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:

Using eclipse viewers
Projecting an image of the Sun onto a white surface, by using binoculars or a small telescope

The same safety precautions apply as for a solar eclipse:

DO NOT look directly at the Sun with the unaided eye or through dark glasses
DO NOT look directly at the Sun through binoculars or a telescope

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:

To know your geographic location to within 1-2 km. A GPS receiver will do this (available from outdoor shops), otherwise you will need to refer to accurate maps.
To be able to measure the times of second and third contact (described below) to an accuracy of 1 second.

There are four 'times of contact':

First contact (contact I) -
The edge of the disk of Venus first touches the edge of the disk of the Sun (at approximately 07h 17m 21s SAST in Johannesburg). Venus will not be visible until this moment.
Second contact (contact II) -
The edge of the disk of Venus just breaks contact with the edge of the disk of the Sun, so that Venus appears fully inside the Sun's disk (at approximately 07h 36m 22s SAST in Johannesburg)
Third contact (contact III) -
The edge of the disk of Venus just touches the edge of the disk of the Sun, on its way out (at approximately 13h 10m 03s SAST in Johannesburg)
Fourth contact (contact IV) -
The edge of the disk of Venus breaks contact with the edge of the Sun and Venus is lost to sight (at approximately 13h 28m 44s SAST in Johannesburg)

How use is made of these measurements is described in the links below.

Can we see the transit from everywhere in South Africa?

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.