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
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
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:
- Using eclipse viewers
- Projecting an image of the Sun onto a white surface, by using
binoculars or a small telescope
Looking at the Sun in either of these ways can damage your eyes permanently.
- 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
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':
- 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
- To be able to measure the times of second and third contact (described
below) to an accuracy of 1 second.
How use is made of these measurements is described in the links below.
- 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
- 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)
Can we see the transit from everywhere in South Africa?
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
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