Don’t Miss This: The Transit of Venus

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Less than one week from today, the (hopefully) clear blue skies of San Antonio will serve as the backdrop for a dramatic and rare astronomical event: the transit of Venus. On June 5, 2012, Venus will pass directly between the Earth and the Sun, seen (through the proper protective gear, of course) as a black dot traversing the face of our star.

As far as predictable cosmic phenomena go, transits are more rare even than total solar eclipses, occurring in pairs eight years apart that are spaced out by 112-year gaps, on average. The last transit, visible in parts of Europe, Asia, and Africa, took place in 2004. The next will occur in the year 2117.

In other words: If seeing a transit of Venus is on your bucket list, you don’t want to miss this. We surely won’t be around for the next one.

Intrigued? Excellent. But before going any further, let me emphasize this one most important point: NEVER look directly at the Sun without proper protective measures. You could actually go permanently blind. Proper protective gear for observing a solar event (transit, eclipse, or otherwise) means eyeballs covered with legitimate solar glasses or number 14 welder’s glass. If you’re viewing it through a telescope, this means a solar filter over the large end of your scope. Another safe way to indirectly observe the Sun is with a pinhole projector, which you can make at home. NASA has a great overview of all these safe solar viewing options here. If NASA says it, I believe it. You can order solar viewing glasses from several venders online, or purchase them at supply stores like Analytical Scientific on Bandera Road. Ok, now let’s proceed.

Do NOT look at the sun without proper protective eyewear!

Tuesday’s transit, the last one to occur during any of our lifetimes, will begin at 5:05 pm in San Antonio and remain visible until the Sun sets. Trinity University professor and chair of the Department of Physics and Astronomy, Dr. David Hough, shed some light on the historical significance of a Venus transit.

“Back in 1761 and 1769, there were a pair of these transits and people were really excited at the opportunity to measure the size of the solar system,” Hough said.

More than a century before, Hough explained, Johannes Kepler articulated the laws of planetary motion, describing the velocity and movement of planets orbiting the Sun. From those laws, Kepler reasoned that one could determine the relative distances of the planets. Determining the actual distance between any two planets would complete the equation and allow scientists to estimate the actual size of our entire solar system.

“The key was you would need a ruler to measure at least one of those distances to determine all the rest of them, to set the scale,” Hough elaborated. “And nobody knew how to do it.”

Fast-forward to the early 1700s: Edmond Halley, best known for calculating the orbit of Halley’s Comet, proposed that the distance from Earth to the Sun could be derived by using measurements from the 1761/1769 transits of Venus and a concept called parallax. “Now maybe you don’t use the term parallax in every day speech,” Hough said, “but we’re used to the idea of parallax.” Dr. Hough gave me a simple demonstration of parallax, which I will now share with you. Don’t be scared.

Part one:

Look at the image above. Close your left eye. Hold your thumb up, about six inches away from your face, and cover up the number 1. Open your left eye and close your right. Now, having switched eyes, your thumb is covering up maybe the number 3 or 4, right?

Part two:

Move your thumb away from your face, closer to your screen, and repeat. This time, your thumb didn’t shift as far when you switched eyes, perhaps only from 1 to 2, correct?

The apparent shift in position of your thumb when looking from two slightly different locations (your right eye and your left eye) and from different distances is a demonstration of parallax in action. In Hough’s simple terms: “Something close to you will seem to shift a lot from one eye to the other, but far away it shifts a little bit.” So, how is this related to the transit of Venus? “In the solar system, there are two things we know that on occasion will pass in front of the Sun, and whose parallax we therefore can measure: Mercury and Venus.” Parallax is measured as the angle between two different lines of sight, and Venus, being closer to Earth, yields more precise measurements. Using the solar parallax angle determined during the transit of Venus, the radius of the Earth, and a little high school trigonometry, the distance from Earth to the Sun was finally estimated in 1771, after the 1761/1769 transits.

“Once you have that,” explained Hough, “then Kepler kicks in. Kepler told us what all the relative distances are, so once you know one distance, you’re in business. You know them all. It sets the scale.

San Antonio has a particular connection to the 1882 transit. In preparation for studying the 1874 and 1882 events, countries from all over the world dispatched Venus transit expeditions to the most ideal observing locations. The U.S. Naval Observatory sent expeditions to eight different sites each year, intending to more accurately measure and calculate the distance from Earth to the Sun. In 1882, Belgium sent its first ever international research expeditions to Chile and Texas with the same purpose. Because of its geographic location and proclivity to clear weather, San Antonio was hailed as the best viewing location in North America that year. Our fair city received both a Belgian expedition, led by astronomer Jean-Charles Houzeau, and a Naval team, stationed at Fort Sam Houston. A historical marker at the Bullis House Inn and a plaque at Fort Sam commemorate the efforts of these Venus-viewing pioneers in the Alamo City, and this article in The Southwestern Historical Quarterly provides excellent detail about this and other significant early astronomy events in Texas.

If the historical importance of the transit of Venus isn’t enough for you, maybe the modern-day scientific implications of this event will get you going. Today, astronomers study Venus transits to better understand and search for extrasolar planets, or planets outside of our solar system, which can be detected by slight fluctuations in light that occur when they transit around their stars. “If you understand better what’s happening in a close up example, in terms of measuring exactly what goes on during the transit, that might help you to interpret distant transits around other stars,” Hough noted. “That’s one modern scientific benefit, but otherwise it’s just a spectacular event that we enjoy seeing because it’s so rare.”

Once you have acquired your protective eyewear or a friend with a filtered telescope, you can look for the transit to occur at these times in San Antonio on Tuesday, June 5, 2012:

  • First contact – 5:05 pm (also called exterior ingress, meaning when Venus first appears tangent to the exterior of Sun)
  • Second contact – 5:22 pm (interior ingress, when Venus first passes just barely but entirely in front of the disk of the Sun)
  • Center of the transit – 8:27 pm
  • Sunset in San Antonio – 8:31 pm

As for viewing location, try to situate yourself somewhere with a clear view of the western horizon. For those of you not in San Antonio, this website provides an interactive map with local transit times, tailored to your specific location. Should something go awry in your pre-transit procurement of welder’s glass, a scope-wielding friend, or a west-facing view, you can also see a webcast of the transit from Mauana Kea, Hawaii on NASA’s website starting at 4:45 pm Central.

The transit of Venus has inspired scientific research, international travel, art, music, and more for hundreds of years. This Tuesday, whether you observe the transit or simply listen to John Philip Sousa’s Transit of Venus march, may you feel inspired by the knowledge of this spectacular and singular event place above you.

Miriam Sitz works for Accion Texas Inc., the nation’s largest non-profit microlender. A graduate of Trinity University, she blogs on and sells handmade goods on Follow her on Twitter at @miriamsitz. [Click here for more stories from Miriam Sitz on the Rivard Report.]

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