Mercury puts on rare show as it passes directly between Earth and the sun
Mercury put on a rare celestial show, parading across the sun in view of most of the world. Stargazers used solar-filtered binoculars and telescopes to spot Mercury — a tiny black dot — as it passed directly between Earth and the sun Monday.
The entire 5 ½-hour event was visible in the eastern U.S. and Canada, and all Central and South America. The rest of the world, except for Asia and Australia, got just a sampling.
Unlike its 2016 transit, Mercury scored a near bull's-eye this time, passing practically dead center in front of our star. Mercury's next transit isn't until 2032, and North America won't get another viewing opportunity until 2049. Earthlings get treated to just 13 or 14 Mercury transits a century.
How to watch
Telescopes or binoculars with solar filters were recommended for viewing the celestial spectacle. As with a solar eclipse, it is not safe to look directly at the sun without special protective eyewear — not just regular sunglasses. There's no harm in pulling out the eclipse glasses from the total solar eclipse across the U.S. two years ago, but it would take "exceptional vision" to spot minuscule Mercury, said NASA solar astrophysicist Alex Young.
During its 2012 transit of the sun, larger and closer Venus was barely detectable by Young with his solar-viewing glasses. "That's really close to the limit of what you can see," he said earlier this week. "So Mercury's going to probably be too small." Venus transits are much rarer. The next one isn't until 2117.
Mercury is 3,000 miles in diameter, compared with the sun's 864,000 miles.
Mercury cut a diagonal path left to right across the sun on Monday, entering at bottom left (around the 8-hour mark on a clock) and exiting top right (around the 2-hour mark).
Although the trek appears slow to viewers, Mercury was actually zooming across the sun at roughly 150,000 mph (241,000 kph).
NASA broadcast the transit as seen from the orbiting Solar Dynamics Observatory, with only a brief lag. Scientists will use the transit to fine-tune telescopes, especially those in space that cannot be adjusted by hand, according to Young.
It's this kind of transit that allows scientists to discover alien worlds. Periodic, fleeting dips of starlight indicate an orbiting planet.
"Transits are a visible demonstration of how the planets move around the sun, and everyone with access to the right equipment should take a look," Mike Cruise, president of the Royal Astronomical Society, said in a statement from England.
