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In astronomical terms, a transit happens when one celestial body passes directly in front of another, making it appear as though it moves across the other's surface from our viewpoint on Earth. A prime example of this is when the moon crosses the sun during a solar eclipse. Even rarer than solar eclipses is the transit of Venus across the Sun. The most recent occurrence was in 2004. Fortunately, you're in luck! The next Venus transit will take place this year, on June 5 to June 6, 2012. Those in the right locations with clear skies will be able to witness this rare event. The best viewing area will be the Pacific Ocean. For those willing to travel, Tahiti offers the ideal location, and the island is preparing to welcome astronomy enthusiasts. Portions of the transit will be visible from Europe and North America, but much of South America and western Africa will miss it. Viewers will see Venus as a small dark dot moving across the Sun's surface. Depending on where you're located on Earth, the transit may take several hours to pass.
The transits of Venus follow a repeating pattern that occurs every 243 years, with transits happening eight years apart, followed by a gap of 121.5 years, and then another eight-year gap (the 2012 transit is the final one in the eight-year sequence that began with the 2004 transit), followed by an extended gap of 105.5 years before the next transit. So, if you miss the Venus transit this June, you’ll need to live to an advanced age to witness the next pair of transits, which will occur in 2117 and 2125. To find out if you can catch part or all of the 2012 Venus transit, visit here (note – a fast internet connection may be required).
10. The First Recorded Observations
Galileo’s first telescope was invented in 1609, but the first opportunity to observe a Venus transit with modern optical devices came in 1631 and 1639. In 1627, Johannes Kepler became the first person to predict a Venus transit. Kepler successfully predicted the 1631 event, but he could not determine the best places to observe it, nor did he realize that the transit would be invisible in much of Europe. As a result, no one planned to travel to locations where they could witness the event, and it was missed.
Eight years later, on December 4, 1639, a young amateur astronomer named Jeremiah Horrocks became the first person in modern history to predict, observe, and document a Venus transit. Horrocks corrected Kepler’s previous calculations and realized that Venus transits occur eight years apart, followed by long waiting periods. It is commonly believed that Horrocks made his observations from Carr House in Much Hoole, near Preston, England. Horrocks also informed his friend, amateur astronomer William Crabtree, about the predicted event, and Crabtree managed to observe the silhouette of Venus on the solar disk, likely from Broughton, Manchester, England. While Horrocks was unsure about the exact timing of the transit, he was fortunate to guess correctly, allowing him to witness part of it. He used a telescope to project the image onto a white card, observing the event safely without damaging his eyes. With his data, Horrocks made the most accurate calculation of the Astronomical Unit (AU) at the time.
9. First to Calculate the Astronomical Unit
One hundred and twenty-two years later, during the next eight-year pair of Venus transits, renowned astronomer Edmond Halley (famous for Halley’s Comet) proposed that scientists could accurately estimate the distance between the Earth and the Sun (known as the Astronomical Unit or AU) using the principle of parallax. Parallax is the apparent shift in an object’s position when viewed from two different locations, measured by the angle between the two observation lines. Halley proposed that if the Venus transit were observed and measured from distant points on Earth, combining those measurements using parallax and trigonometry could determine the precise distance between the Earth and the Sun (AU). Before this, scientists relied on Horrocks' AU calculation but realized more accurate observations were necessary for a better estimate.
The Venus transits of 1761 and 1769 sparked a wave of global scientific observations, marking one of the earliest examples of international scientific cooperation. Scientists, primarily from England, France, and Austria, traveled to remote locations such as Newfoundland, South Africa, Norway, Siberia, and Madagascar to make observations. In South Africa, Jeremiah Dixon and Charles Mason made notable measurements that would later contribute to the historic Mason-Dixon Line in the USA. Key locations for the 1769 transit included Baja, Mexico; Saint Petersburg, Russia; Philadelphia, USA; Hudson Bay, Canada; and Tahiti, where the famous British explorer Captain Cook observed the transit from a site he called “Point Venus.”
Based on data from the two Venus transits, French astronomer Jérôme Lalande calculated the astronomical unit (AU) to be approximately 153 million kilometers. This was a significant improvement over Horrocks’ earlier calculation from his 1639 observations. Today, the modern value of an AU is 149 million kilometers (92,955,807.3 miles).
8. Discovery of Venus’s Atmosphere
Before astronomers observed the Venus transit, it was unknown that Venus had an atmosphere. That all changed during the 1761 Venus transit. From the Petersburg Observatory, Russian scientist Mikhail Lomonosov predicted that Venus had an atmosphere. While observing the transit, Lomonosov noticed the planet’s image refracting solar rays. During the early phase of the transit, he saw a ring of light around the trailing side of Venus, the part that had not yet passed in front of the Sun. Lomonosov correctly concluded that the light refraction could only be explained by an atmosphere surrounding the planet.
7. The Black Drop Effect
The most crucial moments when observing a Venus transit are the four contact points. These are: when Venus’s shadow first touches the Sun's disc (first contact), when Venus’s shadow completely enters the Sun’s disc (second contact), when the shadow's leading edge exits the Sun's disc (third contact), and when the shadow leaves the Sun’s disc entirely (fourth contact). Accurately timing these transitions is vital for obtaining precise data. Unfortunately, an optical phenomenon known as the black drop effect makes it challenging to observe and time the second and third contacts accurately.
Right after second contact and just before third contact during the Venus transit, a small black “teardrop” appears, seemingly linking Venus’s disc to the Sun’s limb. This effect prevents astronomers from pinpointing the exact times of second and third contact. This uncertainty contributed to errors in calculating the true value of the Astronomical Unit (AU) during the 1761 and 1769 transits. Initially, it was thought the black drop effect was due to Venus’s thick atmosphere, but it is now understood to be caused by disturbances in Earth’s atmosphere. Modern telescopes and improved optics are reducing the black drop effect for astronomers observing Venus (and Mercury) transits.
6. Search for Extrasolar Planets
By the time the Venus transits of 2004 and 2012 occurred, more precise methods of measuring the Astronomical Unit (AU) had been developed. However, these transits were still highly anticipated. They provided valuable scientific opportunities, particularly aiding in the search for planets beyond our solar system.
Scientists were eager to study how Venus’s transit dimmed and interfered with the Sun’s light. This data would help refine techniques for detecting extrasolar planets. Currently, various methods are employed to detect these planets, but they mainly identify large planets, like Jupiter-sized ones. Developing a more accurate way to detect smaller planets by measuring the light they block during their transits would allow astronomers to calculate the size of much smaller planets orbiting distant stars. This requires highly precise measurements, as Venus’s transit only causes a tiny 0.001 magnitude drop in the Sun’s light, and the dimming caused by smaller extrasolar planets would be even less noticeable.
5. First Transit of Venus “Movie”
In December 1882, astronomer David Peck Todd traveled from Amherst College in Massachusetts to California to photograph the transit of Venus. The 1874 and 1882 transits were the first to be documented using photography, and Todd’s work marked one of the first photographic records of a Venus transit. From the summit of Mount Hamilton, at what would later become Lick Observatory (still under construction at the time), Todd took 147 glass negative plates on December 6, capturing most of the transit. The conditions were ideal, with no clouds, and the plates were carefully stored, though they were soon forgotten as better methods of documenting the transits emerged.
In 2002, two astronomers from Sky and Telescope magazine rediscovered the long-lost photographic plates, all of which were still intact and in excellent condition. They recognized that the sequence of images could be compiled into the first ever “motion picture” of a Venus transit. The resulting film captures one of the significant historical Venus transit observations. You can view the animation made from the 147 negatives here (note – QuickTime and a fast internet connection are required).
4. Transit Creep and Non-Pairing Transits
The months during which the eight-year pairs of Venus transits occur are slowly shifting forward. Prior to the 1631 transit, these pairs took place in May and November. Currently, Venus transits can only be seen in June or December. Transits typically happen in pairs, about eight years apart, because an eight-year period on Earth is nearly equal to 13 years on Venus, placing the planets in nearly the same relative positions every eight years. However, this small discrepancy is causing the arrival of these eight-year transit pairs to gradually shift forward on the Earth calendar.
Although Earth and Venus typically align for a pair of transits, this does not always happen. The 1396 transit, for instance, did not have a corresponding pair (there was no transit in 1404, and the next one occurred in May 1518). The next solo transit will take place in 3089.
3. Guillaume Le Gentil
Guillaume Joseph Hyacinthe Jean-Baptiste Le Gentil de la Galaisière, more commonly known as Guillaume Le Gentil, was a French scientist and astronomer who set the bar for long names. He made notable contributions to astronomy, particularly with some of the earliest observations of several Messier objects. However, his most significant role was during the international effort to document the 1761 Venus transit, a task that would cement his legacy as both an intriguing and tragic figure in the history of science.
Le Gentil was one of more than a hundred observers who traveled across the globe to secure strategic vantage points for observing the transit and improving the accuracy of calculating the astronomical unit (AU). Many of these expeditions were unsuccessful, hindered by cloudy skies, rain, hostile locals, inaccessible locations, and malfunctioning equipment. Yet, none faced as much misfortune as Le Gentil.
Guillaume Le Gentil departed from Paris in March 1760, intending to reach Pondicherry, a French colony in India. He arrived in Mauritius by July, only to learn that France and Britain were at war. Before his ship could reach Pondicherry, he discovered that the British had taken control of the colony, so his ship was redirected back to Mauritius instead.
On June 6, 1761, the predicted Venus transit occurred, but Le Gentil was still aboard the ship. Although the skies were clear, observing from the rolling deck of a ship at sea was not feasible. He decided to wait for the next transit, scheduled eight years later, thinking it was no big deal after coming so far.
During his wait, Le Gentil kept busy with various tasks, including mapping the coast of Madagascar. He then set sail for Manila in the Philippines to observe the 1769 transit. However, upon arrival, he faced resistance from the Spanish authorities. He then sailed back to Pondicherry, India, arriving in March 1768. There, he constructed a small observatory and waited for the June 4, 1769 transit. Sadly, despite weeks of clear skies, June 4 brought nothing but clouds and rain. Unable to see the transit, Le Gentil became disheartened and decided to return to France. His journey was delayed by dysentery, and his ship was caught in a storm. He was stranded on Reunion Island, east of Madagascar, waiting for a Spanish ship to bring him back to France. Le Gentil finally returned home in 1771, almost 11 years after his departure, only to find he had been declared dead, removed from his position in the Royal Academy of Sciences, and stripped of his fortune by his relatives. To top it off, his wife had remarried. Eventually, his position in the Academy was restored, and he spent the rest of his life in France.
2. Transit of Venus March
The year 1882 saw a Venus transit, and to mark this historic occasion, along with the unveiling of a statue of American physicist Joseph Henry, who developed the first electric motor and served as the first secretary of the Smithsonian Institute, composer and bandleader John Philip Sousa was commissioned to compose a march. The march was written, published by J.W. Pepper Company, and quickly faded into obscurity. However, it was performed for the first time on April 19, 1883, at 4 pm, a time that held Masonic significance for Sousa, a Freemason. The time was connected to the element copper, which was used in electric motors (developed by Henry) and Venus, a connection that makes sense to Masons but may be lost on others.
The march quickly came and went, much like a Venus transit, and was thought to be lost for over a century until it was rediscovered in 2003 in the Library of Congress. Just in time for the 2004 Venus transit, the long-lost Sousa march, 'Transit of Venus March,' was brought to light. In 2004, the Library of Congress, in collaboration with NASA, reintroduced the march to the public, though it seems the excitement surrounding its rediscovery was not much greater than the public's enthusiasm back in 1883. Still, you can now listen to Sousa's 'Transit of Venus March,' which sounds very much like his other marches, in the clip above.
1. Multiple Transits at Once
Although rare, multiple transits happening at once do occur. For instance, it is possible for a solar eclipse and a Venus transit to take place simultaneously. The last occurrence of this rare event happened in 15,607 BC, and the next one will be on April 5, 15,232.
Another rare occurrence is when both Mercury and Venus transit the Sun at the same time. Both inner planetary neighbors align with the Earth’s orbit and the Sun, so observers on Earth could see the tiny shadows of both planets passing in front of the Sun simultaneously. The last time this event took place was in 373,173 BC, and the next will happen on July 26, 69,163. Will humanity still be here to witness this distant transit?
