The transit or passage of a planet across the face of the Sun is a relatively rare occurrence to view.
"This Monday is called the transit of Mercury, where Mercury passes in front of the image of the Sun as we look at it from the Earth". Greg Scheiderer's Seattle Astronomy blog lists several, including a gathering that he's planning to host at Seacrest Marina Park in West Seattle at 7 a.m. PT Monday (weather permitting).
The entire 5 ½-hour event will be visible, weather permitting, in the eastern USA and Canada, and all Central and South America. Asia and Australia will miss out.
According to the Delta College Planetarium, the next transit of Mercury won't be until 2032 and the next time Mercury passes this close to the center of the sun will not be until 2187. The next transit of Mercury is in 2032. Earthlings get treated to just 13 or 14 Mercury transits a century.
If you have a solar scope or filter, take it along. There's no harm in pulling out the eclipse glasses from the total solar eclipse across the USA two years ago, but it would take "exceptional vision" to spot minuscule Mercury, said NASA solar astrophysicist Alex Young.
"That's really close to the limit of what you can see", he said earlier this week.
"I was far from suspecting that Mercury would project such a small shadow", Gassendi wrote. Venusian transits are even rarer, happening on average only once a century. The next one isn't until 2117. When Mercury's leading edge first touches the sun, the planet will appear to grow a narrow neck connecting it to the edge of the sun, making the silhouette look like a teardrop.
Clouds won't be a problem for NASA's Solar Dynamics Observatory, however. Scientists will use the transit to fine-tune telescopes, especially those in space that can not be adjusted by hand, according to Young. From this data, astronomers can then calculate the size, orbit, and even some physical properties of these alien worlds. Periodic, fleeting dips of starlight indicate an orbiting planet.
Astronomers say transits of Mercury are not as historically important as transits of Venus due to its very small, angular size compared to the diameter of the sun.