Question

A light-year is a unit commonly used in astronomy to describe the distances between stars and galaxies. What physical quantity does the light-year actually measure?

Hint:

A light-year is the distance light covers in one year, so it measures length.

Answer 1

A light-year is a unit of distance, not time, even though the word year appears in its name.

Step 1: Definition.
A light-year is defined as the distance that light travels through empty space in one full year, moving at the speed of light, which is about \(3 \times 10^8\) metres per second.

Step 2: Calculating its size.
One year has about \(3.15 \times 10^7\) seconds. Multiplying this by the speed of light gives the distance covered: \(3 \times 10^8 \times 3.15 \times 10^7 \approx 9.46 \times 10^{15}\) metres, which is roughly 9.46 trillion kilometres.

Step 3: Why it is a distance and not a time.
Even though it uses the word year, a light-year measures how far light travels in that time, so the final quantity has units of length, not units of time. It is used because ordinary units like kilometres become impractically small numbers when describing distances between stars.

Step 4: Answer.
So a light-year is a unit of distance, equal to about \(9.46 \times 10^{15}\) metres.