A light-year is a unit of:

Although its name contains the word "year", a light-year is a unit of distance, not time. It is the distance that light travels in a vacuum in one Julian year (365.25 days).

Light travels at a speed of about 3 × 10⁸ metres per second (more precisely, 2,99,792,458 metres per second). In one year, it covers a distance of about 9.46 × 10¹⁵ metres, or about 9.46 lakh crore kilometres (9.46 trillion kilometres).

1 light-year ≈ 9.46 × 10¹² km ≈ 63,241 astronomical units.

The light-year is used to express the enormous distances between stars and galaxies, which would be inconveniently large in kilometres. For example:

The light from the Sun takes about 8 minutes and 20 seconds to reach the Earth; so the Sun is about 8.3 light-minutes away.

Proxima Centauri, the nearest star to the Sun, is about 4.24 light-years away.

The Andromeda Galaxy, the nearest large galaxy to our Milky Way, is about 2.5 million light-years away.

Our Milky Way galaxy is about 1,00,000 light-years across.

Because light from distant stars takes so long to reach us, when we look at them, we see them as they were when the light left them, sometimes thousands or millions of years ago.

Other astronomical units of distance:

Astronomical Unit (AU): the average distance between the Earth and the Sun, about 15 crore kilometres (1.496 × 10⁸ km). It is used within the solar system.

Parsec (pc): about 3.26 light-years (3.08 × 10¹³ km). It is the distance at which one astronomical unit subtends an angle of one arcsecond. It is commonly used by astronomers.

In order of size: 1 AU < 1 light-year < 1 parsec.

Other units that sound like they measure something else: a "nautical mile" measures distance at sea, and a "knot" measures speed (one nautical mile per hour).

Why not the other options?

• ✗Time is measured in seconds, minutes, years and so on.
• ✗Speed is measured in metres per second or kilometres per hour.
• ✗Intensity of light is measured in candela.