Speed of Light

The Speed of Light challenge is now complete.  This observing project was an effort to estimate the speed of light by observations of eclipses of the Galilean satellites of Jupiter, in the spirit of Ole Rømer (1644-1710), who used the technique to demonstrate that the speed of light was finite. (See below for more details).

Picture of Ole Romer
Ole Rømer

To encourage observations, NAW established a website, which went live in January 2014, containing predictions of eclipses and hosting a web form to enable users to report their observations for analysis. The predictions of eclipses spanned the period from the start of NAW 2014 (1st to 8th March 2014) through to the end of the apparition of Jupiter, in late May 2014.

The project was designed to be accessible to all astronomers, from those with very modest telescopes and basic naked eye observing skills through to advanced amateurs with sophisticated imaging equipment. Despite the hoped-for wide appeal of the project NAW 2014 attracted results from just two groups of observers, in both cases doing so as a continuation of projects that had begun before NAW 2014. The observers were affiliated to the Orwell Astronomical Society, Ipswich (OASI) and to the Hampshire Astronomical Group (HAG).  We were disappointed that there was not more participation in what we had hoped would be an observing project with wide appeal.

Nevertheless, from the start of NAW to the end of the apparition of Jupiter, 10 observers reported 32 observations; when added to previously-reported data, the corresponding totals are 17 observers and 119 observations. Analysis of the observations is proceeding. Preliminary results (to be confirmed) show generally encouraging estimates of the speed of light when full orbital details of the Galilean satellites are factored into the analysis through use of an accurate ephemeris. However, a much more insightful approach is obtained by adopting the methods of analysis used by Rømer and colleagues in the late 17th century, assuming only basic knowledge of the orbital dynamics of the bodies concerned. This reveals the effects of the various major factors at play in determining the times of observed eclipse phenomena and shows a major dependency on synodic effects associated with Jupiter’s orbit, in a most surprising way.

We have been most grateful to James Appleton of the Orwell Astronomical Society for his support with this project; he has now produced a very comprehensive report on the project, which is available on the OAS webstie here: http://www.oasi.org.uk/Obsvns/20140518_Romer_revisited.pdf

Details of the Challenge: (this section has not been updated).             Click here to submit your results!

National Astronomy Week 2014 is challenging individuals and groups to re-enact one of the most important discoveries in astronomy, no less than measuring the speed of light!  But how can such a feat be achieved outside of a scientific laboratory?

Everyone has heard about the speed of light, a fundamental constant in studying the nature of the universe.  Light travels at some 300,000 kilometres (186,000 miles) per second which, as far as we know, is the fastest speed that anything can ever travel anywhere in the universe.

The first person to measure the speed of light was Danish astronomer Ole Rømer, back in 1676, albeit only in terms of the diameter of the solar system, which was not known at that time.  Rømer noticed that observed eclipses of Jupiter’s four Galilean moons, when they passed in and out of Jupiter’s shadow, were wrong according to predicted timings.  From this, he brilliantly concluded that the speed of light must be finite (for details take a look at our Jupiter’s Discoverers’ page.)  The critical part was to work out the value.

That is where you come in.  NAW would like your help in repeating Rømer’s experiment. We want you to observe the eclipses of the Galilean moons this year and send in your observations.

The key to the challenge is to time when the moons pass into the shadow of Jupiter and when they emerge again.  Past experience has shown that the innermost one, Io, is the best choice to observe because it whizzes around Jupiter once every 42 hours.

You will need a telescope for this challenge and probably one of at least 125mm (5”) aperture, although a certain amount can depend on optical quality and visual acuity.  The general rule-of-thumb is that every 25mm (1”) of aperture will yield a maximum of 50 times magnification, so a 125mm telescope will comfortably give 200+ times magnification, enough to resolve the Galilean moons as discs rather than dots.  This is an important aspect as it enables greater accuracy.

During 2014, Jupiter is at its nearest to Earth in January and furthest away in July when it passes behind the Sun as seen from Earth.  As the distance between Earth and Jupiter increases, it takes light longer and longer to cross the intervening space.  The differences are small but the result is a growing discrepancy between observation timings and calculated ones.
The more timings that are made, the better the data should be.  Remember that the weather will not always be clear and observations will often need to be made in the small hours.  This means that your observations will ideally be spread over a period of months and at all hours of the night. This is going to need dedication!

All the information and details you need to attempt the challenge have been kindly provided courtesy of a team from Orwell Astronomical Society in Suffolk.  Please look at the following link:  http://www.oasi.org.uk/Obsvns/JoP/JoP.shtml

When you have made your observations, you can complete and submit our online observation form.  This form allows you to submit results for Io, and covers all possible events for the period 1st February to 31st May 2014.  When you submit your observation, we will give you an “instant” calculation of the speed of light, based on your measurements.  For results for other moons or different times, the Orwell Astronomical Society has a suitable submission page.

Your data will be used to calculate a value for the speed of light and the results published on the NAW 2014 website.

Good luck and clear skies!