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Astronomy Cast Episode 223 from Monday, March 7, 2011, the Transit of Venus.

Welcome to Astronomy Cast, our weekly facts based journey through the cosmos, where we help you understand not only what we know, but how we know what we know.

My name is Fraser Cain, I'm the publisher of the universe today, and with me is Dr. Pamela Gay at Professor at Southern Illinois University, Abutsville.

Hi Pamela, how you doing?

I'm doing well Fraser, how are you doing?

I'm doing really well. So since the planet Venus is closer into the sun than Earth, there are rare opportunities to see it pass directly in front of our parent star.

This is known as a planetary transit, and thanks to the geometry of the Earth and Venus, they only happen a couple of times this century.

The transits of Venus have been used by astronomers to unlock the scale of the solar system, and there's one just around the corner.

All right Pamela, so today we're going to talk about the transits of Venus.

So first I think, when we think about the geometry of the solar system, it almost seems obvious, right?

The Earth is further away from the sun than Venus, and so Venus, as Venus orbits more quickly than Earth, you would kind of imagine Venus passing right in front of the sun several times a year, right?

Or once a year anyway, yeah, so why doesn't that happen?

Well the reality is that the orbit of Venus is shifted 3.4 degrees relative to the Earth, so this means that if you had two hula hoops, one smaller than the other, and you lined them up just like Earth and Venus, the one on the inside would be tilted over 3.4 degrees.

And the distance between Venus and the sun is so great that that small shift means that most of the time we see Venus is appearing above or below the sun in the sky, and this difference prevents transits from happening on a regular basis.

Right, so when we are able to see Venus, it's not lost in the glare of the sun, then we're sort of making a direct line from us to Venus to some point on the background sky, you know, it's not passing in front of the sun, but even when Venus is lost in the glare of the sun, it's actually like a little above or a little below the sun, right?

Yeah, that's exactly the problem that we're dealing with, and the only time we ever see a Venus transit is in that magical moment when it just happens to be that the Earth, Venus and sun are lined up just as Venus is at that midway point in its orbit, at the node in its orbit where it crosses from above the sun to below the sun, and that doesn't happen very often.

Right, I can imagine that, right? It's taking this, it's almost like it's sometimes it's above the sun, sometimes it's below the sun, it's just when everything lines up, so how often does this happen?

Well, it happens in a pattern of you will get 105.5 year gap and 8 year gap, a 121.5 year gap and an 8 year gap, and this just keeps basically repeating over and over and over due to the patterns of how the Earth and Venus orbit is.

Orbit against one another, there's a basic resonance in the system, and so it's nice, we happen to live when we can see a pair of these eight conveniently for our adult lives, but there's a whole lot of people that are going to live and die in those hundred plus year gaps that will never have the opportunity to see this, but right now there are some people that got to see the 2004 transit, and there will be others who will get to see the one in 2012.

But that's kind of strange, I mean, you get an 8 year gap, so that's like what we're about to experience, we had one in 2004, and then we're going to see one in 2012, and then you get, what was it, a 100?

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