Information sheet n°07 :


How to observe the transit in order to get a useful measure




J.-E. Arlot (IMCCE)                                             Sunday 2 May 2004







I. Introduction


In order to be able to calculate the distance Earth-Sun, i.e. the Astronomical Unit, it is necessary to have a good measurement of either the timings of the contacts Venus-Sun, or the apparent distances from the disk of Venus to the center of the disk of the Sun. You will find below recommendations for making yourself the measurements either on your observation or on the images of the phenomenon available on our databases.
You are able to calculate yourself the value of the Astronomical Unit from your measurements either using the sheet of calculation n°5 or by sending your measurements of the timings of the contacts through the specific page for automatic calculation of the astronomical unit.



II. Observation of the contacts


The observation itself consists in the determination of the precise time of the contact between Venus and the limb of the Sun. Attention, this timing should be made in Universal Time obtained thanks to GPS or thanks to the clock by phone or by any radio signal. It is not necessary to modify your watch, just note precisely the difference between your watch and the Universal Time. At last, note that the measure must be made with an accuracy better than one second of time. Four contacts have to be measured :


-         The first contact is an exterior contact (labelled 1 on the figure 1) between Venus and the disc of the Sun (near impossible to observe because, when you will see Venus, it will be too late !). Only the observations made with a Hα  filter and recorded on images could  allow to measure the first exterior contact.


-         The second contact is an interior contact (labelled 2 on the figure 1) between Venus and the limb of the Sun (easy to observe : as soon as Venus enters the disk of the Sun, be prepared to measure the instant of the second contact (it takes about 20 minutes for Venus to go from the first contact to the second contact); attention to the « black drop », an effect due to the diffraction which leads Venus to be sticked to the limb of the Sun with some kind of chewing-gum! (see  fig. 2).


-         The third contact (labelled 3 on the figure 1) is an interior contact between Venus and the Solar limb (the easiest to observe since we observed Venus moving in front of the Sun: attention to the “black drop” here also)


-         The fourth contact (labelled 4 on the figure) is an exterior contact between Venus and the Solar limb (easy to observe when Venus definitively leaves the Solar disk)


Fig. 1 Description of the contacts



III. Methods to measure the times of contact


a) Visual method


You observe Venus and try to determine the time of contact. When you think this is time either you switch on a button to record this time or you give a signal to an assistant who can accurately read the time (one second of time is necessary) on a time keeper or a clock. The measure of this time will be delayed of about 1 or two tenth of a second because of your “personal equation” (your reaction time to record an event), but his delay can be determine independently to the observation.


b) The recording of images


If a video camera, a CCD camera or a webcam is available, you can record a series of images which have to be also all referred to the Universal time or other timescale with a known shift to the Universal time. The analysis of such images will allow you to determine the times of contact by interpolation.


Fig 2  The Black Drop




IV. The observation of least distance from Venus to the Solar limb


If you record images, well referred with respect to the  Universal time, all along the transit of Venus, they could be used to compute the Earth-Sun distance by another method than by observing the contacts. Therefore, despite possible bad meteorological conditions at the beginning or at the end of the transit of Venus, you will be able to participate to the computation of the Earth-Sun distance.


For this purpose, your images must allow us to measure the distance between Venus and the Solar limb (see fig. 3). The edge of the Sun, and Venus must be simultaneously recorded on your images, even if Venus is far from the edge (see fig. 4 and 5) . If you carry out this kind of observations, you could send us your images for analysis (send first an e-mail to in order to get instructions). In this case, we also ask you to send us your agreement to keep your images – which will be transformed in a standard format – in a data base in order to be used in a next future for educational purposes. You will have access to the whole data base.

Fig. 3   Image of Venus in transit for a given time: the least distance to the edge is the observational data useful for the analysis.



Fig. 4  Unusable images : we cannot measure the least distance to the Solar limb



Fig. 5 Useful images : the least distance to the Solar limb can be measured.


Note concerning the refraction:

The refraction is an atmospheric phenomenon which reveals to us the stars in the sky higher than they are, actually. Thus, at the time of sunset, the observers still see the Sun whereas it is already below the horizon! This phenomenon of raising of the stars is as stronger as the star is lower on the horizon (i.e. stroner as the atmosphere crossed by the light is thick). Thus, a star observed at the zenith is not affected by the refraction.


We thus see that the Sun that we observe and on whom we measure theVenus distance to its center, is, actually, lower in the sky than it does appear to be. No importance, will you think, since only relative measurement is necessary. In fact, since the refraction varies according to the elevation of the Sun in the sky, and since the Sun is relatively large, the effect of the refraction is stronger in bottom of the Sun than in top! It is noted besides that at the time of the sunset, the Sun appears to be flattened: it is the differential refraction. It will thus be understood that our measurement of distance on the Sun is likely to be affected by this effect if the observation takes place when the Sun is low on the horizon. However,

the method described above allows a relative measure which is not affected by the differential refraction if some precautions are taken:


- to use the values of the theoretical diameters of Venus and the Sun non affected by the differential refraction (values given above)


- to measure the Venus diameter which is used to us as unit on the line which joins the center of the Sun to the center of Venus (Venus may have an ellipsoidal form)


- to avoid the use of an observation made when the Sun is too low on the horizon (less than 20° in height), an additional correction being then necessary, the differential effect between the top and the bottom of the Sun being too large.