Sun at one focus. This means the distance between a planet and the Sun changes as it moves, and the Sun is not at the center of the orbit. A circular orbit would place the Sun at the center, which is not the general description Kepler gave (a circle is a special, degenerate ellipse with equal axes, not the typical case for planetary orbits). The planet’s speed isn’t constant because gravity speeds it up when it’s closer to the Sun and slows it down when it’s farther away. A hyperbola describes an unbound path, not a bound planetary orbit. So the statement that best describes Kepler’s First Law is that the orbit is an ellipse with the Sun at one focus.