Saturday, December 10, 2011

The history of Venus transit

Venus transits occurred throughout history. It is not known however, if someone actually saw such a transit before 1639. It is possible to see the transit with the naked eye if the sun is rising or setting, or if there is a hazy dusty air. It is probable that someone in the early past saw it. The first modern astronomer who calculated the transits was Kepler who predicted  the transit of 1631. Sadly, Kepler's predictions were not accurate enough, the transit was invisible from Europe and no one saw it. Kepler predicted the next transit to be only in 1761. However, a very young English astronomer, Jeremiah Horrocks, calculated from Kepler's Rudolphine Tables, that a transit should happen on the 24th of November, 1639. Sadly, Horrocks reached this conclusion less than a month before the transit. The transit was fully visible from America, but there was no time to send a message to the new colonies. So the first observation of a Venus transit was by only 5 people. Horrocks himself and his colleague William Crabtree and together with Crabtree's family we reach a count of 5 people in all. Horrocks and Crabtree utilized the transit to measure Venus's apparent size compared to the size of the sun and used this data to estimate the distance to the sun. Horrocks was among the first to see that the universe is much larger than ever thought before and estimated the earth-sun distance to be 100 Million kilometers. Although this value is still 50% wrong, it was 4.5 times more than the value Kepler gave, and more or less within the correct magnitude.
Horrocks was the first significant English astronomer and he is often called the Father of British Astronomy. He was both a great observer and a great theoretician, a rare combination, suppressing Tycho Brahe, Copernicus, Kepler and Galileo. Sadly, Horrocks died unexpectedly (the reason of his death is unknown) at the young age of 22.
The next pair of transits were in the years 1761-1769. The measurements and calculations from these transits were used to more accurately determine the distance between the Earth and the sun which by than was called Astronomical Unit (AU). Observations were made from many places, the most famous was in Tahiti by no less than the legendary Captain Cook.  From these observations, a better value which is very similar to the present value was calculated.
The next pair was in 1874 and 1882, and gave another chance to refine the AU distance.
The last transit was in 2004, a marvel to watch, and now we are waiting for the 2012 transit, not to be repeated again before 2117.
Jeremiah Horrocks observing the Venus Transit
Jeremiah Horrocks observing the Venus Transit by Eyre Crowe. The painting name is The founder of English Astronomy. Painted in 1891

For further reading please refer to the following Venus Transits books.
Historical observation data can be found here.

Monday, December 5, 2011

Books about The Transit of Venus

There are several books which deals promptly with the Transit of Venus since its first observation in 1639 and till our days. I will recommend several books about the transit. While the topic is covered in short in many astronomy books, reading a book which is dedicated to the phenomena is more satisfying.

The transit of Venus by Wiliam Sheehan deals with the scientifc importance of the transits. In this unique and fascinating history of science, acclaimed popular science writer William Sheehan and award-winning geographer John Westfall take us back through the centuries to chronicle the intrepid explorations of scientists and adventurers who studied the transits of Venus in the quest for scientific understanding. In exquisite prose, the authors begin their true tale with the first telescopically observable transit in 1639, when Copernicus's vision of the solar system was just gaining acceptance. This "Earth-shattering" observation was of monumental importance, because it helped confirm that the planets, including Earth, revolved around the Sun, and not the reverse as had heretofore been believed. Sheehan and Westfall take us on a journey through time vividly evoking the excitement and adventures of explorers and scientists who braved the elements, wars, and disease to follow the transits of the past. Some succumbed to an early grave. Others were victorious in capturing those precious, fleeting moments when Venus cast its shadow on the Sun. From the courageous voyage of Captain James Cook through later breath-taking adventures to the upcoming 2012 transit (the book includes information about the upcoming 2004 transit which is long past for us), this uniquely invaluable tome will provide a history and a guide for the future on the unparalleled beauty and meaning of experiencing a transit of Venus. Complete with maps showing both historical and contemporary observation points, plus tables of visibility conditions for major cities, this eloquent story of a timeless phenomenon will enchant readers of all ages.
The transit of Venus by Peter Aughton deals with the life of Jeremiah Horrocks which was an early English astronomer, and never really got the fame he is deserves. Horrocks was the first to observe a Venus transit, so viewing the Venus transit 2012, will actually means to continue his way! There is a missing chapter in the history of astronomy--between the work of Galileo and Newton--and it is a chapter that belongs to England. In the period before the English Civil War, Horrocks was the greatest astronomer in the kingdom. He knew the positions and motions of the planets more accurately than any person of his time, and was the first to appreciate the true scale of the solar system and formulate a valid theory for the wanderings of the moon. Yet he was not an elderly grey-bearded sage, but a young man living in provincial obscurity, who on his death had barely come of age but who left a great scientific legacy.
Peter Adds approach to the subject is slightly different and he discussed the influence of the transit, the fact that the Venus transits in the 18th centuries were simultaneously observed to measure our distance from the sun, and actually revealing once more how large is the universe. Going beyond the science of astronomy, this study looks at the history of Venus' rare passings across the face of the sun which have affected the exploration, colonization, and science of our planet for thousands of years. In response to Venus' 2004 transit, the first in 121 years, the scientists and historians in this study look at the extraordinary impact these astronomical events have had on our culture throughout history, and look ahead to the next transit in 2012.