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Transit of Venus: twice-in-a-lifetime phenomenon visible Tuesday over St. Louis

You don’t want to miss this, because it won’t happen again for more than 100 years.

Tuesday afternoon, starting just after 5 p.m., a rare astronomical event will be visible in the skies over St. Louis. It’s known as the transit of Venus. 

St. Louis Public Radio's Véronique LaCapra asked University of Missouri-St. Louis astrophysicist Erika Gibb to help explain this twice-in-a-lifetime phenomenon.

(Music in interview audio:“Transit of Venus March,” by John Philip Sousa) 

Where can I watch it?
You can watch the transit on the web live on Tuesday right here

What is this transit of Venus thing, anyway?

Gibb: So a transit of Venus is when Venus, from our point of view, appears to pass directly in front of the Sun, which is pretty rare because the Sun’s only about half a degree across, and Venus’s orbit is tilted about 3.5 degrees relative to ours. So most of the time it misses — it either passes above the Sun, or below the Sun.

And so when a transit happens what you see is the little black disc of Venus passing right in front of the Sun.

Right, because actually Venus’s orbit passes between Earth and the Sun about every, what is it, 584 days, I think? So the reason we don’t see it so often is because of the tilt in the orbit.

Gibb: Yeah, it’s the tilt that makes it really rare. You get pairs of transits eight years apart, so there was one eight years ago in 2004, and they have gaps of either 121.5 or 105.5 years.

So the last pair of transits was in 1874 and 1882, and the next one won’t be until 2117 and 2125. So these are the last chance for anybody here living currently to see a transit.

The first known sighting, I think, of the transit of Venus was in England in December 1639, and historically it was in the 17th and 18th centuries that the transit started to play an important role in astronomy. Can you talk a little about that?

Gibb: Yeah, so, it was Edmond Halley who first suggested that a transit could be used to measure the size of the solar system.

Basically using triangulation – you would have two people on different parts of the Earth observe the transit, and try to accurately determine when the transit started or ended. And then they could use triangulation techniques to figure out what the actual distance between Earth and the Sun, and Earth and Venus, were.

We didn’t actually know what that distance was.

The distance from the Earth to the Sun.

Gibb: The Earth to the Sun. You know, the scale of the solar system. We knew relative scales, but we didn’t have the absolute numbers. We didn’t know how many kilometers it was, or how many miles.

Here's more on the early uses and history of the transit from NASA:


And that distance actually has a name: the Astronomical Unit. Why is it so important?

Gibb: Well, by definition the distance between the Earth and the Sun is one Astronomical Unit. So it sets the scale. And astronomers use odd scales, instead of using miles or kilometers or something like that, because the distances are so vast.

We use something called Astronomical Units for solar system-sized scales, and we use something called light-years or parsecs for size scales between stars, for example.

And you mentioned it was Edmond Halley who figured out how to do this. That’s Halley of Halley’s comet, right?

Gibb: Yeah, same guy, same guy!

Did astronomers figure out anything else by observing the transit of Venus?

Gibb: Well, they noticed that when Venus passed in front of the sun, that it wasn’t just a sharp cutoff. And so it was predicted that Venus might have an atmosphere that might explain the refraction of the sun.

And that technique is actually used today to study exoplanets, which is why this particular transit’s going to be scientifically interesting for current astronomers. It’s not just a historical thing anymore, we’re actually going to use this transit to see if we can observe how the light decreases as Venus passes in front of the sun. And we use that on exoplanets now to learn about how big they are, you know, how fast does it take for the light to decrease...

So watching those exoplanets pass in front of their stars. Is that what you’re saying?

Gibb: Yes, yes. So the Kepler mission is doing that right now. Seeing what we call transiting exoplanets.

And, what you can observe is not just the star’s light dimming, but you can observe how long it takes the star’s light to dim, and how much the star’s light dims, and you can determine quite a bit about the planets from doing this.

And so we’re going to use Venus, that we know a lot about, to help us learn about these systems that we don’t know very much about.

And here's the rest of the story about the transit from NASA:


And even more, here.


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