Method of Contact Times: Schedule

Never look directly into the Sun through a telescope or binocular!!!!
This can damage your eye.

The equipment
The measurements
Determination of the own geographical position
Publication and Evaluation of your results
Additional activities during the transit

The equipment

Observing the transit using only a pair of solar filters will be very impressive. Nevertheless, for measuring exact contact times it will be necessary to magnify the image of the Sun. There are two safe methods of using a telescope or a binocular and one without any equipment:

  1. Observing Venus with a telescope equipped with a solar filter. The telescope's magnification factor should be at least 30. This method will yield the best results.
  2. Projecting Sun and Venus onto a white screen using a telecope or a binocular without solar filters. (Additional warning: Be careful with the second tube of a binocular!) This method will be less accurate. But the projected image will be visible for several observers!
  3. Looking to the ground under broadleef trees and catching sunspots generated by natural pinholes.

    method of projection projected image of the Sun natural pinhole image

The instrument has to be mounted on a tripod. If it is directed towards the Sun an image of the Sun can be caught on the screen behind the eye-piece. A proper distance of the screen has to be found, where the projected image of the Sun can be focussed as large as possible and the contrast of the image is still sufficient. The contrast should be enlarged by mounting a sheet of cardboard around the objective. If possible the screen should be mounted to the instrument in the proper position with a bracket. While tracking the instrument to the Suns position, the screen will then be always on the right place. If the screen is fixed on the ground (as in the above picture) screen and telescope must newly be adjusted from time to time.

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Measurement of contact times

The contact times measured on different continents are comparable only if they are converted from the time of the local time zone to "Coordinated Universal Time (UTC)". The current local time can continuously be converted into Universal time via internet (for instance with the help of http://www.heavens-above.com/).

Because the measurement of contact times should be exact to seconds, a precise watch has to be used. For this purpose a radio controlled clock is very recommendable. Or, even better, use the above mentioned internet site for this purpose. If an other watch is used it should be set to the time signal of broadcasting or television stations on transit day.

For the project proposed here it will be sufficient measuring the time of only one of Venus' contacts with the Sun's limb. But the chance of finding enough partners for combining the time measurements will be larger if you try to measure the times of all contacts visible for you. Most difficult will be the determination of the contact 1 time in (external ingress), because Venus becomes visible if it has propagated already a little into the Sun's disk. We therefore suggest to concentrate your attention on determining contact 2 and 3 times (internal ingress and internal egress) and, perhaps, the contact 4 time.

From the history of science it is known, that the determination of contact 2 and 3 times was disturbed by the so called "black drop effect". Where Venus touched the Sun's inner limb, a drop like deformation was observed making it difficult to judge the exact moment of contact. This effect has been approved during the 2004 transit. Anyway, you should try to determine the contact time as exact as possible.

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Determination of the own geographical position

For the calulation of the distance between Earth the Sun ( the Astronomical Unit) precise positions of the observers are needed. For this reason you will have to determine the geographical coordinates latitude and longitude of your observing site.

These coordinates can e.g. be derived from an accurate map. More precise data can be obtained, using a gps-receiver. Alternatively the website http://www.heavens-above.com/ may be helpfull.

As an educational project, we propose to determine your geographical position by own measurements (see, for instance, our related project "Geographical positions"). A similar project has been realized in 2009 within the International Year of Astronomy.

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Publication and Evaluation of your results

For uploading your data and comparing them with those of the other participants we offer our data exchange page. There you will find the possibility to inscribe your results, to view the results of all other project partners and to download them for evaluation.

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Additional activities during the transit

During the transit several additional activities are recommended:

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Editors: Udo Backhaus
  last update: last update: 2020-03-19
Stephan Breil