Measuring the Universe

Distances in space are so great that scientists use a special unit to measure them. This unit of measurement is based on how fast light travels.

Light is the fastest moving thing we know. It takes about 8 minutes for light to travel from the sun to Earth. That’s a distance of 150 million kilometers (93 million miles). You might say that the sun is 8 light-minutes from Earth.

It takes years for the light from other stars to reach us. We use a unit called a light-year to measure the distance between Earth and those other stars. A light-year is the distance that light travels in a year.

We cannot travel to the stars. We cannot even send space probes that far away. To learn about other stars, scientists study the light and other energy that travels through space and reaches us on Earth.