Date: June 4, 2012
Title: Transit of Venus
Podcaster: Dr. Christopher Crockett
Organization: United States Naval Observatory
Description: On June 5, Venus passes between the Earth and the Sun in a very rare celestial alignment. Come learn what a “transit of Venus” tells us about our place in the Universe and how astronomers are using this event to look for planets – and life – around other stars in our Galaxy.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.”
Tomorrow afternoon, people around the globe will be able to witness one of the rarest of recurring astronomical events. For over six hours, Earth will pass through the shadow of a neighboring planet as Venus passes directly between us and the Sun. The 2012 transit of Venus is here!
A transit of Venus is just like the recent solar eclipse – some celestial object moves between us and the Sun. As it does, it blocks some sunlight, casting a shadow on Earth. When the moon does this, it can cover all – or nearly all – of the Sun. But while Venus is 4 times bigger than the Moon, it’s also 100 times farther away. So instead of a large bite being taken out of the Sun, we instead see a small hole.
A Venus transit requires a precise alignment of the Sun, Venus and Earth. Get every thing to line up takes some work…and some patience. The orbits of Venus and Earth are not in the same plane: they are tilted relative to one another. This means that Venus spends half of its year a bit above Earth’s orbital plane, and the other half below it. Most times when Earth laps Venus on its way around the Sun, our planetary neighbor appears either above or below the Sun – and so no transit occurs.
But every so often, Venus crosses the plane of Earth’s orbit just as Earth goes speeding past. And because Earth goes around the Sun eight times in the approximately same amount of time it takes Venus to make 13 laps, we can see Venus cross in front of the Sun again eight years later. By after another eight years, the alignment is lost and we have to wait…and wait…and wait. The gap between these eight year pairs alternates between 101 and 121 years. So the Venus transit timing looks like this: get two eight years apart, wait 101 years, then get another two eight years apart, then wait 121 years, then another eight year pair, then back to 101 year wait, and so on. The entire cycle repeats itself every 243 years – which is how long it takes Venus and Earth to simultaneously come back to the same points in their orbits.
A transit of Venus is more than just an opportunity to witness one of the rarest of periodic celestial events. The Venus transit also has enormous historical significance to astronomers. It was a transit of Venus that allowed astronomers to finally figure out how far the Earth is from the Sun which in turn allowed later generations to calculate the distances to the stars. Venus set the scale of the entire Universe!
By 1619, Kepler had worked out the relative distances of the planets from the Sun, but no one knew the physical distances. He knew that Venus was about two-thirds of the way from the Sun to Earth and Jupiter was about 5 times that distance. But that was it. No one had figured out what the actual Earth-Sun distance – what astronomers call an “astronomical unit” – is. Fortunately, once you know the relative distances between a bunch of objects, you just need to measure the distance between two of them to figure out everything else. A transit of Venus provides an opportunity: you can use it to determine the distance between Earth and Venus!
In 1639, English astronomer Jeremiah Horrocks predicted a Venus transit. He watched the event by projecting an image of the sun onto a piece of paper with a simple telescope. While he had the bad luck of having to battle clouds, he was able to measure how large the disk of Venus appeared against the Sun. Knowing the Kepler’s relative distances, he estimated the size of Venus and from there made a reasonable guess at the Earth-Sun distance. He was off by about 30 million miles, or two-thirds of the actual distance. Still, Horrocks’ estimate was the most accurate attempt to date at measuring the size of the solar system.
The next two transits occurred in 1761 and 1769. Famed astronomer Edmund Halley saw an opportunity. Put two astronomers at widely separated locations and have them time when they each see the start and end of the transit. If you compare the time difference, and use how far apart they the astronomers were, you could triangulate the distance to Venus! While Halley never lived to see his prediction, his rallying cry launched a worldwide campaign to watch the transit from every point on the globe. The cooperation of astronomers from dozens of countries is one of the earliest examples of international scientific collaboration.
The measurements of 1761 and 1769 refined the astronomical unit to 153 million km. Repeated attempts in 1874 and 1882 narrowed it down even further.
Today, we measure these distances by bouncing radar off nearby planets and asteroids. The increased precision of radar means we now know the earth-sun distance to an accuracy of 30 meters – or within two-tenths of one thousandth of a percent! Edmund Halley would be proud. But, this means the transit of Venus no longer has any value in determining the size of the solar system.
But, that doesn’t mean it’s lost all scientific significance. Modern astronomers are more interested in another kind of transit – the kind that occurs when a planet in another solar system passes between its sun and Earth. When that happens, the starlight dips momentarily, allowing astronomers to determine the size and orbital period of the planet. But this incredibly fortuitous alignment allows astronomers to do something else. When a distant planet transits its parent star, some of the starlight must pass through the planet’s atmosphere on its way to Earth. When it does that, the molecules of the atmosphere leave their spectral signature in the starlight – which we can then tease apart with telescopes on Earth. The transit of an extrasolar planet is the only way astronomers can measure the chemical makeup of a distant world’s atmosphere. And doing that is the first step to finding evidence of life out there in the Galaxy.
The 2012 Venus transit will allow astronomers to practice and refine their techniques with a participant a bit closer to home. As the transit of Venus occurs, astronomers will carefully monitor the dip in the Sun’s light. Comparing the known size of Venus with estimates from how much sunlight is lost allows researchers to uncover errors in their techniques. Astronomers will also measure the changing solar spectrum as the sun’s light passes through Venus’ stifling atmosphere. From there, we can sniff out the chemical makeup of our sister planet and use what we learn to see if life has taken hold in some other remote corner of the Galaxy.
On Tuesday, find a way to safely view the sun. A solar telescope, a pinhole camera, eclipse glasses – anything that allows you to look at the sun without damaging your eyes. Don’t miss this one, because the next isn’t until 2117. You will witness one of the rarest of celestial phenomena and one that has both a rich history and future. Every time Venus transits the Sun, we learn something new about the Universe, from the size of the solar system to the search for life.
What will we learn the next time Earth and Venus come together?
End of podcast:
365 Days of Astronomy
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