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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

The Transit of Venus

By Jeff Danner Posted June 4, 2012 at 2:06 am

Tuesday June the 5th is the last chance you will have in your lifetime to see the transit of Venus, so don’t miss it!  Not sure what the transit of Venus is or why you should take time out of your busy schedule to observe it?  Common Science is here to help.
 
Everyone is familiar with lunar eclipses, when the moon passes between the earth and the sun.  What is less well known is that every once in a while Venus also passes between the Earth and the Sun.  This is called a transit of Venus.  The upcoming transit on Tuesday will be visible in the Chapelboro skies from 5:30 PM until sundown.  The study of the transit has a long and romantic history and has helped us to understand our place in the solar system and the universe.
 
In addition to being a fascinating astronomical event, observations of the transit of Venus were what allowed mankind to calculate the distance between the Earth and the Sun by taking advantage of the parallax effect.  The definition of a parallax is “the difference in the apparent position of an object when viewed along two different site lines.”  To observe a parallax, hold your thumb in front of your face, look at it through your right eye against a background then look through your left eye.  The apparent position of your thumb against the background will change.  If you know the distance between your eyes and can measure the angle between the two site lines, you can use trigonometry to determine how far away you are from the background.
 
In the case of the transit of Venus, the background is the Sun, Venus is your thumb, and different vantage points from the earth are your eyes.  So if you take careful measurements of the transit of Venus from multiple points on the Earth, you can calculate the distance between the Earth and the Sun.  But there is a catch. 
 
In order to do this calculation you need to know the diameter the earth.  Remember in school that you were taught that, before Christopher Columbus, many people believed the Earth was flat.  This was not always the case.  Around 3000 BC, 18 centuries before Columbus sailed the ocean blue, there was a Greek mathematician named Erastothenes living in Egypt.  Erastothenes noticed that at noon on the summer solstice the sun shone directly down a well in the town of Syene and made a shadow of 7 degrees in Alexandria.  Using this data Erastothenes made a remarkably accurate calculation of the diameter of the Earth.  The key point here is that in order for Erastothenes to calculate the diameter of the Earth, he had to have already known that it was round rather than flat.
 
Please permit me a short reflection on Erastothenes.  History is often written as a narrative of steady and constant progress.  The reality is that periods of peace and stability tend to bring about remarkable scientific, artistic, and cultural progress, much of which is then lost during interspersed dark ages which occur when conflict, disease or famine reduce the population to devoting their energies to basic survival.  Erastothenes lived in the beginning of the Bronze Age, 3000-1000 BC, at time of great civilizations and transcontinental trade.  Then the Bronze Age collapsed, progress halted and then reversed for a while.  OK, back to Venus.
 
Johannes Kepler, a German mathematician born in 1571, was already famous for discovering that planetary orbits were elliptical when, in 1627, he correctly predicted that there would be a transit of Venus in 1631.  Unfortunately he did not live to see it and his calculations were not precise enough to determine that the transit would not be visible from Europe, so no calculations of the Earth-to-Sun distance were made from that transit.  Kepler also correctly predicted that transits would occur in pairs separated by 8 years with the pairs separated by longer intervals of, alternatively, 105.5 and 121.5 years. Kepler was a pretty smart guy.
 
During the next transit in 1639, Jeremiah Horroucks and William Crabtree made observations from two locations in England.  Their view was partially obscured by clouds, it was England after all, and they calculated that the Sun was 62 million miles away. (The actual distance is approximately 93 million miles.)  Their calculations caused a stir as people were astonished that this enormous yellow orb that provided our light and heat was so very far away.
 
The next transit in 1761 brought about one of the first examples of international scientific cooperation.  By now it was known that in order to make an accurate calculation of the Earth-to-Sun distance, multiple observations of the transit needed to be made from widely separated locations on the Earth.  Scientists from Europe were dispatched to Siberia, Newfoundland, Norway, Madagascar, and South Africa. 
 
The observations made in 1761 were reasonably good and also provided evidence that Venus had an atmosphere but the desire for more accurate data drove an even more ambitious effort in 1769.  The trials and tribulations of the expeditions assigned to observe the transits of 1761 and 1769 are some of the most dramatic and interesting in the history of science.  To learn more about the roles of Captain James Cook and the unfortunate Guillaume de Gentil  and their successes and failures in observing he transit, follow this link to listen to my conversation with Amy Sayle of the Morehead Planetarium on Who’s Talking from 5/31/12.  The Earth-to-Sun distance estimate from 1769 was 95 million miles which is quite close to actual distance of 93 million miles. 
 
 
Today scientists continue to use transits to study the universe.  Three years ago NASA launched a telescope named for Johannes Kepler which constantly watches 150,000 stars looking for a dimming of their lights which would indicate a transit of a planet.  The goal of this effort is to look for earth-sized planets in the habitable zone – having temperatures where water would exist as a liquid – which may harbor life.  Somehow I think Erastothenes would be pleased.
 
I hope by now I have piqued your curiosity sufficiently that you will reserve time on Tuesday evening between 6:00 pm and sundown to experience the transit.  Just as in viewing an eclipse, DO NOT look directly at the Sun.  I recommend that my readers head down to Morehead for the event on Tuesday evening starting at 5:30 where “eclipse glasses” will be provided  to allow for safe viewing and images from their telescope will be projected in the theater.  You can also view the transit on the internet on a variety of websites.  Bear in mind that there will not be another transit until 2117, so this is likely your last chance to experience it.  Don’t miss it.
 
Have a comment or question?  Log-in below or send me an e-mail at commonscience@chapelboro.com.

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