Forget blue moons. Even more rare is when the planet Mercury passes right across the middle of the sun, and it's happening Monday. Asia and Australia will pass up a great opportunity.
If you can't get hold of a telescope with solar filters to see for yourself, then you can watch a live broadcast of the transit, such as the one below. This composite image shows the Mercury transit in May 2016.
From our position on Earth, Mercury's transit is evidence of the solar system's clock-like movements.
While the next Mercury transit between Earth and the sun will happen 13 years from Wednesday (in 2032), it won't be visible from North America until the next occurrence in 2049.
While it takes the innermost planet only 88 days to orbit the sun (about nine months less than our world), Mercury's orbit is tilted, so it seldom lines up perfectly with the Sun and Earth.
In Maryland, clouds prevented NASA solar astrophysicist Alex Young from getting a clear peek.
Mercury is 3,000 miles in diameter, compared with the sun's 864,000 miles.
During its 2012 transit of the sun, larger and closer Venus was barely detectable by Young with his solar-viewing glasses. At about 9:20 a.m. CST, Mercury's center will be as close as it is going to get to the Sun's.
If that's not an option, NASA's Solar Dynamics Observatory (SDO) satellite will provide near-real time images of the transit. Scientists will use the transit to fine-tune telescopes, especially those in space that can not be adjusted by hand, according to Young.
Statistically, every star in the galaxy is alleged to have at least one planet orbiting it. Missions such as Kepler have confirmed the presence of thousands of exoplanets with thousands more waiting to be confirmed. Never look at the Sun directly or through a telescope without proper protection. It's this kind of transit that allows scientists to discover alien worlds.