Discovering the Speed of Light!


It is an interesting fact, the far away stars that we see today may not actually exist. Perhaps, their existence may have ended years back. Yet, light carries the information of those stars which is finally decoded by our retina and eureka, that’s when we see those stars. But should we believe what we see? Because what we see, may actually be past, non-existent and irrelevant today.

Lets try and unravel this wonderful phenomena and understand why and how does this happen! Our universe is vast and we still lack proper understanding of the space and its extent. We believe what we see, and we see through the medium of light. Hence it is very important to understand “Light” first.

How Speed of Light was discovered by an accident of astronomy #Physics #Science Click To Tweet

Of all the variables known to mankind, Light has the fastest speed, it is measured at 1,86,000 miles per second (as per modern estimates). Now imagine the vastness of space when someone says, “An exo-planet has been discovered 700 light years away” Which effectively means, that what we see today, is a snapshot of that planet 700 years back and not a true picture of the present day, because light took 700 years to travel from that exo-planet to reach our retina. Yes, we looked back in time!

It’s the sheer vastness of space, huge distances that separate these two bodies and most importantly the “snail pace” speed of light (1,86,000 miles per second) which lead to this wonderful phenomena.

The speed of light was assumed to be infinite until 1676, and it was by the keen observation of Io (one of Jupiter’s moon) by the Danish astronomer Ole Roemer that the speed of light was first measured.

Ole Roemer
Ole Roemer

Roemer, working at the Paris Observatory, was not looking for the speed of light when he found it. Instead, he was compiling extensive observations of the orbit of Io, the innermost of the four big satellites of Jupiter discovered by Galileo in 1610. By timing the eclipses of Io by Jupiter, Roemer hoped to determine a more accurate value for the satellite’s orbital period. Such observations had a practical importance in the seventeenth century. Galileo himself had suggested that tables of the orbital motion of Jupiter’s satellites would provide a kind of “clock” in the sky. Navigators and mapmakers anywhere in the world might use this clock to read the absolute time (the standard time at a place of known longitude, like the Paris Observatory). Then, by determining the local solar time, they could calculate their longitude from the time difference. This method of finding longitude eventually turned out to be impractical and was abandoned after the development of accurate seagoing timepieces. But the Io eclipse data unexpectedly solved another important scientific problem—the speed of light.

The orbital period of Io is now known to be 1.769 Earth days. The satellite is eclipsed by Jupiter once every orbit, as seen from the Earth. By timing these eclipses over many years, Roemer noticed something peculiar. The time interval between successive eclipses became steadily shorter as the Earth in its orbit moved towards Jupiter and became steadily longer as the Earth moved away from Jupiter on its orbit around the Sun. From his data, Roemer estimated that when the Earth was nearest to Jupiter (at E1), eclipses of Io would occur about eleven minutes earlier than predicted based on the average orbital period over many years. And 6.5 months later, when the Earth was farthest from Jupiter (at E2), the eclipses would occur about eleven minutes later than predicted.

Roemer - Speed of LIght

Roemer knew that the true orbital period of Io could have nothing to do with the relative positions of the Earth and Jupiter. In a brilliant insight, he realized that the time difference must be due to the finite speed of light. That is, light from the Jupiter system has to travel farther to reach the Earth when the two planets are on opposite sides of the Sun than when they are closer together. Romer estimated that light required twenty-two minutes to cross the diameter of the Earth’s orbit. The speed of light could then be found by dividing the diameter of the Earth’s orbit by the time difference.

Christiaan Huygens
Christiaan Huygens

The Dutch scientist Christiaan Huygens, who first did the arithmetic, found a value for the speed of light equivalent to 131,000 miles per second. The correct value is 186,000 miles per second that we know today with the help of modern tools like laser. The difference was due to errors in Roemer’s estimate for the maximum time delay (the correct value is 16.7, not 22 minutes), and also to an imprecise knowledge of the Earth’s orbital diameter. More important than the exact answer, however, was the fact that Roemer’s data provided the first quantitative estimate for the speed of light, and it was in the right ballpark.

It was the passion with which Roemer did his work that lead him to this accidental discovery. So, when we talk about distances in light years, we owe it to Ole Roemer.

Godt Klaret Ole Roemer!

Featured Image Source: FLickr

%d bloggers like this: