Jimmy Westlake: Don’t miss rare transit of Venus
May 28, 2012
"By far the noblest (sight) astronomy affords."
That's how Sir Edmund Halley of Halley's Comet fame described one of nature's rarest astronomical events — a transit of the planet Venus across the face of the sun. You won't want to miss the only chance in your lifetime to see this noble sight on June 5.
In centuries past, transits of both Venus and Mercury were important events because astronomers could use the timings to estimate the sizes of these planets and their distances from Earth. Nowadays, these sizes and distances are well established. Modern astronomers search for the transits of undiscovered planets in front of other distant stars. Studying transits in our own solar system helps them know what to look for when observing distant suns.
In effect, a transit is a miniature version of an annular eclipse of the sun, with Venus rather than the moon crossing the sun's face. Even more rare than the once-every-76-year passages of Mr. Halley's comet, transits of Venus occur in pairs separated by more than a century. The two transits in each pair are eight years apart. There were no transits of Venus in the 20th century, with the previous pair happening in 1874 and 1882.
The transit of June 8, 2004, was the first since the Victorian era and, unfortunately, was visible only from the eastern half of North America, bypassing Colorado. The good news is that the second in the current pair of transits on June 5 will be visible from the entire continent.
The transit begins in Northwest Colorado at about 4:10 p.m., when the dark edge of Venus will take a tiny nibble out of the sun near the "one o'clock" position on the sun's fiery disk. Mid-transit occurs at 7:30 p.m., with the sun only 10 degrees above the west-northwest horizon, and then the sun and Venus will set together for us at 8:34 p.m., the transit still in progress.
Like eclipses of the sun, at no time will the transit of Venus be safe to view without a solar filter. The tiny black disk of Venus is barely large enough to be seen with the unaided eye, through a proper solar filter, but a properly filtered telescope will provide the best view. You can also use your unfiltered telescope as a projector to cast an image of the sun safely onto a wall or white card positioned behind the eyepiece, but you should never leave your telescope unattended such that children or others might attempt to look into the eyepiece at the sun. Instant blindness could result!
This transit might be the most rare event that I ever have the pleasure to tell you about. Anyone who misses the transit of June 5 will never get to see another one. The next pair won't happen until 2117 and 2125, events that no one alive today likely will be around to see.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. Check out his astrophotography website at http://www.jwestlake.com.