Last updated 18th January 2014

Speed Of Light Challenge - Submit Your Data

On this page you can submit the results that you obtained for Io. (Our "Speed Of Light" page contains the full details of this challenge). We will next display a page which will give you an "instant" calculation of the speed of light based on your results. There's no need to keep a copy of your input - it will all be displayed on the next page, which you can print. We will also email you a copy.

Please select the Io reappearance event you observed from the drop-down entitled "event". Predicted times are GMT. As it is after opposition during the period of this challenge it is only possible to observe eclipse reappearance events.

We will be publishing a list of results on this website after National Astronomy Week has completed.

The "Speed of Light Challenge" allows readings to be submitted for the period 1st February to 31st May.

Please enter the requested information below and then click SUBMIT. All details are required - the notes below the form give more information.

Please check your email address very carefully!

Email address
Equipment details
TypeReflector     Refractor
Timing method
Observations (Io only)
Please enter one or more results (up to five readings are possible)
 Event date/time (predicted)Time (hh:mm:ss)
Reappearance 1Observation
Reappearance 2Observation
Reappearance 3Observation
Reappearance 4Observation
Reappearance 5Observation

To return to the NAW home page click return home.


  1. When you enter a result for Io, we will calculate the speed of light for you, based on your result. This is only an approximate result to an accuracy of about 2%. The estimate is based on knowledge of the distance the light travelled and the actual time of the event, and will provide some rapid feedback on your observation.
  2. We have listed all event predictions with an approximate time, including ones in daytime in UK - in order for people elsewhere in the world to take part.
  3. Predicted times are in GMT, and your observations must also use GMT.
  4. Please first select the event you observed from the drop-down, and then enter your observed time to the closest second. The format is hh:mm:ss with leading zeros required. Times more than about 5 minutes outside the prediction will be rejected as incorrectly keyed.
  5. You should report the instant when you first detect the reappearance of Io, not the mid-point. Tests have shown this is possible about 1.5 minutes before the mid-point, and our calculations are based on first detection time.
  6. You are welcome to just submit a single result per form. If you have more results to submit later, please just submit a second form.
  7. If you have made observations of other moons and wish to submit them, you can submit them at http://www.oasi.org.uk/Obsvns/JoP/Report_obs.shtml

We are running the "Speed of Light Challenge" in partnership with the Orwell Astronomical Society, who have provided the data and helped develop this form.