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Transit Of Venus: A Once In A Lifetime Event

Sylvie Beland

A rare astronomical event will be visible over the skies of Kansas City at 5:10 p.m. Tuesday, June 5—the transit of Venus.  That’s when Venus, from our viewpoint, will pass across the sun.

Veronique LaCapra spoke with University of Missouri-St. Louis astrophysicist Erika Gibb about the historical significance of this celestial phenomenon.

Places to view the transit of Venus

• Powell Observatory, Louisburg, Kan.

• UMKC Warkoczewski Public Observatory, roof of Royall Hall, 5200 Rockhill Road

• Emporia State University, Cram Science Hall

And remember, never look directly at the sun.


Interview highlights:

On what causes the transit of Venus.

“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.”

On how often it comes around.

“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.”

On why it’s historically important

“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.”

On how the transit of Venus is being used today.

“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.”


Veronique is a science & technology reporter for KWMU in St. Louis.
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