 Most people think transit is getting on the bus or the subway. Here's a way to show what a transit is and how NASA's Kepler mission will be using transits to find planets around other stars. Now, Mercury or Venus sometimes transits the Sun. Let's see what that means. Who wants to be the Sun? Okay, good. Would you kneel down here? And I'm gonna turn on the Sun here and give that to you and could you hold that just on your head like that, okay? Now who wants to be Mercury? Oh cool. All right. Now, this is Mercury. This is the planet Mercury and could you hold Mercury about the same height as the Sun there and silently count to 8 as you orbit it this way. Okay, this way. There you go. Silently count to 8. Good. Now, when a planet passes between an observer and that's all of you and a star, in this case the Sun, we call that a transit. Each of you are standing on Earth and watching Mercury orbit the Sun. Okay? From Earth sometimes everything is lined up so that the Sun, Mercury and the Earth are all lined up and we see a black dot cross the face of the Sun. The planet is silhouetted against the Sun. Now close one eye and position yourself so you can watch Mercury transit the Sun. You may have to move up or down or you know bend over. Yeah, just like you're doing. That's a good job. Okay. Now who can see Mercury transit? Oh, I just saw it. I do. Do you see it now? Yeah. All right. Good. All right. Thank you very much for being the Sun and for being Mercury. You can, we can turn off the Sun now and I'll turn on this star orbiting way over here far away from our Sun. Let's pretend I live on a planet orbiting this star. Let's say my planet orbits my star like this. Would you there on Earth be able to see my planet transiting my star? Can you see it? Good. Okay. How about now? Would you see the transit? No. Good. Right. For you to detect a transit the orbit of the planet has to be oriented in the correct way, right? And that's what the upcoming Kepler mission will be doing. Trying to find stars with planets orbiting them in such a way that their planet transits the star from Earth's perspective. But how far away are the stars? Pretty far, right. So they're so far away that there's no way that we can see a planet transit another star. Now we're here in San Francisco and we'd have to move this light 1500 miles away, say to Texas, to represent a star that was only 10 light years away. We can't do that, but we can move it a little farther away.