Observation Venus and Mercury Transits

A transit, in astronomy, is when an inferior planet, like Venus or Mercury -a planet, that is, which is located between the Earth and the Sun- is seen, in perspective, transiting against the background of the Sun, as seen from the Earth. Transits are rare events as they occur only when the Sun, the planet and the Earth are adequately aligned. Mercury is seen transiting the Sun about 13 times a century, as Venus' transits are much rarer, occurring by rows of two each 105 or 121 years

The Mechanics of a Transit Observing a Transit

More Details About the Coming Venus and Mercury Transits

The Mechanics of a Transit

Although the Venus, or Mercury, transits are working on the same basis, in terms of celestial mechanics, both have some details in particular

How Does a Transit Work? The Venus Transits in Particular

The Mercury Transits in Particular

How Does a Transit Work?

When a transit may occur depends upon the celestial mechanics! A transit in front of the Sun, as seen from Earth, by Venus or Mercury requires two things. First, that the planet be crossing the plane of the Earth's orbit, which is, for such an event, considered the plane of reference. The plane of the Earth's orbit is also called the 'plane of the ecliptic', or the 'ecliptic'. Mercury or Venus may cross that plane up or down (when the planet crosses the ecliptic's plane upwards, that's called the 'ascending node', and downwards, the 'descending node'). Such moments, which exist in the number of two, are called the nodes of the orbit of either Venus or Mercury (for more about the characteristics of the orbit of a planet, see our tutorial 'Orbits'). The second thing required for a transit is that the inferior planet be at its inferior conjunction -it be between the Sun and the Earth, in line with both. So, it's when Venus, or Mercury, are at one of the nodes of their orbit, relatively to the Earth's orbit's plane, and when they are, at the same time, at their inferior conjunction, that there is a possibility for that the transit of an inferior planet occurs. The actual occurrence occurs due to further coincidences -mostly the relative positions between the orbit of the Earth, the one of the inferior planet, and the one of the Sun- provided by the celestial mechanics, leading to that the planet, with those both moments reunited, is seen, perspectively, against the background of the Sun. The theory also works for any planet in the solar system seen from a other one further from the Sun

Interrogations about transits of inferior planets in front of the Sun began with Johannes Kepler's Rudolphine Tables of planetary motion in 1627, as they permitted to predict future positions and interesting alignments. He thus discovered that both Mercury and Venus would transit the Sun's disk by late 1631 but he died before. French astronomer Pierre Gassendi became the first to witness a transit of Mercury that year as he tried to observe a one of Venus the following month. He did not know however that the transit was not observable from Europe. The observation allowed to measure the apparent size of Mercury’s disk, as well as help estimating the distance from Earth to the Sun. As Kepler had predicted that no transit of Venus should then occur before in the 18th century, young British amateur astronomer Jeremiah Horrocks calculated that a other one would occur in 1639. As he ended his calculations just a month before, he had not time to warn the astronomers' community and he and his friend William Crabtree were likely the only ones to observe the transit of Venus on December 4th, 1639. They also accurately measured the apparent diameter of the planet. It was not 40 years after that a young Edmond Halley observed the 1677 transit of Mercury from Saint Helena's Island whence he was compiling a southern hemisphere star catalog. He then realized that the careful timing of transits could be used to determine the distance of Earth from the Sun via the parallax effect from two observation posts located on remote corners of the Earth. That was too to allow to the absolute distance scale of the entire solar system. Venus transits were better suited to that than Mercury's because Venus is closer to Earth and possessesa larger parallax. Unfortunately, his method was somewhat impractical since contact timings of the required accuracy are difficult to make. Nevertheless, the 1761 and 1769 expeditions to observe the transits of Venus gave astronomers their first good value for the Sun's distance. The expedition in 1761 was led by famed English explorer Cook -he was just a lieutenant at that epoch- settling in Tahiti island, now a French possession in the Pacific Ocean, as another expedition looked at the event since Hudson Bay, Canada. The results however could not be achieved as any accurate timings had been impeded by a mysterious 'black drop' effect during which the disk of Venus looked like remaining attached to the solar limb. That effect even affected the more modern expeditions in 1874 and 1882. The black drop phenomenon was indeed associated with the refraction of the atmosphere of Venus as the phenomenon was discovered by Russian astronomer Lomonosov in the 18th century A.D. in St. Petersburg. Venus transits were meant too to measure the distance to the stars using the parallax technique as applied to the Earth's orbit self. Today, Venus transits are remaining useful as transits are part of the methods which are used to detect exoplanets, those planets orbiting around other stars. Transits provide a great opportunity to study the way planets and stars move in space –information that has been used throughout the ages to better understand the solar system- as transits still helps- scientists today calibrate the instruments of the solar exploration space missions

The Venus Transits in Particular

A Venus transit may occur in early June or early December, which are the both times when Venus is crossing one of each its orbit's nodes, relatively to the ecliptic. The celestial mechanics, on the other hand, make that the Venus' transits occur in pair, along a 8-year row, about each century -or, more accurately, between each 105 to 121 years! This means that, when between 105 to 121 years have elapsed, two Venus transits occur in a row, and separated by 8 years. Thus, the Venus transits are much rare astronomical events and mostly the one of a lifetime. As the orbit of Venus is inclined 3.4 degree with respect to the ecliptic, most inferior conjunctions of Venus do not result in a transit because the planet passes too far above or below the ecliptic and does not cross the face of the Sun. Venus transits currently recur at intervals of 8, 105.5, 8 and 121.5 years. Four Venus transits have occurred since the dawn of the telescopic era. Transits occurred in 1631 and 1639, 1761 and 1769, 1874 and 1882, and, eventually, in 2004, with the next of that last row in 2012. After that latter date, no transit of Venus is to occur before 105 years have elapsed, with a new row of two occurring in 2117 and 2125! During a transit, Venus is seen at, for example, 58" of apparent diameter

The Mercury Transits in Particular

The transits of Mercury are not as rare as those of Venus. There are, on average, 13 Mercury transits each century. All Mercury transits occur either in May (about May, 8th) or in November (about November, 10th), the times when the planet is crossing one of both the nodes of its orbit relatively to the ecliptic. When a transit occurs in November, Mercury is at its perihelion (the nearest Sun that is, leading to an apparent diameter of 10") as Mercury, in May, is at aphelion (the farthest Sun, with an apparent diameter of 12"). The November transits occurs each 7, 13 or 33 years. The transits in May are twice less numerous than those in November and they do occur each 13 or 33 years. That lesser probability of a May transit is due to a slower orbital motion of Mercury at aphelion which makes it less likely to cross the node during the critical period. Next Mercury transits will occur in 2016, 2019, and then in 2032 and 2039

Observing a Transit

How Does a Transit Unfold?

Observing a Transit

How Does a Transit Unfold?

Technically a Venus or a Mercury transit are unfolding the same. The process is about similar to a solar eclipse. The transiting planet is seen like a small, black point moving from the East to the West on the background of the solar disk. The transit is divided into two ingress contacts (contact I and II), two egress contacts (III and IV) and a central point (it's the time of the greatest transit, with the planet passing at its nearest of the Sun's center -that moment is also called the 'minimum separation' or 'greatest')

The two ingress and egress contacts are understood like: contact I is when the planet's external limb (or side) arrives to the contact with the Sun's external limb (or side). It's the external tangency of the transit. Contact IV, at the opposite, is the external tangency of the planet's last external limb which is then leaving the Sun's limb

Contact II is the internal tangency for the transit. As the planet already has progressed on the solar disk, its limb is leaving the contact with the solar limb and planet really is beginning its course in front of the Sun. At the opposite, contact III is the internal tangency of the transit, when the planet begins to quit definitively the solar disk. The planet's limb is arriving to the contact of the Sun's limb

The internal tangent contacts (contacts II and III) are precisely the moments when, after or before the so-called "black drop effect", the planet's disk is -or keeps being- entirely encircled by Sun's light as that is due to the refraction of Venus' atmosphere. Before -or after that- the limbs are in contact, with the planetary disk not dissociated from the Sun's limb. After -or before- the contacts II and III, the planet's dark disk is linked, in a drop-shaped look to the solar limb during 20 to 30 seconds. On the other hand, the black drop effect is also a natural optical effect, which you can verify by pinching your thumb and index fingers; just narrow both slowly and you will see, just before they touch, that a shadowy bridge is springing across the gap (it's a matter of dark-to-bright gradients). The contacts II and III are measured when planetary disk becomes completely dissociated (contact II) or when it stops being (contact III). It is these contacts which are usually measured during an observation. Contact I and IV are not easily observable as the transiting planet is still -or back- in the Sun's glare. Both those contacts however may be observed, should one use a H-alpha filter. The transiting planet thus, is seen silhouetting against prominences or the Sun's chromosphere. In the case of Venus, observers since recently have taken notice of that a arc of light is seen around Venus' disc during the first and last minutes of the transit as the planet is at the external limit of the disc, or partially already or still seen against it. That arc is due to the planet's atmosphere refracting sunlight as the study of the arc will help reveal the temperature and density structure of Venus's mesosphere, or middle atmosphere, where refraction occurs, as the mesosphere might be a key also to the physics of Venus's atmosphere super-rotation. The arc of light can be seen both in white light or through H-alpha filters and accessible to backyard telescopes

A Venus or a Mercury transit is taking a long time: Mercury, for example, may take up to 5 hour and 20 minute along its journey against the Sun, or Venus about 6 hour and 12 minute! The 'minimum separation' or 'greatest' is the center of the course, when the planet is getting to its nearest of the center of the solar disk -and farthest from the limb

Observing a Transit

Like recalled repeatedly across this page, observing a transit needs that you'll take in account the fact that the transit is unfolding against the background of the Sun. Which means that a transit has to take in account the dangers related to the Sun to be observed through any of those techniques dedicated to the observation of the Sun. A choice between those techniques may be done function of the planet observed, or the comfort of observation (which is specific to each observer). Venus, for example, may theoretically be observed naked-eye. At about 1 arcminute of apparent diameter the planet is looking like a small sunspot. Binoculars (used with their appropriate solar techniques) or a small telescope at a small power, are bringing a better observation however. In the case of Mercury, the relative smallness of the apparent diameter of the swift planet may lead to discard the projection technique to the benefit of the technique of the solar filters affixed to the instrument's aperture (in that case, a site, once, was advising a power of 50x to 100x). A transit, of course, may be observed photographically, provided you follow the appropriate techniques in terms of protection against the dangers of the Sun. Amateurs can make a useful contribution by timing the four contacts at ingress and egress. Since poor seeing often increases the uncertainty in contact timings, an estimate of the possible error associated with each timing should be included. The 'position angle' is the direction of Mercury at greatest transit with respect to the center of the Sun's disk as measured counterclockwise from the celestial north point on the Sun

In any case, and due to the length of a transit, always think to remain motivated about the dangers of the Sun (never look directly to the Sun, etc. Another good example, in case of an observation when the Sun is low -at dawn, or in the evening- in the sky, is to remember that, even low, the Sun is remaining dangerous, like in any other case of observation! Should your observation attract other people, always describe them those dangers of the observation of the Sun, or with the Sun in the field -and that's especially true for the children, who, further, must never bet let unattended near a telescope!)

this is a diagram of the June 8th, 2004 Venus transit The times for the four contacts of the transit were then (in UT): - contact I at 05:13:29 - contact II at 05:32:55 - minimum separation at 08:19:44 - contact III at 11:06:33 - contact IV at 11:25:59

The transits, often, are the object of some maps, like, for example, at Fred Espenak's Eclipse Wise. Such maps (in English) will allow you to find the trajectory of the transit against the Sun, the line of the ecliptic, the geographical North, East, South and West of the solar disk, and the areas of the world where the transit is observable (a transit may be observable totally as, like for a Moon eclipse, two areas have the transit already en cours when the Sun rises, and the transit interrupted by the sunset; an area too has no transit at all!)

Like for any major astronomical event, the important in the observation of a transit is to get ready before the event. Such events are breathtaking and you might well be caught off guard during your observation session in the case you would not have gotten ready properly! A good way to get prepared is to, some days before the observation, make some review of what is needed and of what is going to happen. Get some maps for the event, get detailed data in terms of time, anticipate about what instruments and techniques you'll be using -and get them prepared. Think about the convenience from where you'll be observing from, and too about what the trajectory of the Sun will be in the sky, as a transit is unfolding on a great length of time (don't have your observation session spoiled for cause of a building, for example, intervening in the process at mid-session!). Review the transit times. Try too to visualize how the transit will unfold. Think too, at last, to be comfortable (in terms of real comfort -chair, some lunch to eat, water, etc.- and of comfort relative to the fact the Sun is involved into the observation -sunglasses, hat, sun cream, etc.). And, don't forget that you're going to observe a rare event. That's another way to motivate yourself! Just think that any lack of serious preparation will have you missing such an event, and that you'll have to wait for years before seeing another one -or, in the case of Venus, that you possibly won't see any other in your lifetime!

A last hint, as far as relatively newbies are concerned, in terms of amateur astronomy, is the question of the sunspots. A unwarned observer might be surprised to see that there are various dark features on the Sun. Those mostly will be sunspots, those dark areas on the Sun which are caused -like the weathered amateur astronomers know well- by the fact that those areas are much lower in temperature than the surrounding areas. Don't confuse such sunspots with the planet transiting! Venus -or Mercury- is looking like a plainly round object, as sunspots are clearly irregular ones

More Details About the Coming Venus and Mercury Transits

The 21st century will provide for 12 transits of Mercury, since 2016 as your last chance to see a transit of Venus in that century will be in June 2012!

The Next Mercury Transits

The Next Venus Transits

The Next Mercury Transits

The following table is giving you the dates and main data for the transits of Mercury in the 21st century (the smaller the 'minimum separation', the most away from the Sun's side the transit will be -hence the most remarkable; the time in UT is when the moment of the 'minimum separation' occurs, heralding the middle of the transit)

this is a view of how large Mercury during a transit may be expected in size (top left) as the spots lower are sunspots

The Next Venus Transits

The following table is giving you the dates and main data for the transits of Venus to come. One only, now, remains to occur during that 21st century, the one on June 6th, 2012, the very last chance, for those in the appropriate area(s) to check such a rare event. The last one had occurred in 2004! The next row of two transits will occur in the 22nd century only, by 2117 and 2125. The smaller the 'minimum separation', the most away from the Sun's side the transit will occur -hence the most remarkable; the time in UT is when the moment of the 'minimum separation' occurs, heralding the middle of the transit; the area where the transit will be observable is a guess only for the transits in 2117 and 2125. A large part of South America and all the western -and southermost- areas of Africa will be totally out of the show only for the transit in 2012

A chart below is showing what the observation areas of the June 2012 transit will be, as a diagram is showing the general aspect of the transits in 2012, 2117 and 2125 (with what the transit's shape was in 2004)

click to Venus June 2012 transit visibility

click to Venus transits 2012, 2117-2125; dotted line is the ecliptic. Inclinations of the Sun are unsignificant