Mohawk Valley Astronomical Society

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Flight of the Messenger

by Perry Pezzolanella

Astronomical events are well predicted thanks to the power of today’s computers. Some events such as conjunctions are more common than a solar eclipse. All events are at the mercy of the weather. Planetary conjunctions may allow for a missed night or two but events such as eclipses and occultations happen fast and are so brief that there is no waiting and no second chance for possibly years, if ever again. There is a do-or-die event in 2016 that is rare and will demand at least a reasonably sunny day.

A transit of a planet across the Sun is a rare event. Only a planet that orbits closer to the Sun than Earth can cross in front of the Sun. Mercury and Venus are the only planets that are capable of showing a silhouette against the Sun but it does not happen too often. These two planets have to line up nearly perfectly between the Sun and Earth, but unfortunately neither planet orbits perfectly on the same orbital plane with Earth. Both planets orbit the Sun at an inclination which means most of the time they pass above or below the Sun instead of in front of it. Venus transits are rare with the last pair occurring June 8, 2004 and June 5, 2012 and the next pair not until December 11, 2117 and December 8, 2125. If you missed the Venus transits you are out of luck, but there is a consolation prize.

Mercury orbits the Sun much faster than Venus, therefore it passes between the Earth and Sun more frequently increasing the chance for a perfect line-up that produces a transit. Although Mercury transits are rare there are more opportunities to see it at least once and possibly several times during a lifetime. The last transit of Mercury on November 8, 2006 was rained out locally but the next one is only about five months away on May 9! The transit will begin at 7:12 A.M. and end at 2:42 P.M., lasting 7½ hours. Unless the day is totally socked in by clouds or a storm, it might be possible to see the tiny black dot of Mercury against the huge Sun. Mercury could be confused for a sunspot so it is best to observe the transit over a period of time. Unlike a sunspot Mercury will be pure black and moving slowly. It would be a fun project to sketch or photograph its changing position. Please exercise great caution with the Sun and do not look at it without a proper solar filter or #14 welder’s glass, or better yet, project the Sun’s image using a simple pinhole projector.

A transit elicits the feeling of depth and makes our 3-D perception within the Solar System real. While watching the transit keep in mind how large the Sun really is compared to the planets. The Sun will be nearly 94 million miles away but Mercury will be about 42 million miles closer to Earth, about 52 million miles away, and yet Mercury will look like an insignificant speck! If this transit is missed, the next one is November 11, 2019, but historically the weather is rarely good here in November. A transit of Mercury is rare, making it special, and should not be missed. Free up some time this coming May 9 to observe and appreciate this rare celestial event!