The planet Mercury is seen in silhouette, lower left, as it transits across the face of the sun on May 9, 2016. Another transit of Mercury — the last one for 30 years — will take place Monday.

The planet Mercury is seen in silhouette, lower left, as it transits across the face of the sun on May 9, 2016. Another transit of Mercury — the last one for 30 years — will take place Monday.