Since the last pair of transits ending in December 1882, there was no living soul has viewed the planet transit the Sun. 5 years after the spectacle and in the International Year of Astronomy we look at the where, why and when’s of this transit and the next. With the use of modest equipment, astronomers around the world have used the transits to find a plethora of data, including the distance of the Earth from the Sun, and the presence of a thick atmosphere around the Venusian planet.The passing of our sister planet in front of the Sun as we see it her from Earth is extremely rare. They are so rare that a baby born just after the second in the pair in 2012 has virtually no chance of ever seeing one. The planet Mercury makes transits more common but are a lot less spectacular. But, however, if you were lucky enough to be born in time to see the 2004 transit, you will almost certainly be able to see two transits in your lifetime! After the second in the current pair occurs on June 6th 2012, the next transit won’t be visible from Earth until 2117! The transits are much more than just a rare conjunction to astronomers. With the passing of Venus (and other celestial objects) between us and the Sun, astronomers can work out the fundamentals of astronomy, such as ‘how far we are away from the Sun’, ‘how big the solar system is’ and ‘ how far is it to the stars’. To be ruthlessly honest, to see the transit again from where you are sitting you would have to wait another 241 years (probably more seen as the UK has a permanent cloud attached to it). So to see these transits, intrepid explorers have traveled the globe in search of this exceptional celestial conjunction.

Until 2004, only 6 transits have been visible to mankind with use of the telescope. Various methods may have been used before this, such as viewing the transit through darkened glass, but the development of the telescope meant that the transit could be viewed in its entirety without damage to the eyes.
The first person to theoretically come up with the notion that planets could pass the Sun in such a way, was Johannes Kepler, using his 3 laws. Not only did he propose this idea, he came up with the exact dates that this would happen for both Mercury and Venus at the end of the year 1631.

Only the Mercury transit was recorded in this year, but re-calculations to Kepler’s “Rudolph Tables” by British Astronomer Jerremiah Horrocks saw that there would be another transit of Venus in 1639! “Therefore, along with William Crabtree they realized the first observation of this phenomenon” .
The 1761 transit almost made many breakthroughs in Astronomy, though variations in time pieces and faults with telescopes made the data inadequate. Though this was the year that Russian astronomer Mikhail V. Lomonosov, made the startling discovery that Venus had an atmosphere. The next in the pair, the 1769 transit, was viewed by “Captain James Cook (1728-1779), called by some “The Great Oceans greatest explorer,” [he] observed the 1769 transit from the black sand beach now called Venus Point, northeast of Papeete on the island of Tahiti” . On this voyage, Cook made the observations and sketches seen on the following page, and then went on to look for the land we now know as Australia.
The observations and data from this transit brought a plethora of information into the lime light. However, it was not until sometime later that Johann Franz Encke, calculated the distance from Earth to the Sun, using Edmund Halley’s idea of parallax. “[H]e deduced a solar parallax of 8” 57” and deliberated that the astronomical unit (A.U.) was 153,000,000km (just 4 million km out).

The 1882 transit saw astronomers discard the parallax method as a direct technique to redefine the AU. Instead, many thousands of photographs were taken and used as a way of calculating the precise AU. The photographs taken were so numerous in quantity, that it would take astronomers Simon Newcomb and William Harkness over a decade to work out the AU to just 95,000 miles of its current value.
So the Galilean method of using calculations of planetary motion can be used to determine the AU, also, the transiting of planets may be used. Radar and by telemetry from space probes now give us a value with an uncertainty of only 1.5km.

To understand why transits of Venus are so rare, you have to know some facts about the planet and some of the geometry behind the planets rotation. All planets of our solar system orbit the planet in the same direction, though each planets ecliptic line is different. The Universal ‘Ecliptic’ that we use, however, is the Sun’s line across the sky as seen from Earth. So if we think of the Earths ecliptic as being zero degrees, Venus orbits at an inclination of almost three and a half degrees! If you now realize that the Sun subtends an angle in the sky of around a half a degree (the entire sky being 180 from horizon to horizon), then you can begin to imagine why Venus rarely journeys across the face of our nearest star. Mercury’s inclination to the ecliptic is just over double that of Venus’ at 7, but the tiny planet orbits at just 0.4 AU’s! This is why the planet transits the Sun many more times than the next planet out.
Our planet orbits the Sun every 365.26 days. Venus orbits on the inside of us, and takes 224.70 days to orbit the Sun. Therefore Venus, in effect, laps the Earth on the inside every 584 days. Like a runner running around the track of Cwmbran Stadium on the inside lane, he/she would circuit the track much faster than a runner in the outside lane. Every time the outside lane runner is lapped, the runner on the inside lane blocks the view of the centre of the stadium, as seen from the outside lane. This is because the runners are on the same plane of inclination. If, however, the runner on the inside lane had an invisible track to run on that ran 3 and a half metres above ground and another 3 and a half metres underground, the runner on the inside would hardly ever block the view to the apparatus in the centre of the track. Although Venus blocks the view to the centre of the solar system, its inclination means that it only blocks out view within 7 of the true centre (the centre of the Sun).

Knowing that the Sun takes up a half a degree of our sky it leaves six and a half degrees for Venus to dance in before it blocks our view of the Sun.
The geometry of this goes a little further. As Venus orbits the Sun on its slightly angular orbit, it intersects the Earths line of orbit at two junctions. These are known as the ascending and descending nodes. So even though Venus may be at inferior conjunction (directly in between the Earth and Sun) it may not actually cross the Sun’s disc. It is only at an inferior conjunction at either node that a transit will occur, and this happens at the beginning of June or December every 115-130 years. It is actually at inferior conjunction again on Friday August 18th in 2007, but it will not cross the Sun until 3 conjunctions later in 2012 .
The last transit we saw of Venus was on June 8th 2004. And it was a spectacle to say the least. On the day of the transit, all amateur and professional astronomers all over Europe and in parts of America, Africa and Asia were all up at the crack of dawn to set up for this chance of a lifetime (not counting 2012). I, myself was up at around 430 GMT, setting up Picture 5for first contact which was due to occur around 515 GMT.

First contact saw the entrance of the Venusian disc onto the face of the Sun. At this point, the aureole of Venus should have been visible but only with significant telescopes. Around 535 GMT, second contact was made; this is the point at which the western limb of the planet crosses the edge of the Sun’s disc. Astronomers before the 2004 Venus Transit assumed that there would be a perfect black disc pass across the line of the Sun’s photosphere, but that’s not actually what happens. As you can see in figures 5 and 6, at contacts 2 and 3 there was vagueness about actually when 2nd and 3rd contact occurred. At these points, the planet seems to be ‘stuck’ to the Sun’s edge, and a tear forms between the limbs of the planet and star. In previous centauries, astronomers thought that this phenomenon may have been linked to another that was discovered in transits, the aureole of Venus (figure 4). Many astronomers, including myself, had no trouble though, taking timings of contacts. This shows that the effect may have been largely (but not totally) due to the inadequate optics of the 19th centaury.
After second contact, the planet travelled along a line that intersected the Sun from WSW to SSE (viewing the Sun as a compass). Venus would become at its greatest angular distance at 820 GMT, but still remain 10 arc minutes south of the Sun’s centre . I was hoping for some Sun Spots at the same time as the transit because 6 hours of watching the planet traverse the Sun can get pretty boring, no matter how rare it is! However, I’m sure that many astronomers didn’t have time to think about Picture 7boredom as all across the world, observing sessions and web casts and scientific
experiments were ago. And with the skies that South Wales had on June 8th 2004 couldn’t have been better! I actually ran an observing session on the school yard of a local primary school, while my University ran a somewhat larger session from the Universities largest car park.
The day ran on for a few hours, but I’m sure all children around the world saw the transit was instantaneously amazed, followed by instantaneous boredom. The third contact arrived at about 1105 GMT and showed much the same as the second. The aureole and black drop effect anticipated, and photo upon photo being taken. The photographs of the Venus transit of 2004 are extravagant. Some of the best can be seen at http://www.vt-2004.org/photos. These include photos of the aureole and black drop, but also photos of intervening objects such as the ISS and Jet Planes transiting the Sun along with Venus.
The 2004 was a success from every angle, especially from the UK. It was the first chance since 1882 to photograph a Venus Transit and it proved to be aesthetically if not scientifically thrilling. However, seen as the 2012 transit is the last for 105 years, if you did miss this one, you might want to catch the next.
Viewing the Sun is surprisingly simple. As in the top left of the pictures below, it was possible to view the transit without the aid of any magnification because Venus’ apparent diameter is quite large (around an arc-minute). But because the planet is around 1/32 the size of the Sun , it was definitely worth using my very modest telescope (even some medium size binoculars can give impressive results). I used three types of viewing at my session. The lady at top right of figure 7 is using a ‘Thousand Oaks Optical’ Solar Viewer, making sure that the children wrap the full length of the card around their eyes while looking at the floor, before Picture 6peering up toward the Sun. The gent in the top right photo of the same figure is using a 60mm refractor with 700mm focal length accompanied by a simple home made viewing box. This scope simply projected the Sun’s disc onto the bottom end of the box for safe viewing with no direct viewing whatsoever. My job (centre) was to operate the 114mm Tasco Starguide Go-to Scope with Baader Filter.

Unfortunately, the next one won’t be until 2016, but in the mean time, we have the 2012 Venus Transit to keep us amateur astronomers as happy as you like!
Viewing transits and eclipses are a lot like every other aspect of astronomy. Those who aren’t involved in the field simply assume that it is a scientific endeavour that is beyond them, but this simply isn’t the case. Celestial alignments, like viewing a planet through your telescope for the first time, is an awe inspiring experience that is unparalleled by much else (if your into that sort of thing). And you don’t have to be a science whiz to do it! Schools around the UK could spend one clear night a year, showing children just what it is they look at when they look up at the sky, and I bet they’ve wondered since they first saw them. So if you know a little about astronomy, especially if you have a modest telescope, share your knowledge with others. Friends, family, even strangers have all asked me just what it’s all about! I just tell them to see for themselves.

The Venus Express mission entered Venus orbit on April 11th 2006. It’s basically a follow up to the Mars Express mission and many of the instruments on the Venus Express are just upgraded versions of those on Mars Express. There hasn’t been a mission to Venus since the NASA probe “Magellen” in that ended in 1994, now ESA is sending its first Venusian explorer. It will take only 155 days to enter Venus’ orbit, but it will take another 5 days to
The Venus express acquired its name because from approval to launch was just a 3 year process. Showing that if you use tested technology, you can send up 3 probes a decade if you want. ESA is very much finding its feet at the moment though. But with a Mars Express, BepiColombo and Venus Express, ESA are fast becoming a top notch Space Agency. They did get a little help off the Russians though, with the launcher being part of the European/Russian association. The mission is being controlled from Germany however, at the European Space Operations Center (ESOC). Other Centers include that at Spain, Australia and French New Guinea. These all help in the operation of this astounding mission that only arrived last month.
It will primarily study the atmosphere of Venus, looking at the dynamics of the planet, deriving answers for questions such as ‘what drives the winds of Venus’, they can reach hurricane speeds but we don’t know much about the complex dynamics of the planets winds, just as we know very little of the cloud systems, chemical compositions and escape processes around the Venusian surface. Like any good space-craft, the Venus Express will be searching for signs of water on Venus, though the outlook is very doubtful, and its not only chemical and atmospheric progressions of the planet that the team will be studying. Like the Earth, Venus undergoes tectonic activity, but unlike the Earth, it’s not a continuous process, it’s a sporadic and chaotic one. The re-surfacing of Venus is a theory that has been on the table for many years, now, scientists may have the chance to prove this theory or find it as a kick in the behind to find a new theory to explain why the entire surface of the planet is only 500 million years old. It goes only to reason that the evidence for this may come from the possibility of there being ongoing volcanic activity on Venus, if this is true, it will join the plethora of Picture 13active bodies we’ve found in our solar system, after the amazingly anthropocentric concept that ours was the only one.
Figure 11 shows a cut away of the Venus Express probe from ESA’s Venus Express mission page, found at www.esa.int/esaMI/Venus_Express/index. It shows ASPERA, the Analyser of Space Plasma and Energetic Atoms, MAG which is the Venus Express Magnetometer, PFS the Planetary Fourier Spectrometer, SPICAV/SOIR, the
oddly named Ultraviolet and Infrared Atmospheric Spectrometer, VeRa, the Venus RadioScience Experiment, the well known VIRTIS (Ultraviolet/visible/near-infrared Mapping Spectrometer) and the tiny VMC, the Venus Monitoring Camera.
All of these instruments add to the weight of the craft, and weight costs money to Space Agencies. The total weight for the rocket though was only 1270kg, the craft itself only taking up one twelfth of that weight, the fuel making up for half of it. However, this fuel (nearly 600kg of it) isn’t going to be used for taking off, this is just stored fuel. This fuel is to break the enormous pull of the Suns gravity at Venus arrival. Once there, it no longer needs fuel, as it can live off the plentiful Sun light that it will get at 108 million km. Now its there, its going to have a busy time of it, perhaps it will even be sending back information at the time of the next transit in 2012.