Being the third rock from the sun, Earth has two neighboring planets that orbit closer to the Sun than she does, Mercury and Venus. Both of these inner worlds can, on occasion, pass directly in front of the sun as viewed from Earth, producing a tiny eclipse called a transit. For this to happen, all of the celestial bodies involved must line up at just the right time and just the right spot. Consequently, transit events are rare.
Transits of Venus happen in June and December, often in pairs separated by eight years, with the pairs spaced about 125 years apart. The last complete pair of Venus transits happened in 1874 and 1882, but we are now in between another pair. On June 8, 2004, Venus passed in front of the sun for the first time in more than a century and was visible from the eastern seaboard of the U.S. The second transit will happen on the afternoon of June 12, 2012, and will be visible from the Western U.S., including Colorado.
With a safe solar filter held in front of your eyes, you can watch a transit of Venus without a telescope. Unlike the moon, which can totally block out the face of the sun during an eclipse, Venus appears as a small, round, black dot silhouetted in front of the much larger bright solar disk. It takes about six hours for Venus to pass completely across the face of the sun during a transit.
Transits of the innermost planet, Mercury, happen more frequently than transits of Venus, but they are more difficult to observe. There are, on average, 13 transits of Mercury a century, occurring in the months of May and November. The last one was May 7, 2003, but this event was not visible from the Western U.S. The last transit of Mercury visible from Colorado was on the afternoon of Nov. 15, 1999.
Now, after seven years, Coloradans are once again in store for a transit of Mercury. On the afternoon of Nov. 8, Mercury will make its appearance against the bright face of the sun, but because Mercury is smaller than Venus and much farther from us, its tiny silhouette requires not only a safe solar filter, but a telescope to view. With an approved solar filter attached to your telescope, you can safely watch the event directly through the telescope eyepiece.
In the event that you own a telescope but not a safe solar filter, you can still watch the event. Carefully align your telescope on the sun by watching its shadow cast on the ground and then project the image of the sun onto a white card or piece of paper held several inches behind the eyepiece. Spectators can simply watch the progress of the transit on the white screen. (Warning: Extreme caution must be exercised when using this technique so that unsuspecting persons do not look into the eyepiece of the telescope. Instant and permanent blindness can result. Never leave an unattended telescope pointed at the sun.)
Mercury will first make its appearance at the eastern edge of the sun at about 12:12 p.m. Nov. 8. Mid-transit occurs about 2:41 p.m., and the transit ends at 5:10 p.m. when Mercury exits the western edge of the sun. The end of this event happens close to sunset and might be difficult to observe low in the southwestern sky. Better to catch it early in the afternoon while the sun and Mercury are still high in the sky.
Would you like to see this rare transit of Mercury for yourself, but you don't have a telescope available? Well, you're in luck! Weather permitting, the students of the Colorado Mountain College SKY Club will have telescopes set up for public viewing of the transit of Mercury from beginning to end. Observing will take place on the patio in front of Bristol Hall on CMC's Alpine Campus on Bob Adams Drive in Steamboat Springs. Bring a camera and you can photograph the transit right off of the projection screen. Bring a friend and you can both experience the thrill of seeing a tiny eclipse of the sun by the planet Mercury.