10 First Historical Observations
As Galileo invented his first telescope in 1609 the first chance to observe a Venus transit using modern optical devices came with the transits of 1631 and 1639. Five years before the 1631 transit, in 1627, Johannes Kepler became the first person to predict a transit of Venus. Kepler successfully predicted the 1631 event. However, Kepler was unable to determine where would be the best location to observe the transit, nor did he realize that in 1631 the transit would not be observable in most of Europe. Therefore, no one made arrangements to travel to where they could see it, and this transit was missed.
Fortunately, 8 years later on December 4, 1639, a young amateur astronomer by the name of Jeremiah Horrocks became the first person in modern history to predict, observe, and record a Venus transit. Horrocks corrected Kepler’s earlier calculations and realized what we now know about Venus transits, that they occur eight years apart after long (very long) waits. It is commonly stated that he performed his observations from Carr House in Much Hoole, near Preston England. Horrocks also told his friend, another amateur astronomer by the name of William Crabtree, about the coming predicted transit and he also spotted the planet’s silhouette on the solar disk. Crabtree probably observed near Broughton, Manchester England. Although Horrocks was uncertain when the transit would begin, he was lucky enough to guess the time so that he was able to observe part of it. He used a telescope to shine the image onto a white card, thus safely observing the transit without harming his eyes. Using his observational data, Horrocks came up with the best calculation for an Astronomical Unit (AU).
9 Used to Calculate an Astronomical Unit
One hundred and twenty-two years later came the next eight-year pair of Venus transits. During that time the noted astronomer Edwin Hubble had proposed that scientists could gain an accurate estimate of the distance between the Earth and the Sun (an Astronomical Unit or AU) using the scientific principal of parallax. Parallax is the difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Hubble correctly reasoned that if the Venus transit was viewed and measured from very distant points on the Earth, that the combined measurements, using parallax, could be used (with trigonometry) to calculate the actual distance between the Earth and the Sun (AU). Up to that time, the scientists were using Horrocks determination of AU, but realized they needed many more accurate observations to get a truer calculation.
Thus the Venus transits of 1761 and 1769 launched an unprecedented wave of scientific observations to the farthest points of the Globe. This was one of the earliest examples of international scientific collaboration. Getting (and surviving the trip) to these locations was as much an adventure as obtaining the first accurate data for a Venus transit. Scientists, mostly from England, France, and Austria, traveled to places as far apart as Newfoundland, South Africa, Norway, Siberia, and Madagascar. In South Africa very good measurements were obtained by Jeremiah Dixon and Charles Mason who would later go on to add their name to the historic Mason-Dixon Line in the USA. Noted points of the globe for the 1769 transit included Baja, Mexico; Saint Petersburg, Russia; Philadelphia Pennsylvania, USA; Hudson Bay, Canada; and from Tahiti, the great British explorer Captain Cook observed the transit from a place he called “Point Venus.”
Using the data obtained from the two transits, French astronomer Jérôme Lalande calculated the astronomical unit to have a value of 153 million kilometers. The calculation was a considerable improvement on Horrocks’ calculations from the 1639 observations. The modern measurement for an AU is 149 million kilometers (92,955,807.3 miles).
8 Discovery of the Atmosphere of Venus
Prior to astronomers viewing the transit of Venus no one knew Venus had an atmosphere. All of that changed with the 1761 Venus transit. Looking from the Petersburg Observatory, Russian scientist Mikhail Lomonosov predicted the existence of an atmosphere on Venus. Lomonosov saw the image of Venus refracting solar rays while he observed the transit. During the initial phase of the transit, he saw a ring of light around the trailing end of the planet (the portion that had not yet transited in front of the sun). He correctly inferred the only thing to explain the light refraction would be an atmosphere around the planet.
When observing the Venus transit, the most critical times are the first, second, third, and fourth contact. Being able to clearly see and time these transitions – from the shadow of Venus not touching, to just first touching the suns disc (first contact) the time the shadow of Venus fully transits into the disc of the Sun (second contact), and then when exiting, the point where the leading edge of the shadow of Venus again touches off the disc of the Sun (third contact), back into outer space, and the time the entire shadow has left the disc of the Sun (fourth contact) and is no longer visible – is important to gain accurate data. Unfortunately, an optical phenomenon called the black drop effect makes it difficult to see the second and third contacts.
Just after second contact, and again just before third contact during the transit, a small black “teardrop” appears to connect Venus’ disk to the limb of the Sun, making it impossible to accurately time the exact moment of second or third contact. This negative impact on the timing of the second and third contact contributed to the error in calculation of the true value of AU, in 1761 and 1769 transits. It was first thought the black drop effect was caused by the thick atmosphere of Venus, but it is now believed it is caused mostly by interference in the Earth’s atmosphere. Today, better telescopes and optics are minimizing the black drop effect for astronomers observing Venus (and Mercury) transits.
6 Search for Extrasolar Planets
By the time the 2004 and 2012 Venus transits rolled around, measurements of AU could be made using other and more accurate measuring techniques. However, that did not mean the 2004 and 2012 transits were not highly anticipated. They could still be used to do very important science, in this case, helping in the search for planets outside our solar system.
Scientists were eager to learn more about how patterns of light were dimmed and interfered with as Venus blocked out the Sun’s light. This would provide data to develop new and better methods to use the same technique to look for planets orbiting distant suns. Right now, a variety of other methods are used to “see” extrasolar planets orbiting distant suns. But most of these methods require the extrasolar planets to be very large – Jupiter-sized planets. Perfecting a way to “see” an extrasolar planet based on the light it blocks, coming from its sun, when it transits, would be a much more accurate way to detect the planet and could be used to ‘see” and calculate the size of much smaller planets orbiting these suns. However, extremely precise measurement is needed: for example, the transit of Venus causes the Sun’s light to drop by a mere 0.001 magnitude, and the dimming produced by small extrasolar planets will be much less.
