Hardly a week goes by that the media isn’t exhorting you to see a “can’t miss” meteor shower, “supermoon,” or other celestial display. In this clickbait cacophony, it can be hard to tell what’s worth a look, and what’s just hype.
A good test is to ask “how often does this happen?” (Supermoons, my personal bugbear, not only occur up to six times a year, but are visibly unremarkable — predictably breathless headlines announcing them notwithstanding.)
Planetary transits (when Mercury or Venus cross the sun’s face) are truly rare. Successive pairs of Venus transits are separated by over a century: The last ones were in 2004 and 2012, and the next won’t be until 2117 and 2125.
Mercury transits average about thirteen per century, separated by as few as 3.5 years, or as many as 13. Because of the relative orientations of Mercury’s and Earth’s orbits, Mercury transits always fall in May or November, the latter being twice as common.
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To see a Mercury transit, which can last over five hours, the sun must be above the horizon. So, even when one occurs, you’ll miss it if it’s at night.
Mercury’s next trans-solar excursion falls on Veterans Day, when the sun will rise with the transit already in progress. Between sunrise (just before 7:30 AM) and 11:04 AM, Mercury’s tiny silhouette will creep across the solar disk, visible only in properly-filtered magnifying optics (binoculars or telescopes). Be warned: Attempting to view this event with eclipse glasses or welding glass between your optics and your eye could result in permanent, irreversible vision damage.
Don’t miss this one! The next two transits, in 2032 and 2039, will only be visible eastward of Canada’s Maritime Provinces. Idahoans will have to wait until May 7, 2049.
Next column: Looking the celestial Bull in the eye.