Mark your calendars! An astronomical phenomenon of great significance will occur this Monday on May 9th. Mercury will cross the sun for the first time since 2006.

The transit of Mercury occurs when Mercury passes between the sun and the Earth (inferior conjunction), since of course Mercury is closer to the sun than we are. In order for the transit to occur, the two points in the orbit of Mercury must intersect the plane of the Earth’s orbit. Mercury periodically achieves its inferior conjunction every 116 days. But, every inferior conjunction does not lead to a transit because the orbital planes of Mercury and Earth are not the same. In fact, Mercury’s orbit is inclined by 7 degrees relative to Earth’s ecliptic plane and it rises and sets to 7 degrees north and south of the sun. Only when the line of Mercury’s nodes happens to coincide with Earth’s orbit does a transit occur during Mercury’s inferior conjunction. Therefore, Mercury must happen to be near a node during its inferior conjunction for a transit to occur because being at the nodes means an intersection with the ecliptic.

Mercury transits occur in May and November and on average, they occur only 13 times a century. In November, when Mercury is at its ascending node (ascending to the north side of the ecliptic) and near perihelion, it tends to move faster. November transits thus tend to be twice as frequent as the May transits since Mercury is now more likely to appear on the sun’s disk given that it is closer to the sun in November than in May. May transits occur when Mercury passes through its descending node and when it is near aphelion (furthest point from the sun). This makes Mercury less likely to transit the sun in May than in November.

The upcoming transit is expected to last 7 hours and 30 minutes between about 7:12 a.m. and 2:42 p.m. EDT. One would need a solar telescope with proper protection to filter out the bright light. The transit will be viewable in North America, South America, Europe, Africa, and most of Asia. However, quite unfortunately, Australia and New Zealand will be facing away from the sun and thus viewers will have to miss out on such an extraordinary event. The event will also be livestreamed here for folks who would much rather want to stay indoors.

Featured image: NASA