Since we speak of light, the speed of light is used in astronomy to estimate the distance of stars, mainly stars and galaxies. Let’s take an example. You know that someone lives 200 meters from your house, but you can also say that she lives within 5 minutes of you. So you can use a length measure or the time to calculate a distance. Astronomers do the same thing with the light to measure the distance of the stars.
Thus, the nearest star, Proxima Centauri lies about 41 trillion km from the Sun. But nobody can imagine such a distance because it is much too big. Knowing that light travels 300,000 km per second, it takes 137 million seconds to reach that star. Converted into hours and years (86344 seconds in a day, 365.25 days in a year and therefore 31.5 million seconds in a year), astronomers prefer to say that Proxima Centaurus is 4.3 light years away (137 million divided by 31.5 million), that is to say that if we travel as fast as the light it would take us 4.3 years to reach this star!
Since light takes a while to reach Earth, the events you see in the sky have actually happened in the past. If the Sun exploded now, you would not see the explosion right away and you’ll have to wait 8 minutes to observe it, it’s the time it takes for the sun’s light to arrive on Earth at 300000 km / s ! Conversely, when you see the last ray of Sun disappear in the evening, in fact the Sun has already gone to bed for 8 minutes!
By observing galaxies that are clusters of stars, we discovered that they were several million light-years away, that is to say that the light of a galaxy that you see now it left several million years ago, the time of the dinosaurs! Looking at the sky in a telescope, so it’s also going back in time and looking at the past!
Surprisingly, if an alien looked at the Earth at the telescope or managed to capture our TV shows from a planet 65 light-years away, he would not see us. Since light travels with limited speed, today it only received images of the events that took place 65 years ago, around 1945. It would see the second world war, the Battle of Britain and the Normandy landings! If he had a very powerful telescope he would even see your grandparents so kids play at recess! Incredible but true ! On the other hand, our extraterrestrial will have to wait 65 years to see what you are currently doing.
The light is a real machine to go back in time, but virtually, because unfortunately we can not use it to really make a trip to the past ! With the light we can look at the past but not touch it!