MR. KOJO NNAMDIEarthlings are going to get a rare astronomical treat tomorrow night, one that's only happened six times since telescopes were invented. Beginning around 6:00 p.m. Tuesday, Venus will pass between the Earth and the sun. In the Washington area you'll be able to watch it for about two hours, but do not look with your naked eye. Unlike a lunar eclipse, Venus won't block out the sun because it's much farther away and therefore appears much smaller. It will look like a black dot as it floats across the sun in the western sky.

MR. KOJO NNAMDIThe event has both modern and historic significance. By measuring the rare transits of Venus across the sun, early astronomers were able to calculate the distance from the Earth to the sun. Tomorrow the event will be photographed for the first time from space and scientists will be watching carefully for clues about what makes up Venus's atmosphere. Joining us to discuss this is James Garvin. He is a chief scientist with the National Aeronautics and Space Administration. He joins from studios at NASA. James, Garvin, thank you for joining us.

MR. JAMES GARVINOh, thanks for having me, Kojo.

NNAMDIAnd joining us by phone is Stephen Redman. He's an astrophysicist, research associate at that National Institute of Standards and Technology. Stephen Redman, thank you for joining us.

MR. STEPHEN REDMANThank you. It's a pleasure to be here.

NNAMDIIf you have questions about this or comments, call now. 800-433-8850. James Garvin, explain the cycle of the transit of Venus. It was first observed in the year 1639, and tomorrow's transit will only be the seventh one since then.

GARVINWell, that's right, Kojo, and this is part of a dance of the planets as we've come to know our solar system. Roughly once every -- well, a couple of time every 243 years, Venus and the Earth and the sun are aligned such that Venus passes like a fly across the big disk of the sun, and the last time we had that kind of event was in 2004, and before that it wasn't until the 1880s. So this is a very rare astronomical event unlike our favorite lunar eclipses, and we can take advantage of it scientifically as well as take advantage of it now from space.

NNAMDIHow long does the transit last, and how much will be able to see?

GARVINWell, here in the Washington D.C. area on the east coast, we'll see it beginning around 6:10 p.m. -- I'm sorry, ten minutes after 6:00 tonight -- tomorrow night, and it will last about four hours, but we'll only see the first couple hours of that, because the sun will set and of course then we won't be able to see it. Of course, we won't want to see it with our naked eyes of course, because viewing the sun directly is not a safe prospect, and so there are special ways of looking at it and seeing this amazing event.

NNAMDIStephen Redman, what is the historic significance of the transit of Venus? How did it allow 18th and 19th century astronomers to measure the distance between the Earth and the sun?

REDMANWell, back in the 1600s and 1700s, astronomers knew the relative distances of the planets from the sun. So for example, they knew that Venus was about .7 astronomical units away from the sun, where an astronomical unit is the distance between the Earth and the sun. But unfortunately, they didn't know what an astronomical -- or how big an astronomical unit was. So this provided them with a brief opportunity to make some very careful measurements and actually measure the physical distance between the Earth and the sun.

NNAMDIHow have changes in technology over the years allowed scientists to arrive at more precise measurements of the transit of Venus, Stephen?

REDMANWell, originally in the 1700s they were using a technique known as parallax. Now everyone listening is familiar with parallax whether they know it or not. If you simply extend your arm and hold up your thumb and then close one eye and then swap open and closed eyes, you'll observe your thumb appear to move back and forth by a small amount. And so this was sort of the basic idea of how they were going to measure the distance between the Earth and the sun by observing the transitive Venus from different locations on the Earth, and by viewing it from different angles, they could make very careful measurements of the distance between the Earth and the sun.

REDMANBut in the 1800s, they used a similar technique, but they were trying to use photographic technology, because in the 1700s they discovered what's known as the black drop effect, where Venus appears to sort of stick to the edge of the sun as it passes in front of the sun, and that really spoiled the measurements they were trying to make.

NNAMDIIf you have questions or comments about viewing the transit of Venue tomorrow evening starting around 6:10 p.m., call us at 800-422-8850 or send us a tweet @kojoshow. We're talking with Stephen Redman. He's an astrophysicist and research associate with the National Institute of Standards and Technology. James Garvin is chief scientist with the National Aeronautics and Space Administration, NASA. 800-433-8850. Jim Garvin, for the first time, the flight engineer on the International Space Station is going to photograph the transit of Venus. How will he do it and how can we, the public, see his pictures?

GARVINWell, Don Pettit, our astronaut scientist on the International Space Station will use various photographic methods, all digital, to record this amazing transit, and at the same time, Kojo, very excitingly, we are going to be trying a new kind of experiment for watching this marvelous event. We're gonna observe the transit of Venus through reflected solar light the moon, using the Hubble Space telescope.

GARVINAnd so as an experiment of opportunity to be very creative with our space borne assets, we will use the powerful capabilities of Hubble to observe little subtleties of how the Venus atmosphere reflects light as its passing reflected off the surface of the moon. So we will be doing a new kind of observation this time for the first time in history as we observe this event.

NNAMDIHubble telescope is going to be pointed at the moon because you can't point it at the sun.

GARVINWell, that's correct, but actually, the engineers who are on Hubble and the scientists that work here and up in Baltimore and in other places, we've even observed the atmosphere of Venus from Hubble directly at times when the sun is not of course in the wrong place, and recent observations as early as -- well, in January of 2011, allowed new measurements of the chemistry of this massive atmosphere of the planet Venus to be made using those sensitive instruments on Hubble. So it's kind of the next best thing to being there.

NNAMDIThe transit of Venus is helping astronomers hone their craft. How will NASA's solar dynamics observatory use tomorrow's transit of Venus to help calibrate its instruments?

GARVINWell, the Solar Dynamics Observatory, which is operated right here out of the Goddard Space Flight Center, is a tool for observing the sun in all of its really high fidelity dynamics, which is really the workings of our solar system, and the instruments on SDO record movie-like data to observe the sun. By watching the (word?) , the variations of the signal as Venus passes across the disc of the sun, will make new measurements of what happens under those kind of circumstances, essentially giving us a little null in a place where normally we have an intense realm of radiation.

GARVINSo this is a new kind of experiment that Mother Nature is giving us an opportunity to use to better understand how good our measurements are, which have been, by the way, fantastic since we launched at satellite.

NNAMDIHere's Al in Washington D.C. Al, you're on the air. Go ahead, please.

ALHow you doing, Kojo? Long time listener.

NNAMDIThank you.

ALHi gentlemen. Is there any place that the public can go within D.C. like an observatory to see it safely, you know, as it's passing through? And I'll hang up and listen.

NNAMDIAny suggestions Jim Garvin, Stephen Redman?

GARVINWell, this is Jim, and I would say right here at the Goddard Space Flight Center we will be having observing events from our facility through the visitor's center which is open to the public, one can watch it. That's one very local place. I'm sure there's many others being coordinated across the Washington area, and I'll turn it to Steve.

NNAMDIAnd you can find other local options posted on wamu.org at the website there. Stephen Redman?

REDMANYeah. There's lots of places to observe the transit. Lots of groups organizing events. Here at NIST we're organizing an event at Blair High School where we'll be observing the transit with a video camera hooked up to a telescope. This is a unique opportunity to view the sun through an H Alpha Filter which will allow you to see some of the activity on the sun while the transit is in progress. But there's several dozen sites throughout the D.C. area that are offering the public opportunities to come see the transit of Venus.

NNAMDIAnd Skip in Damascus, Md. wants to tell us about one. Skip, you're on the air. Go ahead, please.

SKIPWell, thank you. Yeah. We're holding a program at Damascus Regional Park. I'm part of the Westminster Astronomy Club. We will also have hydrogen alpha filter telescope there and every Carroll County branch library will be holding a program.

NNAMDITell us about the telescope out there, Skip.

SKIPWell, like you said, the hydrogen alpha one which shows the narrow band. We also have what we call natural or neutral density filters on the telescopes too. We should have about seven or eight telescopes at the park, and then at each library branch in Carroll County there will be four to five telescopes at each one of those.

NNAMDIOkay. Skip, thank you very much for sharing that with us. Onto Bruce in Reston, Va. Bruce, you're on the air. Go ahead, please.

BRUCEThank you very much for taking my call. Since there's such a short time between tomorrow night, is there anywhere in northern Virginia we could buy those extra dark glasses through which we can look at the sun, and I'll take my answer off the air. Thanks.

NNAMDIAny idea about either Jim or Stephen?

REDMANWell, several telescope stores, stores that sell telescopes or optics -- or photography optics may be offering them. But at this point, you're most likely -- your best bet is to try to attend an event in the area where organizers will probably have a multitude of viewing options, including darkened neutral density filter glasses.

NNAMDIAnd as I said, you can go to -- you can find those places if you go to our website, kojoshow.org, where you will find such a listing. John in Fairfax, Va. Hi John.

JOHNHow's it going?

NNAMDIVery well.

JOHNHow's everybody doing?

NNAMDIWe're doing well.

JOHNI just wanted to know one, in layman's terms what is really happening tomorrow, and two, is there anything I need to know as far as taking pictures with just a basic camera or not just to avoid taking pictures overall. Thanks.

NNAMDIStephen Redman, go over again what is happening tomorrow.

REDMANRight. So the basic idea is that the planet Venus will pass in front of the sun. So much like you would see with a lunar eclipse, the shadow of the planet will appear on the face of the sun. It's not an event that you can photograph without special optics, but the event should be visible if you have a telescope with a neutral density filter, or some other safe viewing methods.

NNAMDIBecause you are not advised to look directly at the sun.

REDMANPrecisely, yes. We should emphasize that there are a variety of safe ways to look at this event, and there are a variety of ways which are not safe, and it's very important that you protect your eyes while you're trying to observe this event.

NNAMDIThank you very much for your call. Jim Garvin, what do you hope to learn about Venus's atmosphere from observing this transit?

GARVINWell, Kojo, again, Venus's atmosphere is the biggest planetary atmosphere that we know of in this inner part of the solar system where our planet and Mars and Venus and Mercury all reside. So it's a virtual laboratory for how planetary atmospheres began, evolve, and get to be the way they are, and, in fact, in Venus's case, we think that Venus as a planet evolved it's atmosphere into a state of what we call a runaway greenhouse. A situation where it's a lot warmer because the solar radiation, the energy from the sun passes into the Venus atmosphere and doesn't get all the way the out like in a greenhouse that you might find anywhere where we have plants growing in the winter.

GARVINSo what we hope to learn is aspects of the real subtle chemistry of that atmosphere, where there's a lot of things we actually haven't measured. The United States last time visited the atmosphere of Venus in 1978 because it's a very difficult planet to operate at. I wanted to add, Kojo, so the observations will tell us more about that atmosphere as we prepare someday to put robotic spacecraft back to the planet Venus, and our last spacecraft there was a radar mapping mission known as Magellan, operated by our jet propulsion lab in the early '90s, so it's almost a generation since we've been there.

GARVINI did want to add one think Kojo, for...

NNAMDISure.

GARVIN...people interested in watching what's happening through the data from NASA, the data will be posted on venustransit.nasa.gov. That's V-E-N-U-S-T-R-A-N-S-I-T.nasa.gov, and also there's a Venus transit, or should I say part of the NASA general website that you can visit to see the images from the International Space Station from Don Pettit from the SDO mission, and so those things will be available for the public if you don't want to go find a telescope with neutral density or H alpha filters, you can watch on the web and experience this unique event, the next of which will occur in 2117, a long way away.

NNAMDIJames Garvin, chief scientist with the National Aeronautics and Space Administration, NASA. Thank you for joining us.

GARVINThank you, Kojo.

NNAMDIStephen Redman, astrophysicist research associate at the National Institute of Standards and Technology, NIST. Thank you for joining us.

REDMANThank you, Kojo.

NNAMDI"The Kojo Nnamdi Show" is produced by Brendan Sweeney, Michael Martinez, Ingalisa Schrobsdorff and Tayla Burney, with help from Kathy Goldgeier and Elizabeth Weinstein. The managing producer is Diane Vogel. The engineer today, the legendary Tobey Schreiner. Natalie Yuralivker is on the phones. Podcasts of all shows, audio archives, CDs and free transcripts are available at our website, kojoshow.org. Thank you all for listening. I'm Kojo Nnamdi.