Most people are aware of eclipses in which the Sun or the Moon is hidden from view, partially or fully, for short periods of time when the two objects are in line with the Earth, as on full moon and new moon days. A solar eclipse takes place when the Moon passes in front of the Sun as viewed from the Earth. This can be partial, annular, or total as described in some of my previous blog posts specifically related to these wonderful natural phenomena.
Very similar and not so well known, but equally interesting and of tremendous historical importance as well, are what are called transits of the inner planets Mercury and Venus (see Figure 1) in which a silhouette of the planet is seen to move slowly across the solar disk as viewed from the Earth. These events are much less frequent than the eclipses and there is no dimming of sunlight since the area of the solar surface hidden from view is only a negligibly small fraction of its overall area.
Figure 1: Inner Planets Mercury and Venus
Venus is the most easily recognizable planet in the evening or morning sky because of its dazzling brightness, the Moon alone being brighter than this. It may not be too much of an exaggeration to say that, barring the blind, everyone will have seen Venus, at least accidentally, but a lot of people who have seen it seem to mistake it blindly for a flying saucer or alien spaceship. Such sightings and UFO claims shoot up when the Moon also happens to be seen close by. I remember several decades ago that, when such an event was reported by observers to the India Meteorological Department, its harried spokesperson issued a press statement that the matter was being investigated! Luckily, he did not imply that it was an UFO. I have had numerous such experiences myself. I immediately ask the person to describe where he saw whatever he saw and discover that the sighting corresponded to the location of Venus at that time. When I explain to him this simple fact he feels greatly let down, having expected some credit for a UFO sighting. How uninteresting and unpalatable truth so often is!
Incidentally, the natural object which has been most commonly mistaken for a UFO is the planet Venus. It is hard to believe that an object which has been a constant and awe inspiring companion in the sky since cavemen left their den and walked on Earth could fool anyone, but such indeed is the case. Even trained observers, like airline pilots, have been fooled when the sightings happened under unusual conditions, such as the proximity to a crescent Moon. Of course there will be no such sightings during the forthcoming transit event!
Venus can be seen up to three hours after sunset in the evening sky or as early as three hours before sunrise in the morning sky. Depending on when it is seen, it is popularly known as the ‘evening star’ or the ‘morning star’. The term ‘star’ came to be (wrongly) used long before the real nature of it as a planet of the solar system came to be understood. When it is closest to the Earth in its orbit around the Sun it is said to be in conjunction.
Mercury, which is the closest to the Sun, is an elusive planet since it stays very close to the Sun, evening or morning, and difficult to locate because of its small size and proximity to the Sun. While almost everyone will have seen Venus sometime or other, the opposite is the case with Mercury. The great Copernicus is said to have developed his heliocentric theory of the solar system without having actually seen this planet.
As with eclipses, the transit of Mercury or Venus occurs when the planet is in line with the Sun and the Earth (at conjunction) and passes between the two. Such a situation is not possible with other planets of the solar system because their orbits around the Sun lie outside the Earth’s orbit which is called the ecliptic. Eclipses are unique because of the fortuitous circumstance that the apparent sizes of the Sun and the Moon as seen by us on Earth are nearly the same. This however is not the case with the planets whose apparent sizes are all much smaller than that of the Sun.
During a transit the planet is seen to first enter the Sun’s disk at one point (ingress) at the circumference, move gradually across the disk in a straight path over several hours and exit at another point (egress) to complete the event. It is during this period that the event can be seen, generally with an optical aid such as a pair of binoculars or a small telescope, but always through a safe filter to protect one’s eyes against the intense solar radiation.
Because of Mercury’s closeness to the Sun and its orbital characteristics, transits of Mercury (ToM) are more frequent than those of Venus – about 13 to 14 per century. The last one to occur was on 8 Nov 2006. In contrast, only seven Transits of Venus (ToV) have occurred since the invention of the telescope – in 1631, 1639, 1761, 1769, 1874, 1882, and 2004, just about two per century. Interestingly, these occur in pairs about 8 years apart, but with each pair spaced over a hundred years apart. After the forthcoming event of 5/6 June this year, one has to wait until 2117 for the next such event. It is highly unlikely that any adult alive today will be still alive to see it. So, here is our last chance! An added motivation is that a transit of Venus is far more visually interesting because of its much larger size as compared to Mercury. It can even be spotted with the naked eye, though this has to be attempted only through a safe filter.
ToV on 8 June 2004
Figure 2 depicts a Transit of Venus as predicted by Fred Espenak/NASA and observed earlier this century, on 8 June 2004. In most parts of India it was observed on a sunny afternoon and lasted about six hours.
Figure 2: ToV on 8 June 2004
The following (Figure 3) is a typical picture of the Sun taken with an appropriate filter attached to a telescope sometime during the ToV on 8 June 2004. It shows the orientations reversed as compared to the drawing in figure 2. The silhouette of Venus is unmistakably visible at upper right against the bland background of the solar disk. Rather improbably, no sunspots were visible at that time. They would have made the picture more attractive. The picture also gives a general idea of the size of the Sun in relation to that of Venus. Actually, the Sun is much larger than the Impression this image creates because it is much farther away from us than Venus.
Figure 3: Venus in transit on 8 June 2004
Prediction for ToV on 5/6 June 2012
Figure 4 depicts the forthcoming ToV as predicted by Fred Espenak/NASA through the usual meticulously accurate calculations (Indian Standard Time is 5.5 hours ahead of Universal Time). For viewers in India it will be a bit of a disappointment in that they will not be able to view the event in its entirety. The transit starts at around 3:40 AM, well before sunrise and lasts just under seven hours. By the time the Sun rises (around 5:50 AM in a place like Mysore) and becomes visible, the transit will be nearly half way through and the dark circular spot corresponding to Venus will have reached the upper middle part of the solar disk. Yet, it is definitely worth watching the rest of the transit assuming that weather doesn’t play spoilsport. If one gets even a few minutes of break from cloudy skies that would be sufficient for any casual observer who is equipped with the proper viewing device. I shall elaborate on this a little later. The Sun will be fairly high in the sky by the time the transit ends around 10:20 AM.
Figure 4: ToV as predicted for 5/6 June 2012
Figure 5 shows the visibility of the event in different parts of the world on 5/6 June 2012.
Figure 5: Visibility of the ToV on 6 June 2012 in different parts of the World
Historical importance of ToVs
The great Johannes Kepler, architect of the laws of planetary motion, was the first to predict the possibility of the transits of both Mercury and Venus though he didn’t live long enough to observe either. A young and highly enterprising British astronomer, Jeremiah Horrocks, and his friend William Crabtree were apparently the only ones to calculate and witness the transit of Venus on 4 Dec 1639 which allowed them to accurately measure the apparent diameter of the planet. It is ironic that Horrocks, who had converted part of his home as a dark room to project an image of the Sun on a screen through a small telescope, missed out on the beginning and the early phase of the transit because of some ‘important assignment’ which turned to be just a Sunday church service. Science had continued to play second fiddle to religion! Despite the pioneering effort of Horrocks, a prominent astronomer later lamented that it was ‘one of the most unfortunate incidents in the history of Astronomy that he missed the critical instant’. It could have been worse if Horrocks had taken the day even more seriously as Sabbath in line with prevailing religious dictats!
Edmund Halley, whose name is permanently enshrined in the history of Astronomy for the discovery of the famous comet named after him, had observed the transit of Mercury in 1677, and realized that the careful timing of transits could be used to determine the distance of the Earth from the Sun. The technique relied on observations made from widely separated locations on earth and measurement of the resultant parallax shifts to calculate the distance. The distance between the points on the Earth could be used as a baseline to calculate the distances to Venus and the Sun employing the geometric technique of triangulation. It is necessary to know the precise location (geographical latitude, longitude, and altitude) of the observer for this technique to yield accurate results. In the olden days this was quite a challenging task, but today it is a trivial matter since such information can be read off on a smart phone from the information fed to it round the clock from GPS satellites.
Because of parallax, the silhouette of the planet will be seen at slightly different positions for different observers. It is like looking at a nearby object against a distant background, both stationary, first with one eye and then with the other eye. The parallax shift is readily noticeable and measurable. Halley reasoned that ToVs were better suited for this purpose than ToMs because Venus is closer to Earth and therefore exhibits a larger parallax.
Halley implored future generations to organize major expeditions to the ends of the Earth to observe the ToVs of 1761 and 1769 and come up with an accurate determination of this distance, called the Astronomical Unit, which would then provide a yardstick for the whole of the solar system. This was indeed achieved, but not with the level of precision expected because of some nagging observational difficulties, much of it due to the Earth’s atmosphere, as well as timing uncertainties and inaccuracies. These exercises are only of historic interest today, superseded by much superior techniques, including direct radar ranging. Incidentally, Captain James Cook observed the 1769 transit of Venus from Tahiti at a location still known as ‘Point Venus’. This promises to be an excellent location for the forthcoming ToV also and many observers will congregate there.
How to view the event
There are several ways of observing the transit of Venus and these are equally applicable to any type of solar eclipse.
Direct Naked Eye Viewing
If the observer has very sharp eyesight, the event can be viewed directly with the naked eye, but only through an appropriate solar filter designed specifically for viewing the Sun. It will be very dangerous to the eye without such a filter or by other means. The filter cuts down the intensity of sunlight to a safe and comfortable level, by as much as one in 100,000. The best filters are made of a vacuum-deposited metallic reflective coating on a substrate with the desired transmittance characteristics. They are available in the form of spectacles as shown in the following picture. Such viewers are however much better suited for solar eclipses than for the transit of Venus. It must be remembered that the Sun itself appears as a small object and the much smaller silhouette of Venus may be hard to spot.
Figure 6: Solar filter for direct naked eye viewing
Viewing through Binoculars
A pair of binoculars with appropriate filters is excellently suited for viewing the event since it gives a good magnification even while showing the entire disk of the Sun in the field of view. This is how I chose to observe the great total solar eclipse of 22 July 2009 from a hilly resort near Anji city in China [see my blog post number 03 (Feb 10)]. The following picture of my improvised setup using a pair of 10x50 binoculars and mounted on a small camera tripod, with a filter meant for a small telescope covering one ocular, is reproduced from this post. It is certainly crude, but eminently functional and I intend to employ the same setup for the ToV on 6 June. Note that the unused ocular is safely covered with its cap and a large sunshade gives protection against direct sunlight on the viewer’s face.
Figure 7: An improvised setup for viewing the Sun anytime
Viewing through a small Telescope
If a small telescope is available it is a simple matter to improvise a setup very similar to the one described above. However, in either case the solar filter should cover the objective of the viewing device, not the eyepiece. Solar filters for eyepieces are commercially available, but they are risky and potentially dangerous for the inexperienced users because of the intense heat they are subjected to.
Projecting an image of the Sun
If an optical aid like binoculars or telescope is available, the best way to see the Sun’s surface is to employ a simple technique discovered by Galileo, who was also the first human to use a telescope for astronomical observations. The device can be mounted on a tripod and sunlight allowed to pass directly through the objective as shown in the following two rather exaggerated figures. The image of the Sun can be captured on a spotlessly clean (to avoid any of those spots being mistaken for a sunspot) white, preferably glossy, cardboard placed some distance behind the eyepiece which is moved to and fro until a sharply focused image is seen on the sheet. The technique is straightforward, but one may have to struggle a bit to learn to orient the instrument’s tube to point directly at the Sun without looking at the Sun itself for obvious reasons. The solar shade (not shown in the first picture but much like the one in figure 7) helps a great deal, particularly to give a bright image on the screen shielded from direct sunlight from around the instrument. The best strategy would be to remove the solar shade initially and play around with the instrument with one hand while looking only at the screen and trying to get a bright circular patch on it which can be later focused to a sharp image.
Figure 8: Projecting an image of the Sun with Binoculars
Figure 9: Projecting an image of the Sun with a small Refractor Telescope
What the casual observer can look for
When the Sun has risen sufficiently above the horizon in the eastern sky in India, the observer should have little difficulty in spotting the silhouette of Venus on the solar disk. In all probability a number of sunspots should also be visible, most of the separately distinguishable spots being significantly smaller and irregularly sized than the perfectly circular spot corresponding to Venus. It is easy to ascertain their locations and sizes in advance by referring to the picture of the solar disk in the following website which gives this information on a daily basis: http://www.spaceweather.com Reproduced here only for illustrative purpose, the following picture corresponds to 15 May 2012. This is a high resolution picture taken with professional quality instruments and rather unusually rich in sunspots. However, only the most prominent regions/spots can be expected to be visible with the devices described earlier. Also, the view on the day of the event will be very different from what this picture shows. The naked eye view may be much less promising and indeed one may not be able to make out any spots at all. However, with the 11 year solar activity cycle due to peak next year, the likelihood of a fair number of sunspots appearing on any day is quite high. Unlike in 2004, the presence of sunspots should make the event more visually attractive.
Figure 10: Sunspots on 15 May 2012
The slowly changing position of Venus tracing out a straight path and disappearing eventually can be readily followed. In contrast, there is unlikely to be any significant change in respect of any of the sunspots during these few hours.
Unlike a total or annular solar eclipse where the action lasts only a few minutes, the viewer can afford to wait for any clouds to clear up and provide at least some glimpses of the event that lasts several hours.
What serious amateurs can do
There is a great deal that any serious amateur astronomer can do, including one’s own measurement of the Astronomical Unit from some precision photographic observations during the transit in collaboration with other similar observers located in faraway places, thousands of kilometers apart. Inspiration can be drawn from documented efforts like the one appearing at: http://www.eso.org/public/outreach/eduoff/vt-2004/auresults/au-1.html. Another very useful website is: http://public.gettysburg.edu/~marschal/clea/Transitlab.html. Large downloadable files with detailed instructions for analyzing data from the 2004 ToV are available here.
A wonderfully produced ‘graphic novel of the story behind the Transit of Venus’ authored by N M Ramanujam, with illustrations by Reshma Barve, and published by the National Centre for Radio Astrophysics, India can be downloaded in English and several Indian languages from the following website: http://mutha.ncra.tifr.res.in/ncra/for-public/transit-of-venus
As with solar eclipse photography, any high zoom (10x or higher optical zoom) DSLR camera, or even a non-SLR camera with Electronic View Finder (EVF), with a good solar filter securely and fully covering the lens and mounted on a tripod is adequate for photographing the event at convenient intervals. The following picture of a DSLR camera set-up with the lens covered by a large solar filter was taken during the great annular solar eclipse of 15 Jan 2010 which I viewed from Dhanushkodi in Tamilnadu. The optimum exposure time, which depends on the transmission characteristics of the filter and the altitude of the Sun, is best determined by trial and error. It is best to switch off the auto focus feature and set the focus manually to infinity. If a good telescope is available, a DSLR camera can be attached to it after removing the eyepiece and high resolution pictures taken. Some of the present day super-zoom non-SLR cameras (with optical zoom levels up to 36 and additional digital zoom levels up to 4, with a minimum picture resolution of 3 MP) now available make such a telescope unnecessary. It is like a telescope by itself. While adjusting the camera, great care needs to be taken not to view the Sun directly with the naked eye. A large sun shield, like the one shown in figure 7, needs to be used while getting the image in the field of view and focusing it.
Figure 11: Photographing the event with a DSLR camera
A Supplementary Resource
An excellent publication (cover pages shown in the following picture) titled “Transit of Venus”, authored by Dr B S Shylaja, presently Director of the Jawaharlal Nehru Planetarium in Bangalore, gives a wealth of information about the event. Covering all essential aspects of the phenomenon, it is particularly good in tracing the historical developments. First published in 2004 by Navakarnataka Publications Private Limited on behalf of the Bangalore Association for Science Education in time for the event that year, and with a reprint brought out this year, it is a valuable resource for all interested and inquisitive readers. Appropriately, a Kannada version of this publication by the same author is also available from the same publishers. The Karnataka Sahitya Academy Award for science writing in Kannada has been conferred to the author for this work.
An article titled “What is Transit?” by the same author appears in the current issue (May 2012, Volume 17, Number 5) of Resonance, a journal of science education of the Indian Academy of Sciences published by Springer. It nicely compliments the contents of the publication cited above.
Transit of Venus is also the name of a play by the Canadian playwright Maureen Hunter in 1992. With liberal artistic license, it is based on the true and tragic life of French nobleman and astronomer Guillaume Le Gentil (1725-1792) who embarked in 1760 on a long drawn out expedition to observe the transit of Venus in 1761 somewhere on the sea off the coast of India and failed. He decided to wait for the next transit on 4 June 1769 and landed in Pondicherry for the purpose only to be totally thwarted by overcast skies on the very day of the event. After recovering from a mental breakdown and going through many privations, he finally returned to Paris eleven years later in 1771, only to find that he had been declared legally dead and been replaced in the Royal Academy of Sciences to which he had belonged. His wife had remarried, and all his relatives had "enthusiastically plundered his estate”. He had to fight a long legal battle before he could reclaim his identity, reestablish himself in society, and regain his place in the Royal Academy. The play was later made into an opera of the same name in 2007, apparently with even greater artistic license.
The sad tale of Le Gentil has a happy sidelight. In addition to Astronomy, he was very much interested in the natural sciences as well. As part of the collection from his Asian adventures he brought a beautiful flowering plant, later named hortensia, into France, and was thus instrumental in introducing it all over Europe. I like the beautiful picture of it on the back page of Dr Shylaja’s publication as much as the contents of the publication itself.