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Venus Transit

The orbits of Venus and Earth tilt about three and a half degrees relative to each other. Usually when the Earth, Venus and the Sun line up, Venus as seen from the Earth is a degree or two above or below the Sun. Since the Sun is only a half degree wide, we do not see Venus transit (cross the face of) the Sun. However twice every 122 years or so, at eight year intervals, Venus, the Sun and the Earth line up near where the orbits of Venus and the Earth cross each other. At these times we can see a small dark disk cross the face of the Sun, marking a transit of Venus. In the twenty first century we will have two transits - one at dawn on June 8th, 2004 and the next in 2012.

Today interest in the transits of Venus are pictorial primarily - an image of a rare event. However until the advent of powerful radars and space craft, transits of Venus were the best method for determining the size of the solar system. Oddly enough, astronomers were able to calculate the relative sizes of the orbits quite accurately. What they could not do was calculate the absolute sizes of the orbits. All distances in the solar system were given in ratios compared to the astronomical unit where 1.0 AUs was the average distance of the Earth from the Sun. What wasn't know very accurately was that 1.0 AUs was 93,192,447.7 miles. In the 17th century this distance wasn't known accurately even in the second digit.

There was a theoretical way to calculate the distance, if we could simply time how long it took a body whose velocity was well known took to cross a distance which was well established. From centuries of observations of Venus, we knew exactly how long it took Venus to complete an orbit. We also knew from solar eclipses, that the Sun was approximately 400 Moon diameters across. Finally we knew how large the Moon's diameter was because the Moon (and only the Moon) was close enough to the Earth to be measured directly by surveying tools.

In 1769 it was known that a transit of Venus would occur over the Pacific Ocean on June 3. Among his other duties (principally mapping the Pacific and establishing British territories) the great English sailor and navigator James Cook was ordered to time the transit from a known location (in this case Tahiti) with the aid of a number of gentlemen and part time scientists who accompanied him. Timings were made but to the consternation of everyone, the timings were widely spaced. It wasn't simply a problem of poor clocks, but errors of several minutes. With errors this great, they could not pin down the size of the AU very well. They came up with a figure which was several percent too large.

The cause of these errors was fascinating in its own right and not totally understood today. When Venus crosses the solar disk, at entrance and again at exit, the round disk of Venus seems to stretch into sort of a tear drop shape. The particular telescope used and the observer's eye entered into the timings. Small telescopes such as those packed for the oceanic voyage were particularly prone to this stretching effect. The largest observatory telescopes were all placed on the wrong side of the globe away from visibility during the transit. Rather than being a source of annoyance today, many observers actually wish to see this effect.

The most accurate measurements of the distances in the solar system come from the time it takes an electromagnetic wave to cross from a planet to the Earth. Electromagnetic waves can be light, x-rays, heat but most commonly the waves used are radio and radar. The source of the waves can be space craft in the vicinity of a planet or radar pulses sent by huge arrays such as the radio telescope at Aricibo Puerto Rico. Our most accurate distance still come from the Moon where the error is on the order of a tenth of an inch. The need for timing the transits of Venus have come and gone.

Yet the technique is not totally gone. Within the last few years, large planets orbiting close to small stars have transited causing a small but measurable dimming of the light of the star. By timing the lengths of the diminutions, we can establish very accurately how far the planet is from the star, and from this data, the masses of the bodies. No Earth based radar is anywhere near powerful enough to bounce a signal from another solar system. At the maximum speed of our space craft, it would takes many millennia for one to reach another system. We shall have to content ourselves on this resurrected technique for determining sizes.

We are not planning any special activities to watch the transit of Venus at Frosty Drew Observatory. We would have to arrive before dawn on Tuesday June 8th to set up. In New England, the transit will already be well underway when the Sun and Venus rise together. We couldn't time the whole transit if we wanted. We would require a horizon which was free of obstructions and which could be relied upon not to have clouds or fog. Neither of these conditions suit Frosty Drew very well. Finally we have the safety issue. Viewing a transit of Venus requires all the precautions of viewing a solar eclipse. With large telescopes this means using special solar shields which exclude almost all of the Sun's radiation including the dangerous infra-red and ultra-violate radiation. A unfiltered telescope as large as the big as our Meade 16" LX200 forms a burning lens that would destroy a human eye in the briefest imaginable instant.

We could obtain such a filter. Several staff member's have telescopes so equipped. However it is hard to explain the differences between proper solar filters on telescopes and sunglasses used with binoculars to children. We simply do not want to encourage potentially dangerous practices. Given our poor location, the likelihood of haze, fog or clouds, the poor timing of the event, and most of all the potential for serious accidents, we are passing up this opportunity for a public viewing.

If you are resolved to view the transit of Venus, please insure that you have a safe way to view the Sun. The best ways are special astronomical solar shields which keep dangerous radiation out of the telescope and eyepiece projection where the Sun's image is projected on a screen. Dangerous techniques involve using homemade shields like exposed film, soot darkened glass, the wrong types of welding goggles or combinations of photographic filters. While eyepiece projection is safe for humans, this is not always the case for the telescope used. Internal focusing of rays can generate temperatures in excess of 1000 degrees. If these focuses touch anything inside, your telescope may be ruined.

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
May 1, 2004
Published Under:
Leslie Coleman's Columns
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