The Transit of Venus!

A very rare celestial spectacle will occur on the late afternoon of Tuesday, June 5th, when the planet Venus will transit the face of the Sun—an event that can be seen even with the (protected) unaided eye!   These transits happen in pairs separated by 8 years (the last one having been in 2004) and then not again for more than a century. The next pair won’t occur until 2117 and 2125, so this one is not to be missed! Let’s take a closer look at the transit itself.

What exactly is a “transit”? It’s when one solar system body (Venus in this case) crosses in front of another one (in this case the Sun) as seen from the Earth.  Being the only two planets inward of us, only Venus and Mercury can transit it.  Transits are not to be confused with “occultations”—during which the Moon, for example, in its perpetual orbital motion eastward around the sky covers up a star, one of the planets, or even the Sun itself during a total solar eclipse, and hides it from our view.

What are the times of this event? “First contact” occurs on the late afternoon of June 5th when the leading (right) edge of Venus first touches the left (northeastern) limb of the Sun’s disk and is predicted for 6:04 p.m. EDT.  “Second contact” happens shortly thereafter at 6:22 p.m., at which time Venus will have entered fully onto the solar disk.  This 18 minute interval is the amount of time it takes our “Sister Planet” (so-called because it’s nearly identical in size to the Earth) to move its apparent diameter along its orbit.  Note that these times will vary by a minute or two depending on exactly where you are within your time zone.  The transit continues for more than six hours from beginning to end, but much of Venus’s slow march across the Sun will happen after our “Daytime Star” sets at 8:21 p.m. for us here on the East Coast.  Having an unobstructed western horizon will help prolong its visibility.  The further west you live, the more of it you will see—with the entire transit from beginning to end being visible for those living in Alaska and Hawaii. As Venus exits the Sun’s northwestern limb, it moves from the evening sky where it shone prior to the transit into the morning one where it will soon become visible before dawn.

What will the transit look like? Venus will be an obvious jet-black dot slowly moving from upper left to lower right across the face of the Sun.  It’s appearance is often compared to that of a sunspot, but these magnetic storms are not as dark nor round as the planet And, yes, as mentioned it will be visible to the unaided eye—but not without proper protection to filter the Sun’s intense glare!  This brings us to the next very important section.

Viewing precautions.  And now we get serious.  June’s transit of Venus is truly a once-in-a-lifetime celestial spectacular not to be missed.  But you must exercise care in watching it.  Looking at the Sun through even the darkest pair of sunglasses will not protect you.  They may filter its visible light, but harmful ultraviolet and infrared rays will still get through.   One traditional recommendation is looking through a #14 welder’s filter.  Better are an inexpensive and safe pair of “Eclipse Shades” similar in construction (only!) to the 3-D glasses being used in theaters, or a hand-held “Eclipse Viewer,” both of which are available from Rainbow Symphony at: http://www.rainbowsymphony.com/soleclipse.html.  For up-close views of the transit, binoculars and telescopes are recommended.  However (and here especially), proper filters are an absolute must to avoid serious eye damage or blindness!  And the filters must be of the type that fit over the front of a telescope or both binocular lenses and NOT over their eyepieces.  Many inexpensive small telescopes (especially imported ones) have an eyepiece filter marked “Sun” on it.    These are absolutely deadly and often crack due to the intense heat being focused by the telescope itself.  Stopping most of the light and heat of the Sun before it enters an instrument is the only safe and sane way to view it directly. One extensive source for optical solar filters is Thousand Oaks Optical at: http://thousandoaksoptical.com/ (click on the “solar filters” icon).  Another safe but not as effective way to view the transit is to project the Sun’s image through a pinhole in a piece of cardboard and onto a white sheet of paper.  The projected image will be quite small, but depending on the hole size and how far back the screen is placed it should reveal Venus. A similar scheme called “eyepiece projection” is sometimes used with telescopes themselves but has been known to crack eyepieces if prolonged too long.  And here, as any time a telescope is being used for solar viewing, its finder scope must be capped since it can also cause damage!

Mention should also be made of another event this month: in the pre-dawn of June 4th the full Moon will be partially eclipsed for those living in the central and western parts of the country.

— James Mullaney

Former assistant editor at Sky & Telescope magazine & author of eight books on stargazing.