alleryKepler's first law of planetary motion states that:
APlanets move in circular orbits around the Sun
BAll planets move in elliptical orbits with the Sun at one focus
CAll planets sweep equal areas in equal times around the Sun
DT² is proportional to the cube of the orbital radius
Answer & Solution
Correct answer: B. All planets move in elliptical orbits with the Sun at one focus
1. Kepler's first law (Law of Orbits) replaced the older circular-orbit picture.
2. Each planet traces an ellipse, with the Sun at one of the two focii.
3. A circle is a degenerate ellipse (both focii merged), so circular orbits are a special case.
4. The other choices state the Law of Areas (2nd) and Law of Periods (3rd).
_Source: syllabus Physics Ch 7 "Gravitation", §7.2 Kepler's Laws, p. 128_
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