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There will be a transit of the planet Venus across the face of the sun on Tuesday, 8 June 2004. Viewed from Durban it will begin at 07:17 and end at 13:28, which means that the full six hours of the transit will be visible as the sun is already up at 06:47.
Venus is the brilliant evening "star" that has been visible in the west after sunset for several months and has been moving lower and closer to the sun over the last few weeks.
A Venus transit is very rare - the last one was in 1882 - and so the small amount of initiative needed to get to view this daytime astronomical occurrence will be well worth it.
What is a transit
The planet Venus is closer to the sun than the earth is. This means that at certain times, Venus will be exactly on an imaginary line between the sun and the earth. At such a time Venus will be seen to "transit" as a small disc travelling across the face of the sun, about 1/30 of the diameter of the sun.
Transits and occultations
A transit happens when there is an alignment of three astronomical bodies, the viewing body being the earth. Transit implies that the body in front appears smaller than the body behind and covers only a part of it. Other examples of transits in the solar system are transits of the four biggest moons of Jupiter across the face of that planet. They are happening all the time only days apart.
An occultation is an alignment of three bodies where the body in front appears larger than the one behind and completely covers it. The moon is continually occulting stars and planets on its passage through the heavens each month. Less often the planets and even asteroids occult stars. A total solar eclipse is another example of an occultation.
Frequency of transits
As Venus is closer to the sun than the earth it travels round the sun in a shorter time. It actually travels at a greater speed and has a shorter distance to go. It takes about 19 months to catch up with the slower-moving earth and so transits could occur every 19 months. However, the orbit of Venus is slightly tilted (about 3º) relative to the earth's orbit round the sun. This means that most times when Venus catches up with the earth, the two planets are not closely enough in line for a transit to occur, and so transits occur much less often. Calculations show that Venus transits come in pairs 8 years apart, but more than 100 years elapse from one pair to the next. The last transit was in 1882. The next transit will be in 8 year's time, but will unfortunately not be visible from southern Africa. And the next after that is again more than a century away.
The other planet that is closer to the sun and can therefore transit the sun is Mercury. These transits occur much more frequently than those of Venus, about 13 in a century as opposed to about 2 per century for Venus. There was a transit of Mercury on 7 May 2003. (This transit was seen by 800 learners at five schools in Durban, with the help of members of the local branch of the Astronomical Society of SA.) The next one will be in 2006. However, during a transit of Mercury that planet appears five times smaller than Venus, and so the phenomenon is not as clearly visible.
All in all, Venus transits are rare, and the more frequent Mercury transits are less impressive. The June 8 Venus transit is at a convenient time of day, especially for schools, and there is the whole morning to fit in a time slot. So this transit is definitely the one to go for!
Viewing methods
Looking directly at the sun will very quickly damage the eyes. So attempting to view the transit is dangerous and great care must be taken. The following options are available.
Eclipse glasses
Many people will have eclipse glasses left over from viewing a partial eclipse of the sun from Durban, or a total eclipse from Musina or Lusaka. Viewing the sun through these glasses is safe but it is important to place the glasses over the eyes before turning to look at the sun. The little disc of Venus should be just visible this way.
Optical instruments - binoculars or telescope
The same principles apply for both binoculars and telescope.
There are two methods to use.
1) Filter viewing: The transit can be viewed directly through the instrument if a suitable filter material (the same as in eclipse glasses) is placed in front to reduce the sun's radiation before it enters the instrument. The filter must be very firmly stuck onto the instrument (both lenses in the case of binoculars). If the filter were to come off, the amount of direct sunlight and heat entering the eye would be enormous and blindness could occur immediately. Also, if inappropriate filter material is used this can damage the eye. (A spare pair of eclipse glasses would be a source of suitable filter material, but the filter must be well mounted and firmly stuck to the binoculars. The material would have to be mounted on a piece of cardboard big enough to cover the whole of each objective lens with no gaps to let direct sunlight through.)
2) Projection viewing: The light coming through the instrument can be projected onto a screen. When using this method no filter is needed. A problem here is that the eyepiece of the instrument can become very hot as the sun's rays are concentrated towards it. This can damage expensive equipment. In the case of the telescope one option is to replace the eyepiece with a cheap lens mounted temporarily.
The instrument needs to be firmly supported. A telescope comes with its own support. Binoculars would have to be clamped onto some kind of stand. The instrument needs to be defocused a little from the normal adjustment in order to focus an image of the sun on the screen. One can experiment with screen distances and defocusing to give a sharp image of a suitable size. The light that has passed through the instrument can be reflected onto a screen off to one side using a mirror placed at an angle of about 45º. This avoids having to shield the screen from sunlight passing along the side of the instrument.
Making your own telescope
A very accessible solution is to make your own telescope and use the method of projection onto a screen. This avoids the dangers of damage to your expensive binoculars or telescope, or to your priceless eyes. It could be a good project for school classes.
The principle of a telescope
A telescope consists of two lenses. The first, the objective, forms a small image of the sun, and the second, the eyepiece, acts as a magnifying glass with which the observer views this image. The larger the focal length (f1) of the objective the larger the image, the smaller the focal length (f2) of the eyepiece, the greater is its magnification. So the total magnification of the instrument is m = f1/f2. This formula is correct for direct viewing through the instrument. When using the method of projection onto a screen, a further factor influencing the size of the projected image is the distance to the screen. This leads to a different formula (formula 7 in the appendix), but the principle still holds that for a large final image the focal length of the objective should be large. The formulae for the projection method are explained in more detail in the appendix.
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