What is a light-year, and why is it not a measure of time?

What is a light-year, and why is it not a measure of time?

• A. A light-year is the time it takes light to travel one year.
• B. A light-year is the distance light travels in one year, about 1 billion kilometers.
• C. A light-year is the distance light travels in one year; about 9.46 trillion kilometers.
• D. A light-year is a measure of light's energy per year.

Answer: (C)

A light-year measures distance, not time: it is the distance light travels in a vacuum in one year. At the speed of light, about 299,792 kilometers per second, that distance is roughly 9.46 trillion kilometers (about 5.88 trillion miles). This unit helps describe the vast scales of space without using ridiculously large numbers of kilometers or years. The other statements mix up the idea: one describes a time interval, another underestimates the distance light travels in a year, and one talks about energy, which isn’t how a light-year is defined.

A light-year measures distance, not time: it is the distance light travels in a vacuum in one year. At the speed of light, about 299,792 kilometers per second, that distance is roughly 9.46 trillion kilometers (about 5.88 trillion miles). This unit helps describe the vast scales of space without using ridiculously large numbers of kilometers or years. The other statements mix up the idea: one describes a time interval, another underestimates the distance light travels in a year, and one talks about energy, which isn’t how a light-year is defined.