European Science & Technology Week 2004Visit the websites of the VT-2004 organisers
European Southern Observatory (ESO)European Association for Astronomy Education (EAAE) Observatoire de ParisInstitut de Mécanique Céleste et de Calcul des Éphémérides (IMCCE)Astronomical Institute of the Academy of Sciences of the Czech Republic

Observing the Venus Transit 2004

.... Timing Hints!


This page provides observers, in particular those who participate actively in the VT-2004 Observing Campaign, with useful hints for making useful timing measurements.

We assume that you have familiarized yourself with how to observe the Sun, with your telescope(s) and instrument(s) and, of course, how to make safe observations of Venus in front of the Sun.

Now you want to make timing measurements that are useful for the calculation of the Astronomical Unit (AU). The better their accuracy, the more precise will be the value of the AU that is calculated on the basis of your timings and the more they contribute to getting the final value deduced by combining the timings made by all the VT-2004 observers right.

You will find more details about how to observe here and also about the subsequent use of the observations on this webpage.

Observations of the four contacts

The observation consists in the determination of the best possible (most accurate) timing of the contacts between Venus and the limb of the Sun. Be careful, the timings must be made in Universal Time (may be obtained by means of GPS, a phone clock or by a radio signal). It is not necessary to modify your watch, just note precisely the difference between the time of your watch and the Universal Time. Please note that you should aim at doing the measurement with an accuracy that is equal to or better than one second of time.

If at all possible, all four contacts should be timed :

  • The first contact is the first exterior contact (indicated as "1" on Figure 1) between Venus and the limb of the Sun. In reality, this contact is nearly impossible to observe because, when you are sure that you see Venus at the edge, it is already too late! In practice, only observations made with an H-alpha filter and recorded on images (video) may allow to measure the first exterior contact with a reasonable precision.
  • The second contact is an interior contact ("2" on Figure 1) between Venus and the limb of the Sun. This one is easy to observe; as soon as Venus enters the solar disk, be prepared to measure the instant of the second contact. It will take about 20 minutes for Venus to move from the first contact to the second contact, so there is plenty of time. However, you must pay attention to the "black drop" phenomenon. This effect is due to the diffraction of light which causes Venus to be "stuck" to the limb of the Sun, as if with some kind of chewing-gum! This is illustrated in Figure 2. The "black drop" phenomenon is further explained on a special page that also contains hints on how to improve the accuracy of your timing despite this problem.
  • The third contact ("3" on Figure 1) is another interior contact between Venus and the solar limb. It is the easiest to observe of all four contacts, since you will have been following Venus as it moved across the solar disc. However, do again pay attention to the "black drop" effect!
  • The fourth contact ("4" on Figure 1) is the final exterior contact between Venus and the solar limb. Also this one is easy to observe as the moment when Venus definitively leaves the solar disk.

Useful methods to measure the times of contact

Note that you may profit by using the STOPER software [click to download the ZIP-file(704k)], developed by Arkadiusz Dudka, active member of the VT-2004 programme in Poland. This programme mimics a traditional stopwatch and allows making precise time measurements with a PC. A good accuracy of the measurements is then possible, thanks to synchronization of a local computer clock with a Time Server via the Internet. The time synchronization is based on Simple Network Time Protocol (SNTP) with an accuracy of at least 50 ms. All labels, texts and the help are in English. This software may be distributed as FREEWARE, provided the author and VT-2004 are credited.

a) The visual method

You observe Venus - on a projected image of the Sun or through a telescope with the proper solar filters - and try to determine the time of a contact. When you think the moment has arrived, you either press a button to record this time or you make a signal to your assistant who can then accurately read the time (to one second of time) on a time keeper or a clock. The measurement of a timing in this way will be delayed by about 0.1 - 0.2 seconds because of your "personal equation" (the astronomical expression for your reaction time when recording an event). This delay will not matter for these observations, but if you want, you may determine it independently of the observations by some simple experiments.

b) Recording of images

If you have a video camera, a CCD camera or a webcam, you may record a series of images which must all be timed to Universal Time or another timescale with a known time difference from the Universal Time. Subsequent inspection and analysis of such images will allow you to determine the times of contact by interpolation to a good accuracy.

Observation of the smallest distance from Venus to the solar limb

If you record images which are well "linked" to Universal Time, all along the transit of Venus, they may be used to compute the Earth-Sun distance by another method than by observing the contacts. Therefore, even if you were so unlucky to have bad meteorological conditions at the beginning or at the end of the transit of Venus, you will still be able to participate in and contribute to the computation of the Earth-Sun distance.

For this purpose, your images must allow to measure the distance between Venus and the Solar limb at a given instant (see Figure 3). The edge of the Sun and Venus must be simultaneously recorded on your images, even if Venus is quite far away from the edge (see Figures 4 and 5).

In this connection, you will find useful the VT-2004 Basic Image Processing Pipeline that is available to everybody! We have created an easy-to-use facility at the Ondrejov Observatory (The Czech Republic) where you may submit your images and have a variety of well documented operations performed on them. The results are immediately displayed on your screen.

If you carry out this kind of observations, you may send your images for analysis to the VT-2004 organisers. However, before you submit your images, please send first an e-mail to in order to receive the detailed instructions. In this case, you will be asked to agree that the VT-2004 programme will keep your images - which will be transferred into a standard format - in a data base in order to be used later for educational purposes. As a contributor, you will be given access to the entire data base.


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