A mini solar eclipse, compliments of Mercury
The planets Jupiter and Saturn have been putting on quite a show in the evening sky since mid-summer, shining among the retreating summer constellations Scorpius and Sagittarius. Unfortunately their performance is winding down. Jupiter is setting just after sunset and Saturn is not far behind in the low southwest sky. Later on this month Jupiter will take a curtain call as it has a close encounter with the bright planet Venus in the evening twilight of the 23rd through the 25th.
The smallest planet in our solar system, Mercury, is going to put on a big show this Monday. It’s going to transit or cross in front of the face of the sun. Call it a very mini solar eclipse! Without a doubt this is a rare event that only happens about a dozen times a century, although we’ve been fortunate because this will be the third transit in the last 15 years. It will be our last one until 2032.
Since Mercury orbits around the sun every 88 days, you would think that we’d see Mercury come between Earth and the sun quite often. That’s not the case though. Transits are rare because the planets in our solar system do not orbit the sun in exactly the same plane. Earth and Mercury literally have to be at the right place at the right time. Mercury usually passes a little below or above the sun when it’s the same general direction as the sun, something called inferior conjunction. Not this time though!
For nearly five-and-a- half hours Mercury will appear as a small dot crossing the disk of the sun from east to west, but it will only be visible in South America, most of North America, and the western tip of Africa. In the United States only about the eastern third of the contiguous 48 states will be able to witness the entire transit. In the rest of U.S. the transit will begin before local sunrise.
In Minnesota and Wisconsin we’ll only see part of this transit.
As you can see on the diagram, by the time our sun rises Monday morning, Mercury will already a little less than a quarter of the way across the sun’s disk. At 12:03 p.m. the show’s over as Mercury completes its journey across sun and the transit will be history.
Watching this transit of Venus is tricky and can be down right dangerous. You never want to stare at the sun with the naked eye and certainly never never never look at the sun directly through binoculars or a telescope. You could burn out the retina in your eye and be blinded permanently in less than a second.
What you can do is put a piece of white cardboard or artist’s paper on the ground and hold a pair of binoculars just above it, maneuvering the binoculars in the general direction of the sun using the sun’s shadow. When the shadow of the binoculars is at its smallest you should be able to see a projection of the sun’s disk on the piece of cardboard. You’ll actually see two disks of the sun because of the binoculars. See if you can spot the dot silhouette of Mercury. It may a bit of a challenge! I have included a photo of this method that I used at during the solar eclipse back in August of 2017. But again, I can’t emphasize this enough … never never never look at the sun directly through binoculars or a telescope and don’t even stare at the sun with just your eyes.
You can also certainly watch it the transit on the Internet. Many sites will be providing live views of something that won’t happen again until 2032. I hope I make it!
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul.