The Venus Transit 2004 European Science & Technology Week 2004Visit the websites of the VT-2004 organisers

Venus Transit 2004

... What Do We Do With the Observations?


Many observers will be satisfied by sending their observations to the VT-2004 Observing Campaign interface site. Here they will be used - together with all the others received - to determine the distance to the Sun (the Astronomical Unit). Timings of the contact will be numerous, and some observers will also make images.

Note also the possibility to form partnerships with other observers in order to exchange data and do individual calculations of the Astronomical Unit, as explained on a separate page "Observing Partners".


We invite all observers to carry out timings of the Venus transit. These observations are relatively easy to perform. Accurate timings (in Universal Time) of the four contacts of Venus with the solar limb will be used to compute the Earth-Sun distance, that is to deduce the Astronomical Unit (AU). How to obtain timing observations which are useful for this purpose is explained on a special webpage with Timing Hints.

The observers who participate in the VT-2004 Observing Campaign must be registered at the registration site in order to be able to send their observations and to receive the results online. Be careful, however, when you register. One observation is considered to be a series of up to four timings (i.e., the four contacts T1 to T4). Remember that one registration only allows you to deposit one observation. Therefore, observers who use several telescopes must register once for each instrument.

It is very likely that many observers will only manage to "time" one or two of the four contact timings, most probably T3 and T4 which are the easiest to catch. There may also be clouds in some places at the moment of some of the timings. Still, all observations are important for the computation of the Earth-Sun distance. So do send in the timings, also if you only obtain one timing!

Assume that you are a registered observer. Once you access the data acquisition facility at the registration site, you will be asked to check your geographical coordinates, and you will get a map with your location in order to check that they are indeed correct. You will be able to correct them if necessary. Then you will be asked to deliver your observed contact timings. If you then ask for the calculation, the following actions will be performed at the VT-2004 computation site:

  • your data will be registered in a database
  • by means of a fast iterative process, using the best available theories of the motions of Venus and the Earth, those values of the Astronomical Unit will be computed that best correspond to each of your contact timings.
  • you will be informed about the "observed" Astronomical Unit and the corresponding Solar Parallax for each of the contacts you timed. Moreover, the average value will be computed if you observed several contacts, as well as the difference from the true values.
  • two diagrammes will then be drawn online in order to show you how different your observed value(s) are from the true value of the Astronomical Unit, and how far they are from the global average value, as computed by using all data deposited by the observers up to this moment.
  • an e-mail will be sent to you with your personal parameters, your observational data, the corresponding results obtained and an acknowledgement to confirm the registration of these data.


A dedicated webpage will be set up which will show the progress of the VT-2004 Observing Campaign. Each personal contribution will be taken into account to produce various statistics. On this page will be displayed several diagrammes in order to indicate the number of participants who have delivered data, the number of observations, the average and dispersion of the observed values of the Astronomical Unit, the difference from the true value, etc.


Many observers will have cameras and will record images during the transit. If they show both Venus and the nearby solar limb (for details, please consult the Timing Hints' webpage), they may be used to measure the shortest distance from the border of the Venus' black shadow to the limb of the solar disc. If the corresponding timing (when the photo was taken) is accurately known in Universal Time, the measured shortest distance can be used like a contact timing to compute the corresponding value of the Astronomical Unit, although with less accuracy.

It is the intention to build an observational database with such "accurately timed" images which will then be deposited at an astronomical data center. If you are interested in participating in this additional activity, please look at the explanatory page set up for this purpose. Participating observers will be asked to agree that these images will be used for an educational project in 2005. They will also be given access to the entire database with these images.

What happens after the Day of the Transit (June 8)?

We will probably not receive all the observational results on the Day of the Transit. The gathering of all observational data will probably take some days and the registered observers will be kept informed about the progress. Starting from June 8, all registered participants will receive regular reports on the status of the VT-2004 Observing Campaign the results obtained for the determination of the Astronomical Unit.

Reduce your own data

Some may like also to do their own calculations, to obtain their own result(s) based on their own observations. While these will most certainly not be so accurate as the "combined" result, the possibility to "reduce your own data" (as astronomers say), will be both interesting and instructive. For this, we suggest that you consult Extended InfoSheet B4.

Access the timings data base

After the Day of the Transit, registered observers will be given access to the entire set of observational data in the timings database (but NOT to the associated personal data!) . By combining the timings obtained at different geographical sites the observers will then be able to compute themselves the Astronomical Unit, for instance by combining their own observations with those from other sites, e.g. by the method explained in Extended InfoSheet B4, or by means of these forms.


If you have questions in this connection, please do not hesitate to contact the organisers via email at