Forget blue moons. Even more rare is when the planet Mercury passes right across the middle of the sun. The transit began on Monday at 7:35 a.m. ET. It takes more than five hours for Mercury to pass from one side of the sun to the other. The transit ended around 1:45 p.m. ET.
A Mercury transit, as it's called, occurs only 13 times in 100 years, according to NASA, and it won't be seen from North America again for another 30 years, or from anywhere until 2032.
The transit time means watchers on the East Coast will be able to see the whole thing, but viewers almost anywhere in North America won't miss out, since Mercury will still be making its journey when the sun is up on the West Coast.
In fact, the only places it can't be seen from are Australia and most of Asia and Alaska, according to NASA's Jet Propulsion Laboratory.
Transits allow astronomers to study the movement of planets and stars. It's also the same method used to detect exoplanets around other stars outside of our solar system.
But, just like during an eclipse, viewers needed a solar filter since looking directly at the sun can cause permanent eye damage.
"Viewing transits and eclipses provide opportunities to engage the public, to encourage one and all to experience the wonders of the universe and to appreciate how precisely science and mathematics can predict celestial events," said Mitzi Adams, a solar scientist in the Heliophysics and Planetary Science Branch at NASA's Marshall Space Flight Center in Huntsville, Alabama. "Of course, safely viewing the Sun is one of my favorite things to do."
Mercury, the smallest planet in our solar system, appeared as a tiny dot on the sun, so NASA recommended using a telescope with a certified solar filter.