Question

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a year is the amount of time it takes a planet to orbit the sun. if earth is 1 astronomical unit (au) away from the sun, and neptune is 30 astronomical units (au) away from the sun, how does the length of a year on neptune compare to a year on earth?
a year is the same amount of time for all planets.
a year on earth is shorter than a year on neptune because earth is closer to the sun.
a year on earth is shorter than a year on neptune because earth is smaller than neptune.
a year on neptune is shorter than on earth, since neptune is bigger and rotates like the sun faster

Explanation:

The time it takes for a planet to complete one orbit around the Sun is its year. According to Kepler's third - law, the farther a planet is from the Sun, the longer its orbital period (year). Earth is closer to the Sun than Neptune (1 AU for Earth and about 30 AU for Neptune). So, Earth has a shorter year than Neptune.

Answer:

A year on Earth is shorter than a year on Neptune because Earth is closer to the Sun.