Sirius is the brightest star in the night sky. Its light reaches us after a journey across time and space.

From our perspective, light travels very fast at about 186,000 miles (299,791 kilometers) per second. We don't usually notice this because everything on Earth is relatively close. But it was noticeable when the astronauts went to the moon. Their radio transmissions took about a second and a half to reach us. This is because radio signals travel at the speed of light, and the Moon is about 250,000 miles (402,336 kilometers) away.

The Sun is much farther away. It is about 93 million miles (147 million kilometers) from the Earth. The Sun is so far that it takes about 500 seconds, or a little more than 8 minutes, for its light to reach us.

Distances between the stars are so vast that astronomers don't usually talk about distances in conventional units like miles or kilometers. The numbers are so big that they are hard to read with so many zeros behind them. So we usually talk about interstellar distances in units of a "light year".

A Light Year is a Measure of Distance

The term "light year" is a little confusing at first because it sounds like a measure of time. But it is really a measure of distance. It is the distance that light travels in a year. A light year is nearly 6 trillion miles or 9 trillion kilometers.

The nearest star to the Sun is called Proxima Centauri. It is 4.3 light years away. That means it is so far away it took the light 4.3 years to reach us. This is about 24 trillion miles (38.6 trillion kilometers).

Sirius, the brightest star in the night sky, lies at a distance of about 8.6 light years (48 trillion miles or 77 trillion kilometers), or twice as far away as Proxima Centauri.

Distances in Space - The Bottom Line As we view celestial objects, we are not only looking across vast distances in space, we are also looking back in time. The Andromeda galaxy is about 2.5 million light years away. That means it took 2.5 million years for the light to get here. So we are not seeing it as it looks now, but as it looked 2.5 million years ago.