 Kepler's first law (law of orbits) states that each planet revolves around the Sun:
AIn a perfect circle with the Sun at its centre
BIn an elliptical orbit with the Sun at one focus
CIn a parabolic orbit
DIn a hyperbolic orbit
Answer & Solution
Correct answer: B. In an elliptical orbit with the Sun at one focus
Kepler's three laws of planetary motion:
1. **Law of orbits (first law)**: each planet moves in an *elliptical* orbit with the Sun at one focus (the other focus is empty space).
2. **Law of areas (second law)**: the line joining a planet to the Sun sweeps equal areas in equal times.
3. **Law of periods (third law)**: $T^2 \propto a^3$, where $a$ is the semi-major axis.
A circle is the special case of an ellipse where both foci coincide at the centre. Most planetary orbits are nearly circular but not exactly so. Mercury and Pluto's orbits have noticeable eccentricity. Parabolic (option C) and hyperbolic orbits are unbound trajectories.
Related questions
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