This very rare astronomical event occurred two hundred years ago, this week. However, the fact is, during the Regency, this event was "occult" in more ways than one. Very few of our Regency ancestors would have seen it, those that did so had to travel nearly halfway around the globe, and there are no known written observational records of this event which are extant today. Nevertheless, due to the extreme rarity of this astronomical event, a Regency author might find it a useful plot device for a special romance set during this extraordinary and spectacular "affair" between Venus and Jupiter.
When Venus occulted Jupiter in the Regency . . .
First, of course, it is necessary to understand the astronomical meaning of the term "occult." An occultation in space occurs when one celestial body passes in front on another, from the perspective of the observer. A classic example is a solar eclipse, when, over certain portions of the Earth, the Moon appears to pass in front of the Sun and partially or totally obscures the light coming from our nearest star. Another common occultation is a lunar eclipse, when the Earth passes between the Sun and the Moon, casting its shadow on the Moon. However, other occultations occur between other bodies in space, including between some of the planets in our solar system. It must be remembered that all of these astronomical events occur from the perspective of the observers, all of whom have been located on Earth, until the end of the last century. The term "eclipse" is perhaps a century older than the term "occultation," but both originated in the Middle Ages and were in use during the Regency. However, the term eclipse is typically used for occultations which involve the Earth, the Moon and/or the Sun.
Astronomers have also used another term when the celestial body crossing in front of another one is much smaller and does not obscure the body behind it. That term is transit, which was often used to refer to the passage of the planets Mercury or Venus across the face of the much larger Sun. In 1769, one of the missions of Captain Cook’s first voyage of exploration was to observe the transit of Venus across the Sun from the island of Tahiti. Because Venus was passing in front of Jupiter and was so much smaller than the giant outer planet behind it, the event which took place on Saturday, 3 January 1818, was referred to by some men of science as a transit rather than an occultation. However, both terms are scientifically correct and would have been understood during the Regency.
The occultation of Jupiter by Venus, as observed from Earth, is an extremely rare astronomical event. Scholars of the history of astronomy note that there were only three before the occultation of 1818. The first would have taken place in what would have been mid-June of 2 BC, the second in mid-September of 1210, and the third in early February of 1570. There are no surviving observational records of any of these events, which suggests that either they were not observed, those who did observe the occultations made no records, or that any records which were made have been lost or destroyed. There are also no known observational records of the occultation which occurred in January of 1818, for which lack scholars speculate that only those living in remote areas might have seen it and did not bother to record their observations. In fact, what we do know of these early occultations of Jupiter by Venus are the result of in-depth research and extensive and detailed calculations by modern astronomers. There will be a gap of nearly two and a half centuries between the occultation of 1818, and the next, which will occur in November of 2065. (It is likely there will be very detailed observational records made of that celestial event.)
Venus is the second planet from the Sun in our solar system, and is the planet closest to Earth. It is also the second brightest celestial body visible from Earth in the night sky, after the Moon. Jupiter is the fifth planet from the Sun and it is also the largest planet in our solar system. Despite its superior size, Jupiter is the third brightest celestial body visible from Earth at night, after the Moon and Venus, because of its much greater distance from Earth. Both Venus and Jupiter are visible from Earth with the naked eye at night, so they have been known since ancient times. They can be distinguished from the surrounding stars because they do not appear to twinkle. That is due to the fact that their light has a significantly shorter distance to travel to Earth than does the light of even the star nearest our solar system. Long before the invention of the telescope, in the seventeenth century, countless humans have tracked the movements of those planets for a plethora of reasons.
By the Regency, both the quality and power of telescopes and the observational skills of astronomers had improved significantly. The musician, composer and amateur astronomer, William Herschel, who discovered the planet Uranus was living at his Windsor home, Observatory House, throughout the Regency. In addition to his continued astronomical observations, Herschel and his son were actively producing high-quality, custom-made telescopes for a select clientele. Another valuable tool for those who were interested in the movements of the celestial bodies in the solar system was the orrery. These clockwork models of the solar system made it possible for even an amateur astronomer to track the movements of the Sun, Moon, and the planets in relation to one another, without the need for complex computer-driven calculations. It would have been possible for an astute astronomer to predict when Venus would occult Jupiter, and even from where on Earth it could best be observed.
On 3 January 1818, when Venus moved across the face of Jupiter, it would be visible from Earth only in the Northern Hemisphere, primarily over the Pacific Ocean. It would also be visible from the western part of North America, and the eastern edge of the Asian continent and the northern islands of Japan. In order to see the disk of Venus completely occult the disk of Jupiter, the observer would have to be located along northern latitude 45º. At that location, the two planets would be just 9º above the horizon. The occultation would not be visible from anywhere in Europe or northern Africa. The best time to observe the occultation would be before sunrise, since the flood of sunlight would obscure the view of the planets for anyone using the telescopes which were available at that time. In order to view the occultation before sunrise, the observer should be located on, or as close as possible, to longitude E 145º. Scholars of astronomical history estimate that the best place from which to have viewed Venus occult Jupiter would have been to the east of the island of Hokkaido. This island is the farthest north of all of the Japanese islands and was probably the most remote part of the country in the early nineteenth century. [Author’s Note: This page, created by Canadian scholar Larry Bogan, provides a number of useful details about the Venus/Jupiter occultation of 1818. He also includes an image of how the occultation would have appeared to someone viewing it with a good telescope, as well as a map of the viewing area on Earth.]
Japan would not officially open its borders to westerners until the mid-nineteenth century. Nevertheless, there were a few intrepid adventurers who risked their lives to visit this exotic land in the early nineteenth century. However, it would not have been necessary for a determined Regency astronomer to travel to the island of Hokkaido itself, since the optimum viewing point for the Venus/Jupiter occultation was actually to the east of the island. Therefore, an astronomer with the necessary wherewithal could have arranged for a ship to take him (or her) to the ideal location to view the occultation. That ship might have sailed from Britain, Australia, or possibly even eastern Russia, which was accessible to westerners in 1818. With a good telescope and the other necessary equipment on board, it would have been possible to observe Venus occult Jupiter from a ship anchored in the optimal viewing location just before sunrise on 3 January 1818.
Though it may seem like a great deal of effort to expend in order to view a celestial event, the effort would almost certainly have been worthwhile. Not only are planetary occultations quite rare, during the course of the observation, the astronomer is likely to gain significant information, particularly about the occulting body. For example, observations of the transit of Venus, in 1769, provided information which made it possible to more accurately calculate the distance from the Earth to the Sun. Observations of the occultation of Jupiter by Venus could well have provided new information about both planets, particularly about Venus.
Dear Regency Authors, if you are seeking a particularly exotic scientific adventure for an upcoming romance, might the observation of the occultation of Jupiter by Venus serve the purpose? Perhaps the hero is a wealthy young gentleman with a deep interest in astronomy. He commissions a ship to travel to the optimal viewing point, and either hires, or invites, other interested astronomers and men of science to accompany him to view the Venus/Jupiter occultation. Will the heroine be part of that group, or, might she, also a committed astronomer, stow away aboard ship? On the other hand, perhaps the driving force for such an expedition is the heroine, a wealthy bluestocking who wants to make her name in the scientific world. Will she commission a ship and invite, or hire, the hero, to help her manage the logistics of the journey?