Which statement correctly defines a light-year?

Which statement correctly defines a light-year?

• A. The distance light travels in 365 days
• B. The time it takes light to travel across the solar system
• C. The distance light travels in one year
• D. The angular separation of stars

Answer: (C)

Light-year measures distance, not time. It is the distance light travels in one year in vacuum. Since light moves about 299,792 kilometers each second, and a year contains roughly 31.56 million seconds, that distance is about 9.46 trillion kilometers (roughly 5.88 trillion miles). So the statement that defines a light-year is the one describing the distance light travels in one year. The other ideas don’t fit because one talks about a fixed number of days, which is imprecise since a year isn’t exactly 365 days; the standard uses a year’s worth of time. Another describes how long it takes light to cross the solar system, which is a measure of time and depends on where you start and end. The last refers to angular separation, which is an angle, not a distance.

Light-year measures distance, not time. It is the distance light travels in one year in vacuum. Since light moves about 299,792 kilometers each second, and a year contains roughly 31.56 million seconds, that distance is about 9.46 trillion kilometers (roughly 5.88 trillion miles). So the statement that defines a light-year is the one describing the distance light travels in one year.

The other ideas don’t fit because one talks about a fixed number of days, which is imprecise since a year isn’t exactly 365 days; the standard uses a year’s worth of time. Another describes how long it takes light to cross the solar system, which is a measure of time and depends on where you start and end. The last refers to angular separation, which is an angle, not a distance.