ΦBK Video Series 2011: Jay M. Pasachoff on the June 5, 2012, Transit of Venus


Uploaded by phibetakappa on 30.08.2011

Transcript:
>>Pasachoff: 2012 is a very exciting year for astronomers. First of all, we have a pair
of solar eclipses: an annular eclipse on May 20th that will be visible in California, through
Texas; and a total eclipse that will be visible in Australia in November. But there are eclipses
every year--a total eclipse every year and a half. What's very unusual about 2012 is
that we will have a transit of Venus. It will be the last transit of Venus until the year
2117. So if you ever want to see a transit of Venus, this is your chance.
So what's a transit of Venus? Well, Copernicus, in 1543, figured out that the sun and not
the earth is at the center of what we, therefore, now call the solar system. (SLIDE 3) He has
a beautiful diagram in his book from 1543 with "Sol," Latin for "sun," at the center,
and this was a major change in people's thinking about the universe. If you look at his diagram
(or any diagram of the solar system) you can see that Mercury and Venus are the only two
planets who orbit within the Earth's orbit, and therefore only Mercury and Venus could
go between the Earth and the Sun. When one of them goes across the face of the sun, we
call that a transit. Now, their orbits are slanted a tiny bit, so they sometimes go a
little above or a little below the line between the Earth and the Sun, so most times when
they go around, they don't go right between the Earth and the Sun.
But a couple of times per hundred or so years, Venus goes in transit across the face of the
Sun, and about 15 or 16 times per century, Mercury goes in transit across the face of
the Sun. Not only is this an odd event, and something that is very difficult to be alive
for when it happens, but also it used to be what was called "The Noble Experiment." It
was the major point in astronomy and astrophysics for hundreds of years because it was the only
way they had for hundreds of years of finding out how big the solar system is. And
since the solar system was essentially the universe, it was essentially the question
of, "how big is the universe?" It's certainly an existential question that everybody wanted
to know [have answered].
Following Copernicus in 1543, at the end of that century, at the end of the 16th century,
Tycho Brahe in Denmark made the major observations of the planets and the positions of the planets,
the positions of the stars, and when he got expelled from Denmark, Tycho brought his observations
to Prague, where he took on a young assistant, Johannes
Kepler. It's Kepler who is key to the story. Tycho died about a year after Kepler came
to work with him, and it took a while for Kepler to actually get ahold of the observations,
but then he did, and he tried to make sense of them. Now, these were pre-telescopic observations,
it was several years before Galileo first turned a telescope at the heavens. But Kepler
couldn't quite make the observations of Tycho, these high quality observations, work with
the assumption of circular orbits. Kepler figured out that the orbits were squashed
circles, what we call ellipses. And of course there are mathematical definitions of this,
but all we need to know is that Kepler in 1609 came out with a book that he called "The
New Astronomy," or Astronomia Nova, and in the Astronomia Nova, he had his first two
laws of planetary orbits: the first, that the planets orbit the sun, and that the sun
is at one focus of the ellipsis, and also a second law that has to do with how fast
the planets move around the sun, depending on whether they're a little closer or a little
further away. And 10 years later he came out with a third
law that links the length of time planets take to orbit the sun with their distances.
And it's this third law that's the key for figuring out how big the universe is. If you
just measure how long the planets take to go around the sun, then you can figure out,
in a proportion, how far they are away. (SLIDE 4) For example, you can tell that Mars is
1.5 times further away from the sun than the Earth is. But all those distances are proportional.
There's no one distance that you actually know in miles or kilometers or meters or whatever
actual unit of distance you want to have. (SLIDE 5) So the key was to find any distance
that you can measure in the solar system, and if you can measure any one distance, then
you can go into the proportionality and you can figure out all the distances. So a key
to finding out how big the universe was, how big the solar system was, was to measure some
distance.
(SLIDE 6) And in 1716, Edmund Halley, of whom we know more from his comet work, figured
out that a transit of Venus would be the key way of figuring out the distance to Venus,
and therefore the distance to the sun, and therefore all the distances in the solar system.
Now, what about a transit of Venus? Kepler had figured out that there'd be one in 1631,
but nobody saw that one. And then Kepler was dead and didn't have any prospect of calculating
more, and from the tables that he'd made, there wasn't one right away. But a very young
man, about 20 years old, named Jeremiah Horrocks in England, in Much Hoole, a small town near
Manchester, went into Kepler's tables and fixed things up and figured out that in 1639
there would be a transit of Venus--Venus would go across the face of the sun. He wrote a
friend of his, Mr. Crabtree, not too far away, in Manchester, and he wrote a friend in London.
And about the time he predicted this second transit in 1639, he started observing. (SLIDE
7) And we know that on a Sunday he didn't observe in the morning. People are still debating
why--he was too young to have been ordained, but maybe he was just doing something else
at church. (SLIDE 8) In any case, he came home at noon and found that there was a black
spot on the surface of the sun, that Venus was in transit. (SLIDE 9) Mr. Crabtree in
Manchester also saw this. They were the only two people to see this; it was cloudy in London.
And Venus looked just smaller than they expected. They had no idea how big things were, how
far away things were in the solar system. This was just a real breakthrough for the
transit of Venus to be seen by Jeremiah Horrocks.
Now if you look at the actual calculations, it turns out that transits of Venus take place
in pairs, separated by only eight years. But then there's a gap of over a hundred years,
either 105.5 years or 122.5 years before the next pair. So after 1639, we had to wait until
1761 for the first of the next pair of transits of Venus, and in that time Halley had made
his prediction and the scientists all over the world and the governments all over the
world took it so seriously, that for each of 1761 and 1769, they sent out over a hundred
scientific expeditions all over the earth. Halley's method worked best if you went as
far north and as far south on the earth as possible to see the difference, to make a
giant triangle and see how far the Earth was away.
The most famous, perhaps, of the expeditions, was that of Captain James Cook, who was actually
a lieutenant at the time, in the British admiralty. They took this young lieutenant, gave him
a ship, and said, "Go to Tahiti to observe the transit of Venus." And he did that. He
took an astronomer with him, Charles Green, and they each observed the transit. There
are lots of stories to do with that. If you want to know more, there are some good books
out about the transit of Venus, and also the transits of Mercury.
In any case, Captain Cook observed the transit very well in 1769 in clear skies from what
was called Point Venus. It's still there if you want to go to Point Venus, Tahiti on some
excursion. And he had orders that after he observed the transit, he could open a secret
envelope, and he opened it and it said: "Go and explore the Southern Continent," and so
we astronomers always consider the mapping of New Zealand and the coast of Australia
to be a spinoff of this astronomical expedition that Captain Cook made. In any case, Captain
Cook and Charles Green had clear weather to observe the transit, but they couldn't time
the transit as accurately as they'd have liked. Halley's method required their timing when
Venus went inside the sun in silhouette and when it exited the sun in silhouette, several--could
be six--hours later. (SLIDE 10) The method required timing it to about a second. (Cropped_Animation
movie) But it turned out that when Venus went inside the sun and went a little further inside,
it was still joined to the edge of the sun by what you could call a ligament, almost
like taffy that pulled out and stretched, and after about a minute it popped. So the
timing was uncertain to about a minute, instead of a second, and that threw off all the calculations.
So he was unhappy, but didn't know at the time that everyone else who saw the transit
saw that same effect. It's called the Black Drop Effect.
Guillaume Le Gentil went from Paris to observe the transit from Pondicherry, India, and he
wanted to time it to the second, but when the Frenchman got there, to the coast of Pondicherry,
he discovered that it was held by the British, who wouldn't let him land, so he had to observe
it from the ship. And at that time he had a pendulum clock, and that was swinging on
the ship, so he couldn't time it. He saw it, but he couldn't make his scientific observations.
So what to do? He decided to stay. It was only eight more years until the next transit.
So Le Gentil stayed, in that part of the world; he went to the Philippines, and he came back,
and eight years later he set up this observatory with major equipment, and it was clear, and
it was clear, and it was clear, and just before the transit started, it was cloudy. So he
failed, than then he tried to get home, and he was shipwrecked and he got dysentery, he
was in the hospital, all kinds of things. He was away 11 years and he got home to find
himself declared dead; his goods were being dispersed to his relatives. His seat in the
French Academy was given away.
People dedicate themselves, a lot, to doing certain kinds of observations, especially
transits of Venus. The story is even worse for Chappe d'Auteroche, another Frenchman,
who went to Siberia in 1761 and hated the people in Siberia so much that even Catherine
the Great wrote a rebuttal to his report. In any case, in 1769, he went to Baja, California,
and by the time he got to where he wanted to observe, it was only a few days before
the transit, and the people said: "Don't stop here. People are dying. They're falling ill."
(We now think it was typhus.) But he said, "Well, I'm going to stay because I don't really
have time to go to some other location, down to Cabo San Lucas." And...he stayed. And...he
died. They all died. Although he did say, after he successfully observed the transit,
that there was an eclipse of the moon in two weeks and that he wanted to stay alive for
that one, and he did manage to stay alive for the eclipse of the moon. It is said he
died happy because he accomplished his goal. Anyway, I hope no one in the current transits
dies, in their astronomical observations.
The situation was much happier in the next pair of transits. I've just discussed 1761
and 1769. Now we jump to 1874 and 1882. By that time photography had been invented and
there are lots of photographs of that one. We still were troubled by the Black Drop Effect,
so the method of finding the size of the solar system from the transit of Venus never really
became the best method. An asteroid came close enough for them to get a good distance to
it, and therefore discover the size and distance of the solar system. But there were lots of
great observations from the transits of Venus of 1874 & 1882. The 1874 transit was especially
well observed in Sydney, in Australia, and there are great drawings,
especially, from that time of the Black Drop Effect and of other effects of the transit.
So now we get up to the next pair, which were 2004 and this coming 2012. So nobody in the
1900s had a transit of Venus to observe at all. And by the time we got to 2004, nobody
alive on Earth had seen one. In fact, I kept up with the Guiness Book of World Records
to see who was the oldest person in the world, just to check, and see that there was no one
alive in the world who could have seen that last transit of Venus.
I learned, a few years before that, at a meeting of the Historical Astronomy Division of the
American Astronomical Society, that a lot of the explanations of the Black Drop Effect
were wrong. They had said it was Venus's atmosphere that caused the Black Drop
Effect, but actually Venus's atmosphere, though substantial, was not thick enough to provide
that big Black Drop Effect that is seen for a minute at the beginning and the end of the
transit. And I knew as a solar astronomer that I had access to spacecraft observations
at the time of the 1999 transit of Mercury. (SLIDE 13) Now, Mercury has no atmosphere,
and the spacecraft was above the Earth's atmosphere, so when we looked at the data and we saw a
Black Drop Effect, we knew that we did not need an atmosphere to form a Black Drop Effect.
And that's true of Venus, too. I worked with a colleague of mine at the University of Arizona,
Glenn Schneider, and we calculated that there were really two effects that went into the
transit of Venus's Black Drop Effect. (VIDEO TOV_2004) One of the effects had to do with
the instrument--just that the telescope had a finite size, and that provided a little
blurring. And the other effect is a subtle effect that is well-known to astronomers--that
the sun and the other stars that you can't see as well are darker near their edges. It's
called limb darkening. The limb is the edge for the sun or an astronomical body. And that
has to do with seeing into the solar atmosphere at a slant, and winding up seeing a higher
level, which is cooler and darker and it's so abrupt at that point very near the edge
of the sun where the Black Drop Effect occurs that that coupled with the blur from the telescope
explains the Black Drop Effect. So we had explained the Black Drop Effect, and it was
a lot of fun for me to participate in solving a problem that was hundreds of years old.
We wanted to see the 2004 transit of Venus. It was all visible in an area of Asia and
Europe and I took my students at Williams College and some colleagues to Greece to observe
the transit so that we could see both Black Drop Effect and make some scientific observations,
too, with a telescope there at the University of Thessaloniki. (SLIDE 14) And we had a fine
view. The transit takes six hours; it was clear almost the whole day. We did have a
few clouds, but in any case we had a very good view.
And then when we got back we were able to get spacecraft observations from a NASA spacecraft
called the Transition Region and Coronal Explorer, known by its initials, TRACE. (SLIDE 15) And
we could see in these TRACE observations that for about 25 minutes before Venus went entirely
on the surface of the sun, you could see a small arc of light around the side of Venus
that was sticking off the edge of the sun. And that really was Venus's atmosphere; that
was no longer just an instrumental effect. That really was Venus's atmosphere.
And we worked with a Frenchman now, Thomas Widermann, who works with a European Space
Agency spacecraft called Venus Express, to check the part of Venus's atmosphere that
we were observing at the time. We published a paper in 2011 in the Astronomical Journal
about these observations of the 2004 transit of Venus and how to observe the atmosphere
and what we detected about Venus's atmosphere then. (SLIDE 16) So it's been a lot of fun
to see that 2004 transit of Venus, both in spacecraft observations and with my own eyes
from the ground, with my own cameras and my students' cameras from the ground.
So we're looking forward to the 2012 transit of Venus. It's listed as June 6th, 2012, but
because of the International Dateline and time zones, in the United States it turns
out to be June 5, 2012. (SLIDE 17) It will be entirely visible in the Pacific region,
including Hawaii and Alaska and then down to Australia. In the continental United States
you'll only see part of the transit, but that's enough to see from anywhere in the continental
United States.
You can't look at the sun directly; it's too bright. You need a solar filter that cuts
the intensity down by a factor of about 100,000. So you can't just use regular sunglasses;
you'd need something like 70 pairs of sunglasses in a row to cut down the brightness enough.
But for 50 cents or a dollar you can buy a solar filter that makes it safe to look at.
Or you can just cut a hole in a piece of paper and hold it up to the sun and project the
image onto a wall or another piece of paper through a little pinhole. Make it about three
or four millimeters across--bigger than a real pinhole. But that's called a pinhole
camera and you can get a projected image of the sun and then you can see a black spot
of Venus on the surface of the sun. Venus is silhouetted against the sun. And it's just
inspiring to be able to see with your own eyes this
very rare effect that you won't be able to see again until 2117. Of course if you have
a telescope or binoculars and you really know how to use those safely and you can project
an image onto a wall with them, that will work too. So you will have a chance in 2012
to see this transit of Venus in the United States.
I will be on the mountain Haleakala in Maui, Hawaii, where there's a solar observatory
of the University of Hawaii. (SLIDE 18) I have a grant from the Committee for Research
and Exploration from the National Geographic Society for this work. We also have
arranged for some telescopes in Sacramento Peak Observatory in Sunspot, New Mexico, that
belongs to the National Solar Observatory, and some other telescopes that belong to the
National Solar Observatory, from the New Solar Telescope at the Big Bear Solar Observatory
of the New Jersey Institute of Technology, and from some spacecraft. There are several
solar spacecraft up now, including a new NASA spacecraft called Solar Dynamics Observatory
that will get really high resolution images of the transit. And
we're working with all of them to amass as big a collection of these data as possible,
both to study right away and to save for posterity.
In fact, when we published our article in the Astronomical Journal in 2011, the editor
had a procedure for putting your data on file. They promise to keep the data for one hundred
years. I guess they think that's good, but I responded: "Well, the next transit of Venus
isn't for 105 years, until 2117 (that is, after 2012), so could you please extend your
promise, and keep the data on file for at least 105 years, not just for 100 years?"
(SLIDE 19) But it is really nice for me to think that these observations we made of the
transit of Venus in 2004 and 2012 will be of interest to astronomers and others as we
come up to the next transit in the year 2117. In the meantime people have been studying
lots of stars and finding planets around them. There are over 1,000 planets around these
other stars and we have measurements of the 1/10 of a percent of sunlight that's held
off by Venus in the transit in 2004 and we'll have to do that again in 2012 and provide
an analog in our solar system of what we're seeing in these other solar systems, so that's
fun too.
I've had a lot of fun studying the transit of Venus and it's something that just came
upon me a few years ago. It's a great opportunity,
and the idea that this is our last chance, on June 5, 2012, to see a transit of Venus
for another 105 years means that I hope that we are all outside, with the proper equipment,
watching safely and seeing this magnificent astronomical event.