The Transit of Venus
June 5th-6th 2012
by Martin J. Powell
On June 5th-6th 2012, a transit of Venus across the disk of the Sun took place. Venus transits are very rare events which can be observed from much of the world, however since they involve viewing the Sun, extreme caution must be taken when attempting to observe them (for details on how to safely view the Sun, see below).
During a transit, Venus is seen as a small, black dot moving slowly in an East-to-West direction across the Sun. The 2012 transit commenced on June 5th at 22:10 UT and ended on June 6th at 04:50 UT, with mid-transit taking place on June 6th at 01:29 UT. The total duration was about 6 hours 40 minutes, however because of the effect of parallax, the exact duration varied by ± 7 minutes depending upon the observer's location on Earth. The 2012 transit lasted 27 minutes longer than the previous one (June 8th 2004) because on this occasion Venus passed closer to the Sun's centre. In 2004 the planet crossed the Southern hemisphere of the Sun; in 2012 it crossed the Northern hemisphere of the Sun (see illustration below right).
Tracks of Venus across the solar disk in 2004 (lower) and 2012 (upper) (click on the thumbnail to see the full-size image, 25 KB). To see the timings of each event, click here (27 KB). Times are shown at hourly intervals in Universal Time (UT), which is equivalent to GMT.
Venus had an apparent diameter of 58".2 (58.2 arcseconds) during the transit. The Sun's apparent diameter at this time of year is 31'.5 arcminutes (i.e. 1890"), so that the Sun's apparent disk was approximately (1890 ÷ 58.2) = 32˝ times larger than that of Venus. Because of parallax, Venus' disk appeared slightly displaced in a vertical sense on the solar disk when seen from locations in the far North and far South of the world. For example, an observer situated in Nome, Alaska, USA saw Venus positioned 1'.2 (75".6) further South on the Sun's disk than an observer situated at Sydney, Australia. All of the diagrams on this page show Venus positioned in a geocentric sense, i.e. from a theoretical position at the Earth's centre.
Note that, although mid-transit occurred on June 6th at 01:29 UT, it did not coincide with the moment of inferior conjunction because the track of Venus did not pass precisely through the Sun's centre, but 9'.2 to the North of it. Inferior conjunction took place on June 6th at 01:08 UT, some 21 minutes before mid-transit.
Because Venus was seen against the solar disk, the transit could be viewed from anywhere on the Earth where the Sun was above the horizon at the time of the event. The 2012 transit could be seen in full from Eastern Asia, the South-east Pacific Ocean (including New Zealand and central/Eastern Australia), the North-western USA (Alaska) and North-western Canada (the Sun being above the horizon throughout). For much of the inhabited world, however, the transit was already in progress at sunrise or sunset so it was not seen in its entirity. Observers in Portugal, South-western Spain, Western and South-western Africa and the Southern and Eastern regions of South America did not see the event because the Sun was below the horizon. At latitudes North of 67° North, the Sun was above horizon throughout the day, hence the entire transit was visible.
Transits of Venus can only occur whenever the planet is close to its ascending node or descending node (the points in its orbit where the planet crosses the ecliptic heading Northwards or Southwards, respectively). For such an event to take place, the planet must be within a few days of inferior conjunction (i.e. positioned directly between the Earth and the Sun) and also be close to crossing one of these nodes. Because of the positioning of the nodes in relation to the Earth's orbit, transits can only take place around June 7th (descending node) or December 8th (ascending node) although the node positions and the event dates slowly change over time. The ascending and descending nodes of the planets are positioned exactly 180° apart - hence the six-month difference between these two dates. Transits at the descending node take place in Eastern Taurus while those at the ascending node take place in Southern Ophiuchus.
Transits of Venus take place at intervals of 113˝ years ± 8 years. In other words, they occur in pairs (one 8-year Venus cycle apart) after an interval of just over a century. The pattern runs as follows: 121˝ years, 8 years, 105˝ years, 8 years, 121˝ years, etc. There were no Venus transits in the 20th century; prior to that, they took place in December 1874 and December 1882. After the only transits of the 21st century (in 2004 and 2012) there will be a 105˝ year gap until the next one takes place on December 11th 2117.
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The Venus Transit 2012 Animation
The animation lasts a little over 5 minutes and plays out the changing events over the course of the transit (a still frame is shown below - click on the image to see the animation, which will open in a new window). As Venus moves across the Sun's disk, the Earth's shadow moves accordingly and the local times across the world are displayed at each stage. The animation commences on June 5th at 22:10 UT (just after the start of the transit), after which there is a 20-minute interval (to 22:30 UT). The next six hours (22:30 UT on June 5th through to 04:30 UT on June 6th) are shown at half-hourly intervals, after which there is another 20-minute interval, bringing the time to 04:50 UT (just after the end of the transit). The animation therefore allows one to estimate the times at which the transit was visible from his/her own location to within about 30 minutes.
The Transit of Venus, June 5th-6th 2012 Animation showing the entire event (click to see the animation, Note: 2.3 MB). The world map shows the regions of the world from which the transit was visible. The animation commences on June 5th at 22:10 UT (just after the start of the transit) and ends on June 6th at 04:50 UT (just after the end of the transit). In the lower half of the graphic, the time zones show the local times at which the event took place, listed by their military time zone designations.
To determine the time zone in which you are situated, locate your Standard Time offset from Greenwich (hours East or West of Greenwich) shown in italics beneath each zone letter (alternatively, refer to the world map at worldtimezone.net). For example, the standard time at Los Angeles, USA is 8 hours behind Greenwich, so the relevant times will be found in the '-8 ' time zone, i.e. zone U ('Uniform'). The time zone abbreviations used in the animation are listed here (32 KB). For more details on how to use the animation for your own time zone, refer to the main text below.
The start of the transit (i.e. when the disk of Venus moves on to the solar disk) is known as the ingress whilst the end (when Venus moves off the solar disk) is known as the egress (see illustration at right). Both ingress and egress are split into exterior contact (where Venus' disk is externally tangential to that of the Sun) and interior contact (where Venus' disk is internally tangential to that of the Sun). Timings of the ingress, mid-transit and egress are given in the box at the upper left of the animation (times are shown in UT or Universal Time, which is equivalent to Greenwich Mean Time). Also shown is the position angle (P.A.) of Venus on the solar disk, i.e. its compass bearing relative to the Sun's centre (measured anti-clockwise from North through East, South and West, where North = 0°, East = 90°, South = 180° etc). Beneath the ingress and egress times the upper section of the Sun's disk is shown, with Venus appearing as a black dot (shown to scale). The world map beside it shows which regions were in daylight or night-time as the transit took place.
Diagram showing the Ingress and Egress stages of a solar transit (click on thumbnail for full-size image, 13 KB). The four stages of ingress and egress (i.e. from upper left to lower right in the diagram) are alternatively referred to as first, second, third and fourth contacts respectively.
Beneath the illustrations, the time zones of the world are shown. Each time zone extends 15° in longitude (i.e. equivalent to one hour of the Earth's rotation). In the animation, the zones are listed according to their military designations (where A = 'Alpha', B = 'Bravo', C = 'Charlie' etc). Under this scheme, time zone Z ('Zulu') represents Greenwich Mean Time; this zone is centred on the Greenwich Meridian (0° longitude). Time zones A to M are situated to the East of Greenwich (i.e. to the right of 'Zulu' time in the animation) whilst zones N to Y are to the West of Greenwich (i.e. to the left of 'Zulu' time in the animation). Because of space limitations, the time zones in both East and West regions are split into two, one above the other. For quicker identification, the approximate locations of each time zone is marked above and below the world map.
Note that, from locations to the West of time zone R ('Romeo') through to the International Date Line, the entire transit took place on June 5th. From locations to the East of time zone A ('Alpha') through to the International Date Line, the entire transit took place on June 6th.
Each time zone is headed by its military letter designation. Beneath it, in italics, is the standard time difference from Greenwich (i.e. not the summertime offset). Hence at New York, USA, the standard time difference from Greenwich is -5 hours (5 hours behind Greenwich) and it therefore falls under time zone R ('Romeo'). Beneath the italicised time offset is displayed the Standard Time operating in that zone; hence in the above example (zone R) it will be 5 hours behind the Greenwich time. Since it is summertime in the Northern hemisphere, Daylight Savings Time operates from numerous locations in the Northern hemisphere at this time of year, and wherever this applies it is shown beneath the Standard Time. Hence in zone R, Eastern Daylight Time (EDT) is operating, which is one hour ahead of the Standard Time.
Where only a handful of countries are situated within a particular time zone, their local time abbreviations are displayed. Hence in time zone Q* ('Quebec star') Venezuela is the only country operating its time offset of -4˝ hours, so only the abbreviation VET (Venezuela Time) is listed. In most cases, however, the number of countries contained within a given time zone are too numerous to list, so only the Standard Time is shown.
The time zone data is colour-coded to indicate whether the transit is wholly visible, partly visible or not visible from any given zone (the time zone letters above and below the world map also change colour accordingly). Because the Earth's shadow falls obliquely across the time zones at this time of the year, in many cases one region of a time zone can see the transit whilst another region cannot, despite the clock times being the same; wherever this situation occurs, the time zone data appears orange. For example, from India (zone E*) the start of the transit was not visible (i.e. the ingress was not seen) so the initial period is shown in grey. The transit started to become visible from around 23:30 UT (05:00 IST) when the Sun rose over the country's North-eastern region. The zone data is shown in orange for the next 1˝ hours, since the event was only visible from its Eastern region during this time. By 01:00 UT (06:30 IST) the whole country was able to view the transit, so the data is shown in yellow through to the end of the event.
The transit was wholly visible from time zones I ('India'), K ('Kilo'), K*, L ('Lima'), L*, M ('Mike'), M*, M**, Y ('Yankee') and X ('X-Ray'); consequently, these zones appear yellow throughout the animation. The transit was not visible from time zone O ('Oscar') since it took place during the local night time (hence it is coloured grey throughout the animation). From all other time zones, the transit visibility changes between not visible, partly visible and/or wholly visible, depending upon where the region was situated in relation to the Earth's shadow at any particular time.
The line (curve) dividing the light and dark regions of the world (or of any planet or moon, for that matter) is known as the terminator. On the world map it is labelled Sunrise on the shadow's Eastern edge and Sunset on the shadow's Western edge. It follows that the closer to the terminator a particular region of the world is positioned, the lower in the sky the Sun will appear from that region.
Close to the Northern edge of the shadow, where daylight is almost continuous at this time of year, the Sun set during the transit and then rose a few hours later with the event still in progress (e.g. in Iceland). Conversely, along the Southern edge of the shadow, the daylight is short at this time of the year. In the Wilkes Land region of Antarctica, the Sun rose during the transit and then set a few hours later, so that the event was visible for the entire local day.
Finally, the zenith position of the Sun (i.e. where it is positioned directly overhead) is shown on the world map by the symbol . On June 5th-6th 2012 the Sun's declination (angle from the celestial equator) is +22°.7, which means that the Sun passes through the zenith around midday from all locations along latitude 22°.7 North. The zenith position of the Moon is likewise shown by the symbol .
More precise transit times for any particular location in the world may be obtained from the following websites:
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The 'Black Drop Effect' and the 'Aureole'
Venus is subject to several mysterious observational phenomena (see 'Venus through the Telescope') and one such mystery has became well-known whenever Venusian solar transits take place. First observed during the transit of 1761, the black drop effect occurs just after moment of second contact (ingress, internal contact) and also just before the moment of third contact (egress, internal contact). The black circular dot of Venus often appears elongated in the direction of the Sun's limb, causing the planet to appear momentarily 'teardrop-shaped'. The apparent 'ligament' between the planet's limb and the Sun's limb appears greyish and fuzzy, often frustrating astronomers' attempts to determine the precise moment of second and third contact. For many years the scientific theory was that the phenomena was caused by sunlight refracting through Venus' atmosphere in the direction of the Earth, thus distorting the planet's apparent shape. However, the high number of observations by amateurs and professionals during the 2004 transit began to cast some doubt on this explanation. Whilst smaller telescopes users were often seeing the 'black drop', many observers using larger instruments did not.
The 'black drop effect' and the 'aureole' of Venus A series of drawings by Italian astronomer Mario Frassati showing the June 2004 transit. Ingress is shown in the left box and egress in the right box (click on thumbnail for full-size image, 30 KB). The 'black drop' is best seen in the series of ingress drawings, one image from the right. The 'aureole' is best seen in the egress drawings, four from right (Image source: Mario Frassati/BAA).
The mystery was largely solved when astronomers studied the results of a satellite observation of a transit of Mercury in 1999. Unlike Venus, Mercury does not have an atmosphere but the effect was nonetheless observed - hence the 'black drop' could not have been caused by refraction in Venus' atmosphere. Neither did the Earth's atmosphere play any significant role (i.e. turbulence or poor seeing conditions) since the satellite observing Mercury was, of course, outside the Earth's atmosphere! The astronomers concluded that the 'black drop' was caused by a combination of two factors: a telescope's inherent optical defects (causing any image seen through it to be slightly blurry) and solar limb darkening, i.e. the darkening effect around the Sun's circumference (limb) caused by the gas in that region being more opaque - and therefore darker. These two factors combine to cause a dark blurriness at the point where the limbs of Venus and the Sun touch - hence causing the 'black drop effect'. The fact that the effect was less widely reported in 2004 than in previous centuries is testament to the much-improved optics of modern-day telescopes.
Much less of a mystery is another effect often reported by observers during a solar transit of Venus: namely, the aureole. Like the 'black drop', it was first observed in 1761, although this effect requires superlative telescope optics and excellent seeing conditions to detect. The aureole is a bright arc which is seen around the circumference of the planet which may appear partial or complete. At ingress, the aureole begins to show itself between first and second contacts, i.e. when about half of the Venusian disk has crossed the solar limb. At egress, it usually starts just after the moment of third contact. It begins as a spot of light (apparently, near one of the Venusian poles) which slowly extends as the planet moves off the Sun.
The aureole is caused by the refraction of sunlight through the Venusian atmosphere. The extent of the arc is determined by the density of the Venusian atmosphere at the time of the transit. The effect diminishes when the planet is positioned more than half its angular width away from the solar limb. A short Quicktime movie of the effect, taken by the Swedish 1-metre telescope on La Palma in 2004, can be seen at the transitofvenus.nl site.
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Observing the Venus Transit
The safest way to observe a transit of Venus (or Mercury) is to project the image of the Sun through a refracting telescope on to a piece of white card (i.e. the image of the Sun is projected backward through the telescope, from the main object glass through to the eyepiece and on to the card). In practice, a second piece of card is normally attached to the telescope, positioned just ahead of the eyepiece and perpendicular to the telescope's axis, in order to create a shadow around the projected image and thereby improving its contrast. The solar image on the card appears pale white, the silhouette of Venus looking like a small black dot (rather like a large sunspot). The projected image may be flipped horizontally and/or vertically, depending upon the telescope's optical arrangement.
Binoculars can similarly be used to project the Sun on to a piece of card. The resulting image is naturally smaller than that from a telescope and, of course - unless one of the lenses is capped - the binoculars produce two identical images.
Transits can also be observed safely through a telescope by attaching an aluminised mylar solar filter to the front of the telescope, ahead of the object glass (always ensuring beforehand that the filter has not been damaged in any way!!). If a mylar filter is used the solar disk appears pale blue. Filters can also be purchased which can be attached to conventional cameras for photographing the event.
Methods of safely viewing the Sun Four techniques by which the transit of Venus can be safely viewed (click on each thumbnail for a larger version, 12 KB / 4 KB / 13 KB / 15 KB): (Left) solar projection by telescope (Centre left) projecting the Sun using a pair of binoculars (Centre right) attaching an aluminised mylar filter to the front of a telescope and (Right) wearing solar viewers (Image sources: solar projection by telescope from Sky At Night Magazine; binocular projection by Nils Ölmedal; telescope with mylar filter by Don Cross; children wearing eclipse shades from Xinhua/Li Xiang/People's Daily Online).
Similar filters for observing Venus transits are commercially available which allow direct observing of the Sun using a pair of cardboard 'spectacles' (referred to as solar viewers or eclipse shades); these are commonly sold to the general public in advance of a total eclipse of the Sun. Again, extreme caution must be taken when using them.
When wearing eye protection, Venus can be seen against the Sun using just the naked-eye. During the June 2004 transit, the author was fortunate enough to have clear skies for the event from his observing location in the south-western United Kingdom. Looking through solar viewer 'glasses', Venus could just be discerned as a tiny dot - at the threshold of visibility - against the solar disk. The apparent size of Mercury's disk is, however, too small to be seen with the naked eye whenever it transits the Sun.
More information on how to safely view the Sun can be found at NASA's website.
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Copyright © Martin J Powell October 2011 / July 2012
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