Rømer’s speed of light

By Matthew Allen

What’s the fastest thing you can think of? A Cheetah? A Formula 1 car? A space rocket? Well, there’s one thing which is faster than all of these and is in fact the fastest thing in the Universe: Light! It was in 1676 that the Danish astronomer Ole Rømer first measured the speed of light.

Image form Google Doodle of the day

Image from Google Doodle of the day, marking 340 since the determination of the speed of light.

When we talk about light and the speed of light, we are talking about the light you see with your eyes, such as from a light bulb or the Sun. But, we’re also talking about the different types of light that you can’t see with your eyes, such as X-Rays, Infrared and Radio waves. They’re all different types of light and they all travel at the same speed: 300 million metres per second!

It’s because light travels at a finite speed (as opposed to an infinite speed) that means it takes around 8.5 minutes for light from the Sun to reach us down here on Earth. If the Sun suddenly disappeared this second, then we would still be able to see it and the light from it in the sky for a further 8.5 minutes, before it went very dark. The speed of light and how light travels isn’t really any different to you driving a car down the road – it’s just a lot faster!

300 million metres per second sounds fast. But, how fast? Well, the NASA Space Shuttle was able to travel at around 8,000 metres per second. That’s 37,500 time slower than light! If you were travelling around the Earth at the speed of light, you would travel all the way around it around 7 and a half times… every second! So, yes, the speed of light is pretty fast!

Who is Ole Rømer and why is he the Google Doodle of the day?

Ole Rømer (By Jacob Coning (circa 1647–1724) - Frederiksborg Museum, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11617410

Ole Rømer (By Jacob Coning (circa 1647–1724) – Frederiksborg Museum, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11617410

Good question! You’ve probably heard of the scientists Isaac Newton, Galileo Galilei and Albert Einstein, but you might not have heard of the Danish astronomer Ole Rømer. He was born in 1644 and is famous for inventing the modern-day thermometer as well as the inventing the first street lights in Copenhagen! However, the thing that Rømer is most famous for is for measuring the speed of light.

So, how did he calculate it? Well, he started with a very simple idea that the speed of light was finite, which means it has a measurable speed. Some people believed that light travelled instantly between two places, meaning that it has an infinite speed, but Rømer believed that it’s speed was less than infinity.

He studied Jupiter’s moon Io, which travels once around Jupiter every 42 hours. Io is frequently eclipsed by Jupiter as it passes into Jupiter’s shadow. The length of this eclipse could be measured very accurately, and if light travelled infinitely fast, then that length of the eclipse would be the same with every time. However, as the Earth orbits around the Sun, sometimes it is moving towards Jupiter during Io’s eclipse and sometimes it is moving away from Jupiter. Rømer discovered that when the Earth is moving towards Jupiter, the length of the eclipse was shorter, as the light from Jupiter (which is sunlight reflected off Jupiter) had a shorter distance to travel. On the other hand, when the Earth was travelling away from Jupiter, the eclipse took longer as the light had further to travel.

Rømer had proved that the speed of light was finite and took a certain amount of time to travel from point A to point B! He had also been able to estimate how fast the speed of light was, which was around 220 million metres per second, about 25% slower than the real value. However, it was a pretty good estimate, considering he did it in 1676 based on the positions of one of Jupiter’s moons!

Why would we want to know how fast light travels?

Albert Einstein was very interested in the speed of light. He published some work called The Special Theory of Relativity. In it he showed that the fastest that anything can travel is the speed of light. That means nothing, not even the Universe itself, can travel faster than the speed of light. Even gravity, the force which holds our Solar System together, only travels at the speed of light!

The actual value of the speed of light, which scientists often refer to by the letter ‘c’, is used in many areas of physics and astronomy. In fact, it’s so important that the length of a metre and the length of one second are defined from the speed of light. The speed of light pops up everywhere in physics, so it’s worth knowing what it is!


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