On 8th June 2004 the planet Venus will cross the Sun's disk as seen from Earth. It will cut into the solar disk at about 5:20am in the morning and will pass off the other limb at about 11:23am. The last occasion on which this happened was 1882, over a century ago.

A transit of Venus is like a solar eclipse, but instead of the Moon being in line between the Earth and Sun it is the planet Venus. You can hardly fail to notice a solar eclipse because the Moon, being about the same apparent size as the Sun, blocks out its light.

Venus, on the other hand, looks very much smaller from Earth and so you would have to be specifically observing the Sun to see the small disc of Venus passing across its disk. You would also have to wait a very long time. Solar eclipses occur every year or so somewhere on Earth, but you could have been observing the Sun for the last century and still not have seen a transit of Venus.

Venus appears very small (Venus is only about 58'' - the Sun is about 1890'') as seen from Earth, so as it passes across the Sun, it will be just big enough to be seen without any magnification with the unaided eye through a solar filter. Using image projection onto a card will also show the planet's slow progress but perhaps a little to set up in say, a school playground.

What are transits?
To start to understand what is going on, imagine a circular race track and two runners, Eartha and Vera. Eartha can do a lap of the track in one minute. Vera is quite a bit faster, and also has the advantage of the inside track, she can do a lap in just under 37 seconds. The question is, if they start off together how long is it before Vera catches up with Eartha so they are together again?

To work out the answer it is convenient if the runners' speeds are in a whole number ratio, so let us assume Vera can lap the track in 8/13 minutes (36.92 seconds). That means Vera is going 13/8 times faster than Eartha, so Vera is catching Eartha up at 5/8 laps per minute (13/8 minus 1). Vera will therefore catch up Eartha after 8/5 minutes, ie 1.6 minutes or 1 minute 36 seconds. Check it out and you will see that in 1.6 minutes Eartha has gone round the track once plus 3/5 of the track while Vera has gone round the track twice and 3/5 of the track, so they meet up.

Translate runners into planets and minutes into years and the picture is close to that for the actual planets, Earth and Venus. It is not quite as simple as that because the tracks are a bit elliptical rather than circular and the runners do not go at quite a constant speed. Also Venus orbits the Sun just a bit faster than 8 thirteenths of a year, more like 7.997. As we shall see this is a small but crucial difference.

The time of 1.6 years between Sun, Venus and Earth lining up is called the synodic period (synod = meeting). Fig 1 shows the lined-up positions from 1996 to 2002. You can see that there is a five spoke pattern. (Ignore for the moment the dotted line). The planets travel anti-clockwise and each lined-up position is 3/5 of the circle on from the previous one.

If we continue the pattern we can see that the next lining-up after 2002 will be in June 2004 in the same place as June 1996, 8 years previously. Almost, but not quite. Since Venus travels a little bit faster than one orbit in 8/13 of a year it catches up with the Earth a little bit before the meeting place 8 years previous. Consequently, if you draw the five spoke pattern 8 years on from Fig 1 it will look like Fig 1 but with the spokes rotated slightly clockwise.

We can start to see that 8 years might be a significant interval but it is still not clear how it relates to transits. For a start we have said that Earth, Venus and the Sun are lined up every 1.6 years, so why is there not a transit of Venus every 1.6 years? The answer is there would be if the orbital planes were exactly aligned. However, the orbit of Venus is at a small angle (3.4 degrees) to that of Earth. The result is that, although the planets appear lined up when looking down on the orbital planes, most of the time Venus viewed from the Earth will be either above or below the Sun. For a transit to occur the planets not only have to line up as in Fig 1 but they have to do it just in the place where the orbital planes cross, which makes it a much rarer event.

The same thing happens with a solar eclipse. Every month the Earth, Moon and Sun are lined up at New Moon. However the Moon/Earth orbit plane is at an angle to the Sun/Earth orbit plane and it is only when New Moon occurs at the orbit plane crossing that there is a solar eclipse. Otherwise, seen from Earth, the New Moon is either above or below the Sun.

Referring back to Fig 1 the dotted line shows where the orbit planes of Earth and Venus cross. The crossing line corresponds to about 7 June and 8 December. We can see that the lining-up in 1996 was very close to the orbit-crossing line but was not quite there. Venus was near to the Sun but passed just south of it. On 8 June 2004 the lining-up will be even closer to the orbit-crossing line, in fact so close that Venus will transit the south side of the Sun for some 6 hours from about 5h 15m to 11h 15m UT. Eight years later in 2012 the lining-up will occur just the other side of the orbit-crossing line and there will be another transit, this time with Venus transiting the north of the Sun. Fig 2 illustrates the position of the Sun seen from Earth relative to the orbit path of Venus on the three occasions. (The orbit angle has been exaggerated so Fig 2 is an illustration rather than a precise picture).

After that there is a long wait until the January 1998 "spoke" in Fig 1 slowly makes its way round clockwise in eight year ticks until it gets close to the December orbit-crossing line in 2117. Eight years later in 2125 there will be another transit when the "spoke" ticks through to the other side of the orbit-crossing line. This time the orbit path rises from south to north so the first transit will be north of the ecliptic and the second transit south. There is then a long wait while the "spokes" tick round until the lower one arrives close to the June orbit-crossing line again. And so the process continues.

On a final note we saw that because the orbit time of Venus (measured in units of Earth-orbit time, ie one year) was close to 8/13 it led to 8 year cycles. A closer approximation to the orbit time of Venus is 243/395 years, which by similar logic leads to a longer run 243 year cycle.

Copyright Peter M Langford, September 1998

What will we expect to see?
The two images below show the area of Earth that can view the 2004 transit of Venus. The left image shows the start, on the right is the position at the end.

How to view the transit in safety
Firstly, you have to get up early! The Sun will be low down in the east.

Projection of the Sun onto card is by far the safest method. Even a modest telescope will show Venus.

To observe the transit you will need either a pair of binoculars or a small telescope, two pieces of white card and some kind of photographic tripod (a solid chair will do).

With one of the pieces of card cut one hole in the centre if you have a telescope or two holes if you have binoculars for the eye pieces. Push the card onto the telescope or binoculars and fix with masking tape (sellotape is okay but it is transparent). On binoculars, use the lens cap (or something else opaque) to block one of the objectives so that no sunlight can get in, and use the other.

Mount the binoculars or small telescope onto the tripod and fix them in such a way that they can be pointed into the sky towards the Sun. Get someone to hold the other piece of white card a short distance away from the eye piece (as shown in the photograph). Slacken the mounting so that you can move the telescope with ease.

DO NOT UNDER ANY CIRCUMSTANCES TRY TO ALIGN THE BINOCULARS OR TELESCOPE WITH THE SUN, BY LOOKING THROUGH THEM.

Move the telescope around until the 'smallest shadow' has been achieved onto the white card, you will quickly notice that every time you get the 'smallest shadow' the image of the Sun jumps across the card. The trick is to lock the mounting once the image of the Sun can be seen on the white card. There you have it, the right equipment for the right job.

AT NO TIME DURING THE TRANSIT SHOULD THE APPARATUS BE LEFT UNATTENDED - BLINDNESS IS NO FUN AND THE DAMAGE CANNOT BE UNDONE!!

During the six hour transit you will have to move the set up in order to follow the Sun. However, because you are using a small telescope or binoculars you will find several minutes of comfortable viewing before the apparatus has to be reset.

For those who do not own a tripod, a high backed wooden chair is also ideal, but following the Sun may prove a little tricky.

Of course, if you have a nice sharp image you can 'peek-a-boo' the camera over the edge of the masking card to capture images of the transit as it progresses.

External Ingress

Dec. 1631