To our eyes light appears to be everywhere, instantaneously.
As I look out at the view, there seems to be no time lag and no delay while I wait for the light to reach me.
But towards the end of the 17th century, it was discovered that our senses are mistaken.
The Danish astronomer Ole Rømer (1644–1710) observed the moons of Jupiter. Working at the Royal Observatory in Paris at the time, he demonstrated in 1676 that light has a finite speed, and so doesn't travel instantaneously.
Jupiter's innermost moon, Io, is known to make a complete circuit or orbit around the gas giant (i.e. Jupiter) once every 1.77 Earth days; that's every 42 ½ hours.
The plane of Io's orbit is very close to the plane of Jupiter's orbit around the Sun. This means that it passes much of each orbit in the shadow of Jupiter – an eclipse.
Now from Earth, you can see Io disappear behind Jupiter and then re-emerge around the other side of Jupiter as it travels around in its orbit.
But in Paris in the 1660's, Giovanni Cassini had noticed that the timing of these eclipses seemed to vary; sometimes the eclipses were sooner and sometimes later than expected.
But remember that Io takes 42 ½ hours to orbit around Jupiter.
Rømer noticed that these fluctuations weren't happening at random. When the Earth was closer to Jupiter, Io would be seen to disappear and re-emerge earlier. But as the year went by, and the Earth moved in its orbit around the Sun so that the Earth was further away from Jupiter, then the eclipses appeared to happen later than expected.
Here's now it works:
The eclipses of Io appear later than expected when the Earth is further from Jupiter because light takes a longer time to cover the greater distance.
But the eclipses of Io appear earlier when the Earth is closer to Jupiter because light needs less time to reach the Earth.
|The Earth shown being closer to Jupiter during its orbit around the Sun. With this, the eclipse of Io was perceived by our eyes (or senses) to occur earlier.|
|The Earth shown being further away from Jupiter during its orbit around the Sun. With this, the eclipse of Io was perceived by our eyes (or senses) to occur later than expected.|
|Io, Jupiter's natural satellite (i.e. moon). orbiting around Jupiter. The time Io takes to orbit Jupiter is 42 ½ hours.|
Light isn't instantaneous; it travels at a finite speed.
3 x 10^8 m/s is the speed of light, that is, three times ten to the power of eight metres per second. Or 300,000 km/s, that is, three hundred thousand kilometres per second.
|The speed of light.|
Looking at a mirror one metre away, you see yourself as you were 6 nano seconds ago.
From Earth, the moon appears as it was 1 second ago, and the Sun 8 minutes in the past.
The further you look out in space, the further you look back in time.
['Light and Dark', Series 1 Episode 1, by Jim Al-Khalili]
"We will show them Our Signs in the universe, and in their ownselves, until it becomes manifest to them that this (the Quran) is the truth." (Quran 41:53)