Dr. Sten Odenwald
Although the transit of Venus is certainly a rare event in the heavens, why would a scientist be interested in it? Why was it that so many hundreds of scientists in the 1700's and 1800's voyaged thousands of miles from home to observe it, at a cost of millions of dollars? It all had to do with something as simple as the size of the solar system.
The revolutionary solar systems models created by Nicolas Copernicus, and improved by Johannes Kepler and other astronomers of the 1600's, were able to calculate the positions of the known planets, but only relative to the Earth-Sun distance. This important distance was simply '1.0' and all other distances were specified relative to this fundamental unit.
When you make a scale model of the solar system with its nine planets, how do you know that in this model, the actual distance from Sun to Earth is 93 million miles and not, say, 153 million or 23 million? The answer is that you have to come up with a way to actually measure this physical distance.
In 1663, Rev. James Gregory (1638 - 1675) who was well-versed in mathematics and astronomy, and considered one of the most important mathematicians of the 17th century, suggested that a more accurate measurement of the Earth-Sun distance. He published this idea in his 1668 book Geometriae pars universalis, (or in Opticae promota in 1663?) he showed how this distance could be measured from observations of the transit of Venus made from various widely separate geographical locations. In addition, in 1664 he is also credited with inventing the first reflecting telescope based on a design we now call Gregorian, but it never worked because of inaccurate polishing. Also, Sir Isaac Newton declared this design unworkable and offered his own 'Newtonian' design which ultimately became more popular. In fact, there was nothing wrong with the Gregorian design and his 'first discovery' has been unfairly forgotten in favor of Newton's later accomplishment. This is sometimes the way credit for discoveries go in science research! As one Scottish history site about Gregory declairs:
"There is yet another discovery of the very highest importance to the science of astronomy, which is falsely and, we would hope, unknowingly attributed to another philosopher, whose manifold brilliant discoveries throw an additional lustre over the country which gave him birth. We allude to the employment of the transits of Mercury and Venus, in the determination of the sun's parallax, the merit of which is always ascribed to Dr Halley, even by that eminent astronomer Laplace. But it is plainly pointed out in the scholium to the 28th proposition of Gregory's work, published many years prior to Halley's supposed discovery."
Sir Edmund Halley, the namesake for Halley's Comet, later made the same suggestion 14 years later in 1677 and published an important paper on the details of this technique in 1716. He had originally read this paper before the Royal Society of London in 1691, entitled 'A new Method of determining the Parallax of the Sun, or his Distance from the Earth'. In the paper he described in detail how scientists of various nations, observing the upcoming 1761 and 1769 transits of Venus from many parts of the world, would be able to measure this distance to near-perfect accuracy. Although the distance did get refined enormously, this measurement turned out to be harder to perfect than scientists had anticipated. But the focus on Venus at this crucial moment led to another, even more exciting discovery.
Figure 2 - The atmosphere of Venus seen during a transit by Lomonosov and sketched as a ring in fig.6 and fig.7
In 1761, the Russian astronomer Mikhail V. Lomonosov (1711-1765) watched the first of these two transits and discovered a very strange thing. Instead of the very black disk of Venus sliding into the Sun's bright edge, it actually grew a beautiful halo of light all around its dark edge. The halo lasted only a few minutes and then vanished. With great insight, he figured out that this is exactly what you would expect to see if Venus had an atmosphere! Lomonosov is often regarded as one of the most eminent physicists of the 18th century. He was the founder of the prestigeous Moscow State University in 1755, and a monument to him, erected in 1877 can be found in the main square of the university.
This transit, and the next one in 1769 were observed by international teams of scientists all over the world. Over 400 measurements were made world-wide during the 1769 transit alone. After collecting the data and working on calculations for several years, they announced a new value for the Earth-Sun distance close to 85 million miles, but the uncertainty was still quite large because of a multitude of observation and measurement 'errors'. Could they ever hope to measure it even better?
They were already beginning to be frustrated by the atmosphere of Venus itself, which gave the edge to Venus a slightly indistinct shape, which confused the ultra-precise measuring times for the transit. A one-second error in measuring the precise 'contact' times during the transit could lead to thousands of miles of error in the final distance. The skill of the observer could only overcome this problem to a certain degree, then it became an issue out of the astronomers hands entirely. Even with the use of photographs had its fill of problems.
Some observers during the 1874 transit preferred to stick to the older method of measuring the four 'contact times' when Venus entered and left the Sun. In Figure 3 we see the transit moving from left to right across the sun. When the disk of Venus first 'contacts' the face of the sun, this is called First Contact. When it is just inside the disk of the sun and is last in contact with the limb of the sun, this is called Second Contact. After it transits across the face of the sun it reaches Third Contact where the edge of the Venus disk first contacts the right-hand edge of the sun's disk. Then after passing across this edge it reached Fourth Contact and the transit officially ends. Because of our atmosphere distortions, optical effects and the accuracy of your clock, the actual times reported by many observers of the same Contact event can differ by several seconds.
Other astronomers began taking photographic plates by the hundreds hoping to capture the transit in enough detail to work out the exact Contact Times, and the interplanetary geometry. But the methods during the 1874 transit were still primitive, and many of the photographs were useless. During the 1882 transit, direct timing of the transit was all-but abandoned, and thousands of photographs were taken instead - with better calibration techniques. In the end, it took nearly ten years for all of these photographs to be properly measured, and their data pushed through complex mathematical calculations, which had to be done entirely by hand. In the end, Simon Newcombe in 1891 published his conclusions in the 'Report of the Council to the Seventy-Fifth Annual General Assembly'. Based on his exhaustive analysis of all records since 1761, he concluded that the solar parallax was 8.79 +/- 0.051". This works out to an Earth-Sun distance between 92,372,500 and 93,450,700 miles. This uncertainty in the calculations spanned a range of about 1 million miles!
The 1882 transit was the heyday of international studies such as the above image of the U.S. Naval Observatory in Washington D.C. But despite all of the hard work and preparation, the final determination of the Earth-sun distance, now called the Astronomical Unit (AU), was hardly better than previous ones, not because of poor instruments, but because of the atmosphere of Venus itself. Its indistinct edge made it very hard to measure the transit times to the level of accuracy that was technically possible, and which would have resulted in a distance uncertainty well below a half-million miles.
In 1931, a close passage of the asteroid Eros to Earth allowed astronomers to triangulate the distance to this object as well, which resulted in a second estimate for the AU of 93 million miles.
Beginning in the 1960's, radar pulses from the Goldstone Tracking Station, the Haystack Radio Observatory in Massachusetts and from the giant Arecibo telescope in Puerto Rico were reflected from Venus and their travel times calculated. After 40 years of measurements the current value for the AU is now 92,957,209 with an uncertainty of only a few miles!
|1595||Tycho Brahe||5 million miles|
|1610||Kepler||15 million miles|
|1672||Giovanni Cassini||87 million miles|
|1882||Venus Transit||92.5 million miles|
|1931||Eros flyby||93 million miles|
|Venus Radar studies||92,957,209 miles.|
The Atmosphere of Venus - The earlier discovery of the atmosphere of Venus by Lomonosov inspired astronomers during the 1874 and 1882 transits to use a new technology called spectroscopy to detect the elements in the atmosphere. But all they could conclude, and wrongly, was that there seemed to be a lot of water in the atmosphere. In 1922, astronomers St. John and Nicholson investigated the spectrum of Venus at wavelengths where oxygen should be detectable, but no trace of this gas was found. Then in 1934, astronomers Walter Adams and Theodore Dunham used even more refined spectroscopic instruments to finally detect a gaseous component to the atmosphere. It wasn't nitrogen, oxygen or any other simple gas, instead it was carbon dioxide. Lots of it! In fact so far as they could tell, carbon dioxide was the only constituent of the atmosphere. Decades later, astronomers also discovered traces of sulfuric acid, and went on to make the first measurement of the surface temperature of Venus. It was hardly a moist, tropical and humid planet like some science fiction stories had claimed. Instead, its surface was at a sizzling temperature of over 800 Fahrenheit with an atmosphere thick enough to crush any human or machine not properly protected.
Figure 5 - Ultraviolet image of Venus taken by the Mariner 10 spacecraft in 1973 during a fly-by. Although it appears featureless to Earth-bound optical telescopes ,the atmosphere has a distinct, banded structure to it, produced by atmospheric winds and solar heating.
Venus is often called a 'Twin' to Earth, but we know that that really isn't the case except for its diameter and mass. Ancient observers once called it Lucifer ('The goddess of Light') who later became associated with the ruler of Hell. In fact given the surface temperature of Venus, it is not a bad analogy for this horrific abode within our own solar system. Rather than a twin of Earth, it is more like the antithesis of Earth in nearly ever way we can now scientifically quantify.
In 1934, astronomers Walter Adams and Theodore Dunham used sophisticated spectroscopic instruments to finally detect a gaseous component to the atmosphere, some 173 years after Lomonosov had discovered the atmosphere during a transit. Venus's atmosphere wasn't nitrogen, oxygen or any other simple gas, instead it was carbon dioxide. In fact so far as they could tell, carbon dioxide was the only constituent of the atmosphere. Decades later, astronomers also discovered traces of sulfuric acid, and went on to make the first measurement of the surface temperature of Venus. It was hardly a moist, tropical and humid planet like some science fiction stories had claimed. Instead, its surface was at a sizzling temperature of over 800 Fahrenheit with an atmosphere thick enough to crush any human or machine not properly protected.
Venus is often called a 'Twin' to Earth, but we know that this really isn't the case except for its diameter and mass. Rather than a twin of Earth, it is more like the antithesis of Earth in nearly ever way we can now scientifically quantify. But why should this be so? How could two planets in nearly the same part of the solar system end up so very different? Is there something about Venus today that might tell us what Earth was like in the past, or may become in the future?
Figure 6 - A Magellan spacecraft view of the surface of Venus using radar mapping techniques. It has a heavily cratered and volcanic surface.
Venus has no magnetic field, but Earth has a very strong one. We also know that Earth's field changes in time. Not only does it wander around in its direction on earth's surface, but its intensity also changes. During the last 125 million years, geologists have found that there have been hundreds of times when our magnetic field has reversed its polarity. We also know that during the last 150 years, the strength of its field has decreased by nearly 10%. In only a few thousand years, Earth's field may once again 'flip' over. There will be no hazards for us to worry about on the ground because these reversals have happened before with no Mass Extinctions or other obvious effects that can be seen in the fossil record.
The atmosphere of a planet is a delicate thing indeed. It is held captive by a planet's gravity, but as it gets heated by the sun, some of the atoms and molecules can reach escape speed and be lost into interplanetary space. The solar wind, as it streams by, has atoms that can collide with atmospheric atoms, and like a cosmic pool game, eject the atoms by direct collision. As we look at the inner planets, Mercury, Venus, Earth and Mars, we see worlds whose atmospheres have been changed by the Sun, but in strikingly different ways. By comparing them, we can learn about how solar interactions alter a planet. We can also learn something about how our own planet may change in the future.
The largest, single, challenge for astronauts traveling to Mars will be to overcome exposure to solar storms and radiation.