The moons of Jupiter and the speed of light

Recently, I was setting up my telescope to image the great planet Jupiter. I was interested in capturing an eclipse of one its largest moons, Io. Everything was ready, all the batteries were charged, the telescope was aligned and tracking the planet, but there was a problem. The eclipse just wasn’t happening. My computer programme predicted it to start at 21:10 on the 12th March 2017, but nothing happened. I was more than surprised, my computer is normally accurate to the second. So I checked the settings, the time is internet controlled so no problem there, the computer showed other stars in their correct positions so I knew it was not having problems with other parts of the sky. Then at about 21:48, Io started to cast a dark circle on Jupiter. I was amazed, I have never seen a total eclipse on Earth but I can now see one on Jupiter. But why was it more than 30 minutes late? It turns out that my confusion was shared by astronomers in the 17th century and, in an effort to explain the discrepancies of Io’s eclipse times, they inadvertently measured the speed of light.

It was the 17th century astronomers Giovanni Domenico Cassini, Ole Rømer and Jean Picard (not from Star Trek) who first studied the eclipses of Io on Jupiter whilst trying to solve the famous longitude problem: before the invention of accurate clocks, there was little way of knowing how far east or west you were sailing from a given location (normally Paris or London). Galileo himself proposed to use the predicable orbits of Jupiter’s moons to calculate the time on Earth, which can then be used to calculate longitude.

Ole Rømer (left) and Giovanni Cassini (right). Along with Jean Picard these pioneering 17th century astronomers observed and studied hundreds of Jovian eclipses. (Wikipedia Commons)

Unsurprisingly, this proved too difficult a task to do on a moving ship with the primitive optical equipment available at the time. On land, however, this method could be used to improve maps and navigation. So Cassini and Rømer set to work. They observed hundreds of Jovian eclipses over several months and were able to determine the difference in longitude between Paris and their location. Unfortunately, there was a problem; after accurately calculating the orbit of Io, Cassini found that sometimes during the year, eclipses were occurring earlier while at other times eclipses  happened later than predicted. Cassini logically surmised that light had to travel at a finite speed instead of instantaneously spanning the distance from Jupiter to Earth. For instance, when the Earth and Jupiter are on near opposite sides of the Sun, the light traveling from Jupiter will take longer to reach Earth (around 54 minutes). This causes the Io eclipses to appear delayed. When the Earth is between the Sun and Jupiter (a period called Opposition), then light from Jupiter takes only about 37 minutes to reach Earth making eclipses of Io happen earlier than expected.

An eclipse of Io imaged by my myself on 12-13/03/2017. The Io eclipse cases a dark spot on Jupiters northern cloud band. The delay of this event caused by the speed of light prompted me to write this post! (My own work)

Strangely, Cassini never followed up his discovery, Rømer continued observing and recording Io eclipses and defined an equation that related the delay caused by the speed of light to the angle between Earth and Jupiter. However, it would not have been possible to publish an actual speed of light because the distances between the planets were not known then. Interestingly, Rømer could have shown the speed of light as a ratio of Earth’s orbital speed…but for some reason he didn’t. It was another famous astronomer, Christian Huygens, who took that credit. He used Rømer’s detailed observations and formula to define the speed of light as 7600 times faster than Earth’s orbital speed.  This equates to a speed of 226328 km/s which is only 25% lower than the true value of light speed.

Christian Huygens, a leader in 17th century science. He was the first person to define the speed of light using the eclipses of Io. (Wikipedia commons)

This was the first time a universal constant had been calculated quantitatively and since then the speed of light has played a huge role in James Clerk Maxwell’s theory of electromagnetism and Einstein’s theories of relativity. But for anyone peering into the night sky, the work of these great men more than 300 years ago shows us that starlight is old…and by looking at it we are looking back in time. We see Jupiter as it was 40-50 minutes ago, the nearest star 4 years ago and the relatively nearby Andromeda galaxy 2.56 million years ago. Not bad for 17th century science.

I think next time I’m sitting by my telescope waiting for an Io eclipse, I’ll be a bit more appreciative of the significance that 30 minute delay had on our understanding of the universe.

Post by: Dan Elijah.


I come in peace: Engaging life on a flat Earth

Did you know that the Earth is actually flat, not round and that NASA and the government fuel the round Earth conspiracy?….No, neither did I but this mind-boggling world view is currently gaining momentum on the internet and has recently found its way onto my radar.

To give you a bit of background:

Alongside my vociferous online academic rantings and day job helping researchers and the lay public work together to design and implement health research, I also spend a fair bit of time volunteering with the British Science Association (the BSA). The BSA is a charity and learned society founded in 1831 with many strings to its academic bow; including the standardisation of electrical units (including the Ohm, Volt and Amp). Today it is supported by a huge backbone of volunteers working tirelessly across the country to improve the public perception of science – letting everyone know that there is much more to science than just mind boiling equations and stuffy white haired professors.

Our small group of Mancunian volunteers meet monthly to mastermind and implement a huge range of engagement activities. Over the years I’ve been with the group I’ve found myself designing an endangered species treasure hunt (based on a mash-up of Pokemon Go and geocashing), baking cake pops for an astronomy and art crossover event held on the site of Manchester City centre’s oldest observatory and, just last week, hosting over 40 AS/A-level students at a science journalism workshop.

As a group we work hard to make sure our activities are fun and open to everyone – no matter what their academic background. But, we’re not naive, so we recognise that our reach is still pretty small and that there are many communities in our home city who will never have heard of us. This is why we have been working with a BSA volunteer from our Birmingham branch who’s role has been to help us find out more about Manchester’s hard to reach communities and discover how we can offer them meaningful engagement. It was during one of our meetings she said that she had been in contact with someone who runs a computer coding club for local teenagers and had noticed that some of these youngsters were adamant supporters of the ‘flat Earth’ theory – which is apparently backed up by a number of celebrities including rapper B.o.B who recently went on a amusing and disturbing Twitter rant about the topic.

This got me thinking. If science has never really been your thing, which is fine by the way just like P.E was never my thing, how do you avoid falling down the black hole of conspiracy theories (Illuminati, anti-vaccination, flat Earth)?

These theories offer an alternative world view which can, at first glance, appear to fit much better with the world we see and experience around us every day than the complex and often invisible world of science. Take flat Earth as a example. In our everyday lives we interact with both flat and round objects (compare a table top with a yoga ball) and, from these interactions, we build up an understanding of how these objects work. On a very basic level we see that things fall off a ball, you can’t really balance things on it like you can a table and it has an obvious curvature. Then take a look at the Earth. We can stand and walk along it with no obvious indication of its curvature, water sits flat in rivers and oceans it doesn’t run down the sides of the Earth as you would see if you spilled a glass of water onto a yoga ball. So, assuming you have little or no interest in astronomy (perhaps you live in the city center so don’t get a good view of the night sky anyway) and the mathematics of gravity and scale makes your head hurt, it’s easy to understand why you may choose to mistrust theories which you cannot test or see for yourself.

So, with this in mind, my question is: Is it possible to design activities and interactions that don’t patronise or assume knowledge but enable people to test scientific theories in ways that make sense and allow them to simply observe the outcomes with their own eyes?

We are now hoping to meet with this community, attend some of their activities, make friends and let them know scientists are just ordinary people. Then we want to jump in and put together a small accessible science festival where everyone can have fun and hopefully engage with science on a small scale. I get the feeling it’s not going to be an easy sell but will undoubtedly be worth it if done properly.

My mind is bubbling with ideas, including the possibility of sending a Go-Pro camera up on a balloon and playing back the footage – the possibilities are endless…although sadly our budget isn’t. Whatever happens, I’m excited and will keep you all updated on our progress as things move forward.

For now I want to invite anyone reading this to drop me a line in the comments below. Perhaps you’re an academic who has worked on a similar event and has some ideas, or maybe you’re keen on the flat Earth theory and want to tell us more about what you believe? Either way I’d love to hear from you.

Post by: Sarah Fox

Update: A pretty interesting gif image of a few pictures my telescope loving partner took last night showing Jupiter spinning on its axis – notice how the great red spot moves round. Perhaps we could bring our telescopes along to the festival and have a play 🙂