May 1, 2012 at 9:37 PM ET
Over the next several weeks, skywatchers will thrill to a couple of astronomical wonders playing out in daytime skies: a solar eclipse on May 20 — and then, on June 5, a "micro-eclipse" of the sun that involves the planet Venus.
As senior staff scientist at San Francisco's Exploratorium, Paul Doherty will be keeping a careful eye on both of those wonders (with appropriate eye protection, of course). He'll let you in on his eclipse-chasing secrets on Wednesday night during a "Virtually Speaking Science" chat on BlogTalkRadio and in the Second Life virtual world. I'll be the host for the hourlong show, starting at 9 p.m. (6 p.m. PT/SLT).
Doherty says the solar eclipse and the micro-eclipse will be well worth chasing.
"By all means, go out of your way to see 'em," he told me this week. "They are rare events ... You'll have things to talk about for years after."
Each event offers a special kind of rarity: This month's eclipse is of the annular variety, which means that the moon's disk isn't quite big enough to cover the sun completely. At its darkest, the moon's black circle will be surrounded by a spectacular "ring of fire." That sight can be seen only along a narrow track running along Earth's surface, from China across the Pacific to the western United States. But a partial solar eclipse will be visible across a much wider swath of territory. It'll take about three and a half hours for the moon's umbral shadow to race across Earth's surface, from 6:06 to 9:39 p.m. ET.
Next month's micro-eclipse, more formally known as a transit of Venus, is even rarer. The planet's tiny black disk will march across the sun over the course of several hours (roughly from 6 p.m. ET June 5 to 12:50 a.m. ET June 6). Most of the world will get in on at least part of the show, but the best viewing will be had once again from the Asia-Pacific region. This will be the last transit of Venus most of us will ever get a chance to see: The next one is due in the year 2117.
Even if you're not in the viewing zone, or the skies are cloudy, you can still get in on these events via online presentations that will be sponsored by a variety of organizations — including the Exploratorium, of course. The combination museum and science center is planning a webcast of the transit as well as a Second Life teach-in timed to coincide with the annular eclipse. Doherty will be in on all the action — in Nevada for the eclipse, and at San Francisco HQ for the transit.
Over the past 40 years, Doherty has been chasing astronomical events in locales ranging from Cape Cod to Zaire. In this first of two postings about the coming attractions, Doherty discusses the appeal of eclipses and transits, plus a little bit of the science behind them. Here's an edited transcript of the Q&A:
Cosmic Log: You've been in on a number of solar eclipses in your time — what is it that draws someone to become an eclipse-chaser? A lot of people talk about how sun-observing satellites have come so far that eclipses aren't as crucial for scientific observations as they used to be. So what's the appeal?
Paul Doherty: It's certainly true that an eclipse is not a scientifically significant as it was a century ago, but I will tell you there's something special about the experience of an eclipse — even a partial eclipse. You'll be outdoors, looking around, and you'll notice that the light is getting very strange. There's a beautiful clear sky, and it starts to get dim. Your body knows that something really different is happening. It's great for people to know what's causing that feeling: The moon is blocking the sun. (But don't look at it directly with your unprotected eyes.)
Then, for totality, you have the experience of seeing a clear sky go dark at midday, so that the brightest planets and stars are visible. You see that black disk where the sun should be, surrounded by bright rays like big fat spider legs of white around the sun. That will set the hair on your back standing up, even if you're a scientist and you know what this is. It is just so awesome to see it.
So I look at modern eclipses as places for people to come in contact with a great event in our solar system, and it shouldn't be missed.
Q: Do you find that it's an inspirational experience? Do eclipse-chasers go on to "harder drugs" in astronomy?
A: I've met so many eclipse-chasers who are already into the harder stuff. You just can't tell which came first. But I'm sure that the sight of a total eclipse just inspires them. I've seen eight solar eclipses myself, and I keep running across the same people. I know that they're inspired to really take time out of their lives, and take money out of their bank accounts, and invest it in these few minutes of a great experience, surrounded by an hour of interesting shading in the sky, surrounded by days of travel to wonderful places on Earth and meeting people with the same passion they have.
Q: What are the differences between a total solar eclipse like the one in November, and an annular eclipse like the one this month?
A: The moon is in an elliptical orbit around the earth. When the moon is at its farthest point from the earth, its angular size in the sky is a little smaller, and when it's close to the earth, it's a little bigger. Also, the earth is in an elliptical orbit around the sun. It gets smaller and bigger. When the moon is farthest from the earth, it's small enough in the sky that it cannot totally block the bright part of the surface of the sun called the photosphere. That's where the bright light comes from on the sun. At that point, the rim of the photosphere shines around the edge of the moon. That light is so bright that it can damage your eyes if you look at it without eye protection. It overwhelms the dim light coming from the chromosphere and the corona of the sun.
But during a total solar eclipse, the moon is big enough in angular size to block out the photosphere completely, and with your naked eye you can see this million-degree gas glowing in the corona, like rays reaching out quite a ways from the sun. You can see the red chromosphere quite near the edge of the moon. You can even see prominences reaching out from the sun and moving during the course of the eclipse. The total eclipse offers many more things to see than the annular eclipse does.
However, an annular eclipse provides you with a bright circle of the sun. Let's say that sunlight is streaming through the leaves of a tree, and you look at the ground. Normally, you'd see round circles of light at your feet. During an annular eclipse, those all become the letter "O." That is really something, to look at the ground underneath a tree and see it covered with bright Cheerios.
If you're not on the center line of annularity, and it's a partial solar eclipse, then those images become the letter "D," or a really nice letter "C."
There are more total solar eclipses than there are annular eclipses, because it takes this special combination of the moon being a little farther away from the earth than average, and the sun a little closer, to create this annular eclipse opportunity.
Q: It seems as if people in the know are getting as excited about next month's transit of Venus as they are about the eclipse. Why is that?
A: Well, the transit of Venus is a much rarer occurrence. And it is kind of a micro-eclipse. Venus is one-thirtieth the diameter of the sun in angular size, and it's moving across the sun's disk in this stately procession that lasts six hours. It happens in this amazing pattern: There'll be one transit, and then eight years later there'll be another one, and then it's 121.5 years, then eight years, then 105.5 years. It's a really weird pattern.
You wouldn't even notice a transit was happening unless someone told you. The light on the ground is not going to change. You're not going to see anything. But it's different if you know it's happening, like Jeremiah Horrocks did in 1639. He noted that Johannes Kepler, his hero, missed the calculation that Venus was going to transit the face of the sun. Horrocks actually set up a telescope to project the sun's image into his darkened room, and became the first person ever to see a transit of Venus and record it.
Now, knowing what we do, we can tell you that on this day, at this time, if you project an image of the sun safely or use a sun-viewing filter, you can see this tiny black disk going across the face of the sun. That's interesting — but what's really interesting is that in 1761, they used the transit observations to measure the size of the solar system. Good old Halley, of Halley's Comet, figured out how to do that. They did it kind of roughly, during transits.
Q: And even today, astronomers are using alien transits to learn about new planets beyond our solar system...
A: That's right. In 1761, a Russian astronomer named Mikhail Lomonosov discovered the atmosphere of Venus during a transit. He noted that as Venus approached first contact with the sun's disk, it was completely surrounded by a bright glow, which was the sunlight being refracted by Venus' atmosphere and being sent to Earth. That's how scientists first detected Venus' atmosphere. And during the transit eight years ago, scientists used the sunlight going through the atmosphere of Venus to measure its composition.
We're doing that exact same thing with exoplanets. We're studying the light going through the atmosphere of those exoplanets, and we've actually found the constituents of the atmospheres of some of those exoplanets. They've found water vapor, and carbon dioxide, and sodium gas. We're using the very same techniques we used on Venus to study the air of exoplanets.
On Wednesday, we'll talk about some of the best places in America to see this month's annular eclipse, how to pick out an advantageous viewing spot near you, and how to make sure you see the eclipse and the transit safely. Then, be sure to tune in "Virtually Speaking Science" on BlogTalkRadio or in Second Life — and bring lots of questions. Paul Doherty (a.k.a. Patio Plasma) and I will be at the StellaNova Small Auditorium, courtesy of the Meta Institute for Computational Astrophysics, starting at 9 p.m. ET (6 p.m. PT/SLT) on Wednesday. If you miss the live event, don't worry: It'll be archived by "Virtually Speaking" on BlogTalkRadio as well as iTunes.
On Friday, head on over to the Cosmic Log Facebook page for our weekly "Where in the Cosmos" picture puzzle. If you're the first to solve the riddle, you'll be eligible to receive a pair of sun-viewing safety glasses for this month's eclipse and next month's transit. In the meantime, check out these podcasts from previous episodes of "Virtually Speaking Science," plus links to eclipse-related resources:
More about eclipses:
Alan Boyle is msnbc.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter or adding Cosmic Log's Google+ page to your circle. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for other worlds.