Right now, Earth is changing speed. Every January it moves along its orbit about 3% faster than it does in July. Nobody steers it.
This lesson states Kepler's second law: how a planet's speed changes around its ellipse.
Take the line from the Sun to a planet. As the planet moves, that line sweeps out an area, the way a windshield wiper sweeps a windshield.
Near the Sun, that line is short. A short line sweeping the same area in the same time must swing through more of the orbit. So the planet covers more of its orbit each day when it is near the Sun: it moves faster there.
Far from the Sun, the line is long. The same area needs less orbit covered, and the planet moves slower.
Over any two equal times, how do the areas swept by the Sun-to-planet line compare?
Where on its elliptical orbit is a planet's speed greatest?
Earth obeys the law: nearest the Sun in January, Earth moves fastest; farthest in July, it moves slowest. The law holds for every orbiting body: planets, moons, and comets alike.
An asteroid's elliptical orbit brings it near the Sun each spring. What does Kepler's second law say about its speed then?
Near the Sun, the Sun-to-asteroid line is short.
Sweeping equal area in equal time, a short line must swing through more of the orbit.
The asteroid moves fastest when it is near the Sun.
Osei says Earth moves fastest along its orbit in January, at its closest to the Sun. Is he correct?
The diagram shows a planet's current position on its orbit around a star. How does its speed there compare with its speed elsewhere on the orbit?
Near the Sun, the Sun-to-planet line is short. To sweep the same area in the same time, what must the planet do there?
Which statement is Kepler's second law?