6.
The speed of the Earth orbiting the sun is
a. the same at all points along its orbit
b. fastest at a single fixed point on its orbit
c. fastest at two fixed points on its orbit
d. slowest at two fixed points on its orbit
The correct option is: b. fastest at a single fixed point on its orbit.
Concept: Elliptical orbit and varying speed
Earth travels around the Sun in an elliptical (oval-shaped) orbit, not a perfect circle. Because the orbit is elliptical, the distance between Earth and the Sun changes during the year, giving two special points: perihelion (closest) and aphelion (farthest).
Kepler’s second law (the law of areas) states that a line joining a planet and the Sun sweeps out equal areas in equal times, which implies the planet moves faster when closer to the Sun and slower when farther away. Thus, Earth’s orbital speed is greatest at perihelion and least at aphelion, so it is not constant along the orbit.
Evaluating each option
Option (a): “the same at all points along its orbit”
This would be true only if Earth moved in a perfectly circular orbit with a perfectly uniform gravitational field, so that the centripetal and gravitational forces balance at a constant speed. In reality, Earth’s orbit is slightly elliptical, and its speed changes between about 29–30 km/s as it moves closer to or farther from the Sun.
Therefore, the speed is not the same at all points along Earth’s orbit, so option (a) is incorrect.
Option (b): “fastest at a single fixed point on its orbit” (correct)
In an ellipse, there is one unique closest point to the Sun, called perihelion. At perihelion, Earth has its minimum distance to the Sun, so by conservation of angular momentum and Kepler’s laws, its orbital speed reaches a maximum only at that single point.
Hence, Earth is fastest at one fixed point (perihelion) on its orbit, making option (b) the correct answer.
Option (c): “fastest at two fixed points on its orbit”
If Earth were fastest at two distinct points, the orbit would need some special symmetry where there are two equal “closest” positions to the Sun at the same distance and energy, which is not true for a standard elliptical orbit. An ellipse has exactly one perihelion, so there is only one point of maximum speed, not two.
Thus, option (c) contradicts the geometry of an ellipse and Kepler’s laws and is incorrect.
Option (d): “slowest at two fixed points on its orbit”
Earth is slowest at aphelion, the single farthest point from the Sun, where its gravitational potential energy is highest and kinetic energy (and thus speed) is lowest. As with perihelion, an ellipse has only one aphelion point, not two separate, equal “farthest” points.
Therefore, Earth is not slowest at two fixed points; it is slowest at one fixed point (aphelion), so option (d) is also incorrect.
Key SEO points in the introduction
The speed of the Earth orbiting the sun changes because Earth follows an elliptical path and not a perfect circle. Earth’s orbital speed around the sun is maximum at perihelion (closest point) and minimum at aphelion (farthest point), so exam questions asking where it is “fastest” are testing Kepler’s second law and the idea of varying orbital velocity.
