On Nov. 8, 2006, Mercury Slipped Across the Face of the Sun, and Astronomers Around the Pacific Watched the Smallest Planet Make a Rare Crossing

On Nov. 8, 2006, the innermost planet did something it manages only about a dozen times a century: it passed directly between Earth and the Sun. From western North America across the Pacific, anyone with proper solar filters could watch a speck of Mercury inch across the solar disk.

A Dot on the Sun

A transit of Mercury is easy to describe and hard to see. The planet is so small and so far away that it appears as a tiny, perfectly round black dot, far too minuscule to be seen without magnification. Looking at the Sun without proper protection is dangerous, so observers used solar-filtered telescopes or projected the Sun's image onto a screen. The 2006 crossing took several hours, and the best views were from the Pacific region and the western Americas, where the Sun was well above the horizon for at least part of the event. Backyard enthusiasts and professional observatories alike lined up their instruments, and the pinpoint silhouette drifted steadily from one edge of the Sun toward the other. Unlike a solar eclipse, nothing about the sky darkened. The Sun carried on exactly as bright as ever, with one small, steady blemish making its way across the glare. For viewers expecting spectacle, the drama was in the patience.

The event also carried scientific history. Astronomers have long used planetary transits to refine measurements of the solar system, and timing Mercury's contacts with the Sun's edge has been a classic test of orbital predictions for centuries. In 2006, the Sun was also being watched from space by newer tools, including the Japanese-led Hinode observatory, launched only weeks earlier, and the long-running SOHO spacecraft. The irony was hard to miss: with spacecraft studying the Sun in extraordinary detail, one of the afternoon's most closely followed sights was a small dark dot that anyone with a filtered lens could follow by eye. Mercury's next crossing came in 2016, and then again in 2019, which made the 2006 transit a reminder that some of astronomy's best shows arrive on a schedule, and then make you wait.