The astronomers Bradley, Fizeau and Roemer created a series of procedures to set the speed of light. This was established at 299 792 kilometers per second in space, and is lower when it comes to measuring it in another medium (air or water). Thus, when viewed in light of a star, he does not look that has at the moment but that was at the time of issue it, perhaps hundreds of thousands of years ago. For example, sunlight takes eight minutes to reach the ground but the light of the Andromeda galaxy takes 800,000 years. It is for this reason that the science of astronomy uses the light-year as a measure of length. That is, a light-year is the distance light travels in one year, 299 792 kilometers per second.
The Sun, the center of the planetary system, is a luminous star, while Venus or the moon reflecting the sun and its light is only apparent.
Many objects can be seen because they have their own light as, for example, flames or stars. To cast light on the bodies have to be subjected to high temperatures.