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With Johannes Kepler’s discovery of the elliptical nature of planets’ orbits in the opening decades of the 17th century came the ability to predict their motions with unprecedented accuracy. With this newfound power, Kepler began to calculate when celestial bodies would draw near one another in the sky, motivated by the perceived astrological significance of such conjunctions.

In 1629, Kepler found that in November and December, two years hence, Mercury and Venus, respectively, would pass directly between the Earth and sun, known as a transit. Unfortunately, Kepler’s death in 1630 prevented him from verifying his predictions (although he would have missed the Venus transit in any case, since it occurred during the nighttime hours in Europe).

Mercury transits are relatively common, averaging 13 per century (the last was in 2006). Venus transits are much rarer, occurring just four times every 243 years. Since the ability to predict transits, as well as the technology to observe them (i.e. the telescope) didn’t exist before Kepler’s time, only six transits of Venus have been observed in all of human history.

On June 5, southern Idaho will see most of the next transit, which commences just after 4 p.m., its end truncated by sunset after 9 p.m.

Observing the transit safely requires the same precautions as viewing a partial solar eclipse. Sharp-eyed viewers, with proper filters, should see Venus as a tiny speck slowly traversing the sun’s upper right quadrant. Solar telescopes will provide a better look — CSI’s Centennial Observatory will be open for free views.

Parents: If your child is old enough to comprehend it, make every effort to let them safely view the transit. With medical science and a bit of luck, they might just live long enough to see the next one, in 2117.

Next column: Venus’s vanishing moon.

Chris Anderson manages the College of Southern Idaho’s Centennial Observatory in Twin Falls. He can be reached at 732-6663 or canderson@csi.edu.

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