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Venus to cross the face of the sun

Few sciences are as intertwined and rooted in history as astronomy is. On the morning of Tuesday, June 8, most of the world?s population will be able to witness at least a portion of a celestial event that is not only scientifically significant, but one that is steeped with historical connections back to the early 1600s. The upcoming transit of Venus is not only an observational event, it is also an opportunity to look back and see that four hundred years ago, astronomy was an important part of peoples? lives, just as it is now.

If you sample some of the activities that the June 8 transit is initiating, you will find that then as now, investing in space research netted gains for society in general from one end of the practicality spectrum to the other. Knowing about what happens out there, can make our lives better, down here.

On the historical side, around 1620, Johannes Kepler, a German astronomer had just put forth a set of natural laws that described the motion of the planets.

Kepler?s three laws of planetary motion, as they came to be known as, greatly influenced later scientists like Isaac Newton, Edmond Halley and others. Kepler?s third law of planetary motion simplified by the expression P2/A3=K and translated into words says that if you knew a planet?s period of revolution around the sun ? the length of its year (P), you could calculate its average distance to the Sun (A).

In the expression for Kepler?s third law, P, the period is expressed in years and A, the average distance to the sun, is expressed in astronomical units, or AU for short.

The AU was and still is the astronomical yardstick for measuring distances between the planets-within our solar system and was defined as the average distance between the sun and Earth. What astronomers of the 1600s did not know, was the actual, physical length of the AU. Nobody really knew how far the planets were from each other in absolute numbers, only in units of the AU.

The determination of the length of the AU was the holy grail of astronomy at the time. The elusive quantity remained an extremely important goal for astronomers all over the world into the late 1800s.

The British, Astronomer Royal, once referred to the determination of the AU, as "the noblest problem in astronomy."

Not only was Kepler the first to formulate the fundamental laws governing planetary motion, he was also the first to understand that Venus and Mercury would occasionally transit, that is pass across, the sun?s disc. Since they are the only planets that orbit the sun inside of Earth?s orbit, they are the only planets that can pass across its disc as viewed from Earth.

In September of 1627, Kepler published a set of tables that predicted that a Venus transit would occur in December 1631. Kepler died in 1630, and as far as we can tell, no one witnessed this transit. What made matters worse, was that at the time of Kepler?s death, the next transit of Venus was not to happen until the middle of the 18th century.

Kepler also knew that if teams of observers could accurately observe and record the passage of Venus across the sun from different vantage points on Earth, the actual value of the AU could be determined using the effects of parallax. Parallax can be easily demonstrated by holding a finger out in front of your face.

By alternately closing one eye, then the other, your finger appears to shift in position against the background. By measuring the angle of that shift, one can calculate the distance from your face to your finger.

If Venus? passage across the disc of the sun was observed by several different expeditions from widely separated locations on Earth, Venus would appear in slightly different positions silhouetted by the sun.

If the distance between observers was accurately known, the Earth-Venus distance could be solved for using geometry.

Since astronomers already knew that Venus was 0.7 AU from the sun, it was a very simple feat to solve for the value of the AU, once the Earth-Venus distance had been determined. This fact was also realized by Edmond Halley (The Halley in Halley?s comet). Halley reasoned that if the expeditions could accurately observe and time the duration of the transit, a very exact measure of the Earth-Sun distance (the AU) could be made. And so until 1882, there was a "space race" of sorts to determine the value of the AU-to determine just how big our universe really was.

Due to a small error in his calculations, Kepler did not realize that there was to be another transit of Venus in 1639. This omission was uncovered by a twenty-one year-old, English amateur astronomer, Jeremiah Horrocks only a few weeks before the December 4 transit. Despite being called away from his home on the day of the transit, Horrocks observed at least some of the transit and later wrote up his observations.

Early the following January, he was about to publish his findings and set up a meeting with astronomer and friend, William Crabtree, quite possibly the only other person that observed the 1639 transit.

As fate would have it, the day before Horrocks and Crabtree were to meet, Horrocks died unexpectedly and his work went unpublished until 1662, when Isaac Newton got a hold of it and actually may have used it to formulate his own theories of planetary motion.

While no one witnessed the 1631 transit of Venus, two witnessed the 1639 transit, Crabtree and Horrocks. Certainly not enough came out of their partial observations to lead to the determination of the value of the AU.

But, the importance of future transits was now clear to astronomers around the world. Accurately observe the transit, and with that data the size of the solar system could be determined.

Over the next 243 years, Venus transited the Sun four more times, in 1761, 1769, 1874, and again in 1882.

The years leading up to each transit brought with them a treasure trove of stories and adventures of the teams that set out to observe the upcoming transits.

Before each transit, nations allocated large sums of money and planned extensive, scientific expeditions to cover each one-to extract the data that lay buried in the difficult and tedious observations.

But each time, something intervened to thwart their efforts. Often times it was the weather?other times it was stormy seas or resentful natives or warring factions. Sometimes it was just plain bad luck. As it turns out, the transits of Venus never gave astronomers the true value of the AU to the accuracy they were looking for-they came close, but not close enough.

As other technologies developed like radar and laser ranging, astronomers used them to measure the true distances to the planets.

With radar, an antenna directs pulses of electromagnetic waves(radio waves) at a target and receives a return pulse after the wave bounces off of the target.

The time it takes for the pulse to return is accurately measured and since we know the speed of the wave, we simply multiply the speed of the wave times the time it took the pulse to return. When this is done, we get a very accurate distance to the object.

Laser ranging is another distance measuring technique used by astronomers. A powerful laser beam is fired at a target. The faint laser reflection is recorded and timed. Again, we are left with a very accurate distance measurement to the target in question. The McDonald Observatory, operated by the University of Texas, has utilized its laser ranging facility for many years to measure the distance to the Moon to an accuracy of the width of your hand.

Well, if scientists do not need transits of Venus to determine the AU any longer, what?s all the fuss about? Why do we care about transits? First, transits are extremely rare events. The last transit occurred in December of 1882. That alone makes them interesting. Second, transits allow us to see the inner planets, Mercury and Venus, in a different way.

If you will recall, all through the spring, Venus has graced our western, evening sky. Its brilliant, white, star-like appearance has captured our attention for the last few months. Between now and the June 8 transit, Venus will move closer and closer to the Sun.

Another reason transits of Venus create such a stir is because for over 400 years, Scientists, navigators, and the curious have been chasing transits around the globe. The June 8 transit will be no different. Hundreds of expeditions have been organized and thousands have signed up to view the silhouetted image of Venus against the Sun.

Some will be fortunate and will watch Venus traverse the Sun?s disc. Others will travel thousands of miles and will be clouded out. But all will have their stories to tell. All who partake in the adventure will be changed by it, just like the transit chasers of the past.

Besides the aesthetic and historical aspects of the transit, there are still scientific justifications for studying these events.

For example, astronomers have discovered planets orbiting other stars. So far, they have discovered over 100 stars that have planets orbiting them. Imagine that, there are already more than 100 solar systems astronomers have cataloged besides our own.

Recently, astronomers have discovered some of these extrasolar planets, as they transit across their parent star?s disc, much like Venus will do on June 8. If an extrasolar planet?s orbit is lined up so we see it cross its sun?s disc, the star?s light diminishes by a very small amount. Scientists can detect this small light drop and plot it as a function of time. They can even determine the transiting planet?s size and make estimates of its mass, density, and temperature. Astronomers will be using the upcoming Venus transit to hone their skills for studying planets orbiting nearby stars.

What can we

expect to see?

Venus will first touch the Sun?s disc at 1:19:57 a.m. on Tuesday, June 8. Of course, you?ll probably be sleeping at that time, but don?t worry, you will get your turn. While the east coast will miss the ingress, Venus? entry on to the sun?s disc, when the sun rises around 5:04 a.m. Venus will be a little more than halfway across the sun. From that point, over two hours will pass before the transit ends.

The best way to view this event is to use the special solar sunglasses that are manufactured especially for viewing the sun.

These filtered glasses will not only block out the sun?s intensity, but also the damaging infrared and ultraviolet portion of the sun?s spectrum. You eyes cannot see these wavelengths of light, but they are there.

Another safe way to view the transit is to use a small telescope to project the sun?s image on to a white card set up behind the telescope.

If the telescope has a finder scope, don?t forget to put its lens cap on to protect it from the sun. Put a low to medium power eyepiece into the telescope and focus the sun on to the white card. Venus will be easy to identify as a perfectly round, black dot near the bottom of the sun.

Depending on what kind of telescope you have, Venus may be near the top of the sun. Also, don?t be confused by sunspots. It is quite normal for the sun to have other black dots on it surface.

Usually they are irregularly shaped-not like the crisp round shape that Venus will have. One additional word of caution if you plan on projecting the sun on to a white surface; NEVER look through the telescope to sight the sun. Instead, point the scope at the sun without looking through it.

When the telescope tube casts the smallest shadow onto the card or ground, lock the scope into place. You should be close enough to carefully center the sun.

As Venus approaches egress, its exit from the disc of the sun, watch very carefully. This is where the real science will be done during this transit. Just before 7 a.m. make sure you have a good clear view of Venus and don?t take you eyes off it. Prepare to witness the "black-drop" effect. A poorly understood phenomenon that prevented astronomers from accurately timing the previous transits of Venus.

No one really understands for sure what causes this interesting effect. It actually looks like the perfectly round shape of Venus, distorts like two drops of water will when merging together on a flat surface.

This only happens when Venus is very close to fully entering or leaving the sun?s disc. This aspect of the transit is the other scientific goal for this transit of Venus: to find out what causes the "black-drop" effect?

It has been 121½ years since the last transit of Venus. No one alive has ever seen one. The next one occurs in June of 2012. If you miss that one, you have to wait until December 2117. That will be a long wait. If you are interested in viewing this historic, celestial event, I suggest you join astronomers from the University of New Hampshire observatory and Physics department, Representatives from Rivers Camera and the Seacoast Science Center, at the Seacoast Science Center in Rye, as we train our telescopes on the Sun and provide the public with a great location from which to view the transit.

The UNH observatory staff will be handing out solar sunglasses while supplies last.

This free, public session will begin at 5 a.m. and end as Venus leaves the Sun?s disc around 7:25 a.m. You can view this grand astronomical event and you don?t even have to be late for work or school.

Now how can you beat that? To make your observing of the 2004 transit of Venus memorable, the astronomy staff will email participants a digital photograph of the event.

The Seacoast Science Center is located at 570 Ocean Blvd. in Rye, New Hampshire.

For more information you can call the science center at 436-8043, or send inquiries to John Gianforte at jsg00027@aol.com. If it is raining the morning of June 8, the session is cancelled. We hope to see you at sunrise on June 8.

Posted by thinkum at June 1, 2004 03:44 PM