Transits of Mercury

Just as Venus does, Mercury on rare occasion passes across the face of the Sun when Mercury reaches inferior conjunction within a few days of passing the ascending or descending node of its orbit. Like with transits of Venus, there is a regularity to transits of Mercury. Successive transits of the planet are separated by periods of 3.5, 7, 9.5,10 or 13 years. This is because of a harmonic that brings Mercury and Earth back to the same position each 16,802 Earth days or 46 Earth years with Mercury arriving at inferior conjunction 0.37 days later at the end of each cycle. There are several mathematical harmonics within this more precise group that are not as exact which cause the uneven spacing of transits due to the elliptical nature of Mercury's orbit. Because of this, transits of Mercury can be neatly organized into groups much like solar and lunar eclipses fall into saros series. For example the transit of May 2003 is part of a family that includes transits that occurred in May 1957 and May 2049. The November 2006 transit is part of a family that includes transits in November 1960 and November 2052. There are not nearly so many series in progress as there are with eclipses and they do not last as long either. Currently there are six series of transits in progress. Transits also are very different depending upon which side of the Sun they occur.

Mercury passes the descending node of its orbit at a position that corresponds to May on Earth and passes the ascending node on the opposite side of the Sun at a position corresponding to November. Thus transits can only occur within a pair of windows currently centered on about May 10 and November 10, respectively. When Mercury transits the Sun at the descending node in May, it is also only 30 degrees of orbital travel past aphelion and thus travels relatively slowly in its orbit. Being far from the Sun also means it is closer to Earth so the planet appears larger than in November events, about 12 seconds across. Each time a new member of a transit series takes place, the track shifts about 200 arc seconds further south because Mercury arrives at inferior conjunction about eight hours later than it did at the event 46 years earlier. After approximately 10 events, the track shifts off the Sun and the series ends after a run of approximately 400-450 Earth years. November events are different. November events take place with Mercury about 30 degrees of orbital travel past perihelion, so the planet is traveling at near its maximum velocity in space and scoots across the Sun at twice the speed that it does in a May event. If Mercury were to travel directly across the center of the Sun in both a May and a November event, the November event would last only half as long. Also Mercury's speed means that the planet's track across the Sun shifts only 100 arc seconds northward across the Sun with each successive event in a series. As a result of this, a November transit series will have about twice as many members and last twice as long as a May series. Overall therefore there are twice as many transits of Mercury that take place in November as take place in May. Mercury is also much farther from Earth during November events so the planet is somewhat smaller, only about ten arc seconds across during November transits.

Observing a transit of Mercury is a bit tougher than Venus in that optical aid is likely needed to pick out Mercury's tiny disk. Venus can be more than one full minute across when at inferior conjunction but Mercury is only ten to twelve arc seconds across when it transits the Sun. You certainly have more opportunities to view Mercury transits in that they will occur about 13 or 14 times per century, so you will have opportunities to view a few in a lifetime, unlike those of Venus which are once in a lifetime events.

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