To look at the sky is to see the past
To look at the sky is to see the past of celestial bodies, as light takes a certain time interval to leave the observed objects and reach the Earth.
As much as the images of the various celestial bodies were captured recently, they do not reveal the current state of the object. Some show the shape that the body under analysis had millions of years ago.
Speed of light
All electromagnetic waves have the same propagation speed if the medium is a vacuum. This speed is incredibly large and equates to 300,000 kilometers per second. In material media, light has its speed reduced . Such reduction occurs according to the characteristics of the material in which there is propagation.
The light year is a unit of distance used in astronomy and refers to the distance traveled by light in the time interval of one year . Due to the enormous distances between the Earth and the countless celestial bodies in the universe, this unit of measurement is more conventional than the units kilometer (km) or meter (m).
1 light year ≈ 10 trillion km
look at the past
We can only see a certain object if the light, produced or reflected, leaves the object and, reaching our eyes , stimulates them. Therefore, we can understand that light will always spend a certain time interval to leave the observed body and reach the observer.
The distances between the heavenly bodies and the Earth are so great that the time spent by light cannot be disregarded. Thus, the images captured by telescopes are not current, but represent the state of the observed object taking into account the time of arrival of light. The image of an object 5,000 light-years from Earth represents the state of that object 5,000 years ago!
Celestial objects and their distances
The following images are of some celestial objects. The distance of each of them in relation to the Earth, represented in light years, corresponds to the time required for the journey made by light.