Astronomy Practice Exam

How far does light travel in one year, which defines a light year?

1. The distance of a year on Earth

2. The distance light travels in one year

3. The distance between the Earth and the Moon

4. The distance light travels in a day

The correct answer is: The distance light travels in one year

A light year is a unit of distance that reflects the distance light travels in one year in a vacuum. Light moves at a speed of approximately 299,792 kilometers per second (186,282 miles per second). When this speed is multiplied by the number of seconds in a year—specifically, about 31.56 million seconds—one finds that a light year amounts to roughly 9.46 trillion kilometers (or about 5.88 trillion miles). The definition of a light year is critical in astronomy as it helps to express vast distances in the universe. It serves as a standard measurement when discussing the distance to stars and galaxies, allowing astronomers to effectively communicate the immense scale of space we observe. The other options do not accurately reflect the definition of a light year: one year on Earth refers to the orbital period of the Earth around the Sun, while the distance between the Earth and the Moon is much shorter than a light year. The distance light travels in a day is also significantly less than a light year and does not convey the concept of distance measured over a year. Therefore, the most precise and correct understanding of a light year is indeed the distance light travels in one year.