That's right you read in the title, to look at the sky is to travel in time. In the universe there are thousands of stars, planets and moons and other infinity of stars. These objects are at unimaginable distances, which obviously our mind cannot process them. And for you to have an idea, not even the image of the Moon in the sky is "live", because the light reflected in it takes about 1 second to get here on Earth.
That's where we wanted to go: time. Light-years measurement is used in astronomy to determine distances between stars, since using ordinary measurements would be unfeasible by the number of numbers we would have. For example, only 1 light-year would be equivalent to 9,460,730,472,580.8 km (approximately 9 trillion). That way, the light with a speed of 300 thousand km / s in the vacuum, would take a year to cover all that mileage.
Each star you in the sky is a few tens or thousands of light-years away from Earth. The star Sirius, the brightest of the night sky, is located 8 light-years from our planet, and that means that its light takes 8 years to get from there to here. Therefore, the image we see of it is currently passed, that is, emitted 8 years ago.
But what story is this of the Moon not being live? Our natural satellite is about 380,000 km from Earth, light travels at 300,000 km per second, and so, for a photon (particle of light) to leave the lunar surface and get here, it has to travel this distance which equals almost its speed per second. The same holds true for the Sun, Jupiter, Saturn, Mars or any other star we see in the night sky. The sun, for example, the light takes 8 minutes to get here.
The next time you look at the night sky or even our Sun, remember that we are seeing a picture with delay, or rather, we are seeing the past.