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Venus transits the Sun - 2004 June 8

The transit was observed by most HartRAO staff at Hartebeesthoek and at the Johannesburg Planetarium and they produced this photographic report.

HartRAO staff astronomer Sarah Buchner and educators Daphne Kedibone Lekgwathi and Mzi Taele helped run the mass viewing of the transit from the Johannesburg Planetarium, which was attended by hundreds of learners and public.

Background to the Venus Transit

On 2004 June 8, Venus passes in front of the Sun for the first time since 1882. Historically, these "Transits of Venus" were of great importance, as they enabled the size of the solar system to be measured. Today this is done using radar. However it is possible for individuals across most of Africa to make simple observations of this transit and so establish for themselves the size of the solar system. This makes it a valuable educational exercise.

How can we watch the transit?

It is possible to watch the transit using the same methods used to watch the recent total eclipses of the Sun. These methods include: The same safety precautions apply as for a solar eclipse: Looking at the Sun in either of these ways can damage your eyes permanently.

What will we see?

Venus appears as a small black dot passing across the face of the Sun. The angular diameter of the Sun is 30 minutes of arc (= 0.5 degrees), while the angular diameter of Venus will just under 1 minute of arc (this changes as the distance of Venus from the Earth changes). Hence the diameter of Venus during the transit will appear to be 3 % of the diameter of the Sun.

What should we measure?

To make measurements that will let you make useful calculations you will need: There are four 'times of contact': How use is made of these measurements is described in the links below.

Can we see the transit from everywhere in South Africa?

africa map
Cropped from Fred Espenak's Venus Transit webpages

First Contact occurs when the Sun is below the horizon for the south-western part of South Africa. Only places (shown in yellow above) that are east of a line approximately from East London through Kimberley will be able to see first contact from a suitable site. For Johannesburg, the Sun will be at an elevation of 4 degrees at First Contact. it will be 1 degree at East London and 2 degrees at Bloemfontein.

By second contact, the Sun is rather higher, and it will be visible from places east of a line approximately through Port Elizabeth and Upington (marked as II on the map). For Johannesburg, the elevation of the Sun will be 8 degrees for Second Contact.

These angles are very low! Hence it is ESSENTIAL to find a viewing site with a clear view of the distant horizon from the east to the north-east in order to see first and second contacts.

All of South Africa will be able to see the end of the transit (weather permitting!)


Several excellent websites have been set up that cover this event in detail, and show you how to make calculations from your measurements:

Last revised 2004/06/09 by Mike Gaylard e-mail: mike@hartrao.ac.za
since 2004/05/17