If you missed Mercury's journey across the sun Monday (Nov. 11), you're in for a bit of a wait for the next sun-transiting planetary occasion.
It's going to be more than a decade until the next transit of Mercury, which will occur on Nov. 13, 2032. But that's not as bad as the wait for Venus' next transit, which is still 92 years away.
These rare events are only possible through tricky alignments in planetary orbits. Mercury and Venus (like all the planets in our solar system) circle the sun in a plane, somewhat like a pancake. But it's not a perfectly flat pancake, and each planet has a slight tilt in its orbit.
Mercury's path, for example, is inclined at 7 degrees to the ecliptic (the plane in which the planets on average orbit in our sky). That means that from Earth's perspective, Mercury passes above or below the sun most of the time, only crossing the sun's disk 13 or 14 times a century. For Venus, the phenomenon is even rarer — it occurs in transit pairs separated by eight years, with each "pairing" happening about a century apart.
Only Venus and Mercury can pass across the sun in our sky, since they are on the inner side of the solar system, compared with Earth. Transits of Mercury tend to fall around the same time of year — either November or May. Venus' are more scattered.
Although Mercury's last transit was in 2016, if you missed today's, you'll need to wait an awfully long time, according to NASA. The next two transits will take place in 2032 and 2039, but these will not be visible from North America. U.S. skywatchers will have their next chance across the country in 2049 and from the West Coast in 2052.
As for Venus transits, make sure you have a reliable calendar (or memory) to mark down these dates from NASA: Dec. 10 to 11, 2117 (visible in southern and western North America, among other locations); and Dec. 8, 2125 (visible for at least part of the journey in the U.S.). Then the next pairing will be in 2247 and 2255.
Fortunately, there are plenty of opportunities to see the moon transit across the sun during a solar eclipse, or to see the moon pass into Earth's shadow during a lunar eclipse, a transit from the moon's point of view. Eclipses tend to take place every few months, although visibility varies considerably — especially for solar eclipses.
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