The transit or passage of a planet across the face of the Sun is a relatively rare occurrence. As seen from Earth, only transits of Mercury and Venus are possible. On average, there are 13 transits of Mercury each century. In contrast, transits of Venus occur in pairs with more than a century separating each pair.
The last Venus transit was in 2004 so the second event of the pair will occur on Wednesday, June 6 (Tuesday, June 5 from the Western Hemisphere). The entire event will be widely visible from the western Pacific, eastern Asia and eastern Australia as shown in Figure 1. Most of North and Central America, and northern South America will witness the beginning of the transit (on June 5) but the Sun will set before the event ends. Similarly, observers in Europe, western and central Asia, eastern Africa and western Australia will see the end of the event since the transit will already be in progress at sunrise from those locations.
For Northern Hemisphere locations above latitude ~67° north, all of the transit is visible regardless of the longitude. Northern Canada and all of Alaska will also see the entire event. Residents of Iceland are in a unique wedge-shaped part of the path (Region X in Figure 1). They will see both the start and end of the transit but the Sun will set for a short period around greatest transit. A similarly shaped region exists south of Australia (Region Y in Figure 1), but here, the Sun rises after the transit begins and sets before the event ends.
The principal events occurring during a transit are conveniently characterized by contacts, analogous to the contacts of an annular solar eclipse. The transit begins with contact I, the instant the planet's disk is externally tangent to the Sun. Shortly after contact I, the planet can be seen as a small notch along the solar limb. The entire disk of the planet is first seen at contact II when the planet is internally tangent to the Sun. Over the course of several hours, the silhouetted planet slowly traverses the solar disk. At contact III, the planet reaches the opposite limb and once again is internally tangent to the Sun. Finally, the transit ends at contact IV when the planet's limb is externally tangent to the Sun. Contacts I and II define the phase called ingress while contacts III and IV are known as egress. Position angles for Venus at each contact are measured counterclockwise from the north point on the Sun's disk.
Table 1 Geocentric Phases of the 2012 Transit of Venus Event Universal Position Time Angle Contact I 22:09:38 41° Contact II 22:27:34 38° Greatest 01:29:36 345° Contact III 04:31:39 293° Contact IV 04:49:35 290°
Table 1 gives the geocentric times of major events during the transit. Greatest transit is the instant when Venus passes closest to the Sun's center (i.e. - minimum separation).
During the 2012 transit, Venus's minimum separation from the Sun is 554 arc-seconds (During the 2004 transit, the minimum separation was 627 arc-seconds). The position angle is defined as the direction of Venus with respect to the center of the Sun's disk, measured counterclockwise from the celestial north point on the Sun. Figure 2 shows the path of Venus across the Sun's disk and the scale gives the Universal Time of Venus's position at any point during the transit. The celestial coordinates of the Sun and Venus are provided at greatest transit as well as the times of the major contacts.
Note that these times are for an observer at Earth's center. The actual contact times for any given observer may differ by up to ±7 minutes. This is due to effects of parallax since Venus's 58 arc-second diameter disk may be shifted up to 30 arc-seconds from its geocentric coordinates depending on the observer's exact position on Earth. Table 2 and Table 3 list predicted contact times and corresponding altitudes of the Sun for locations throughout Canada and the United States, respectively. Table 4 provides similar predictions for a number of cities around the world.
Since the apparent diameter of Venus is nearly 1 arc-minute, it is just possible to see without optical magnification (but using solar filter protection) as it crosses the Sun. Nevertheless, the planet appears to be only 1/32 of the Sun's apparent diameter so a pair of binoculars or a small telescope at modest power will offer a much more satisfying view. All binoculars and telescopes must be suitably equipped with adequate filtration to ensure safe solar viewing. The visual and photographic requirements for observing a transit are identical to those for solar viewing. Amateurs can make a scientific contribution by timing the four contacts at ingress and egress. Observing techniques and equipment are similar to those used for lunar occultations. Poor seeing often increases the uncertainty in contact timings, so an estimate of the possible error associated with each timing should be included. Transit timings and geographic coordinates of the observing site (measured with a topographic map or GPS receiver) should be sent to: ALPO Transit Section, c/o Dr. John E Westfall, P.O. Box 2447, Antioch, CA 94531-2447, USA.
White light observations of contacts I and IV are not technically possible since Venus is only visible after contact I and before contact IV. However, if Hydrogen-alpha filtration is available, the planet will be visible against either prominences or the chromosphere before and after contacts I and IV, respectively. Observations of contacts II and III also require amplification. They are defined as the two instants when the planet appears internally tangent to the Sun. However, just before contact II, the so-called black drop effect is seen. At that time, the transiting planet seems to be attached to the Sun's limb by a thin column or thread. When the thread breaks and the planet is completely surrounded by sunlight, this marks the true instant of contact II. Contact III occurs in exactly the reverse order. Atmospheric seeing often makes it difficult to measure contact timings with a precision better than several seconds (see "black drop" effect below).
The orbit of Venus is inclined 3.4° with respect to Earth's orbit. It intersects the ecliptic at two points or nodes that cross the Sun each year during early June and December. If Venus happens to pass through inferior conjunction at that time, a transit will occur. Although Venus's orbital period is only 224.7 days, its synodic period (conjunction to conjunction) is 583.9 days. Due to its inclination, 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. Since the invention of the telescope (1610), there have only been seven transits as listed in Table 5.
Table 5 Transits of Venus: 1601-2200 Date Universal Separation Time 1631 Dec 07 05:19 939 " 1639 Dec 04 18:26 524 " 1761 Jun 06 05:19 570 " 1769 Jun 03 22:25 609 " 1874 Dec 09 04:07 830 " 1882 Dec 06 17:06 637 " 2004 Jun 08 08:20 627 " 2012 Jun 06 01:28 553 " 2117 Dec 11 02:48 724 " 2125 Dec 08 16:01 733 "
The 2004 and 2012 transits form a contemporary pair separated by 8 years. More than a century will elapse before the next pair of transits in 2117 and 2125. During the 6,000-year period from 2000 BC to AD 4000, a total of 81 transits of Venus occur. A catalog of these events containing additional details is available online at:
Additional information on transits of both Mercury and Venus can be found at:
When Johannes Kepler published the Rudolphine Tables of planetary motion in 1627, they permitted him to make detailed and fairly accurate predictions of the future positions and interesting alignments of the planets. Much to his surprise, he discovered that both Mercury and Venus would transit the Sun's disk in late 1631. Kepler died before the transits, but French astronomer Pierre Gassendi succeeded in becoming the first to witness a transit of Mercury. The following month, he tried to observe the transit of Venus, but modern calculations show that it was not visible from Europe. Although Kepler's predictions suggested that the next Venus transit would not occur until the following century, a promising, young British amateur astronomer named Jeremiah Horrocks believed that another transit would occur in 1639. His calculations were completed just a month before the event so there was little time to spread the word. Horrocks and his friend William Crabtree were apparently the only ones to witness the transit of Venus on 1639 Dec 04 which allowed them to accurately measure the apparent diameter of the planet. Unfortunately, Horrocks and Crabtree both died young before reachinbg their full potentials.
Nearly forty years later a young Edmond Halley observed the 1677 transit of Mercury while completing a southern hemisphere star catalog from Saint Helena's Island. Halley realized that the careful timing of transits could be used to determine the distance of Earth from the Sun. The technique relied on observations made from the far corners of the globe. The effect of parallax on the remote observers would allow them to derive the absolute distance scale of the entire solar system. Venus transits were better suited to this goal than were Mercury transits because Venus is closer to Earth and consequently exhibits a larger parallax. Halley challenged future generations to organize major expeditions to the ends of Earth in order to observe the transits of 1761 and 1769.
Many scientific expeditions were mounted but the results were disappointing. The accurate timings needed were not possible due to a mysterious "black drop" effect in which the edge of Venus's disk appeared to deform and cling to the limb of the Sun. Undeterred by the results, another major observing campaign was mounted by many nations for the Venus transits of 1874 and 1882. Again, the "black drop" limited the precision of the observations and the determination of the Sun's distance. Modern analyses show that the "black drop" is the result of seeing effects due to Earth's turbulent atmosphere.
The distance to the Sun and planets can now be measured extremely accurately using radar, so the 2004 and 2012 transits are of minor scientific importance. Still, they are remarkably rare events that were of great value during the early the history of modern astronomy.
As an aid to historical research, two Excel 97 spreadsheet files have been prepared that can perform calculations for any geographic position. Simply enter the location name, latitude and longitude. Each of the tables then calculates the altitude of the Sun at that location for every contact and for every transit in the table. The two tables are similar but cover different time periods for Transits of Venus:
These files will not open properly with versions older than Excel 97. Each spreadsheet is protected so the user can not accidently delete or edit any information that is required by the calculations. Only the name and geographic coordinates fields (green cells in the spreadsheets) may be modified.
The 2012 transit predictions were generated on a Apple Power Mac G4 computer using algorithms developed from Meeus  and the Explanatory Supplement . Ephemerides for the Sun and Venus were generated from VSOP87.
All calculations, diagrams, tables and opinions presented in this paper are those of the author and he assumes full responsibility for their accuracy.
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