Hey, sky fans. This coming Monday, May 9, is going to be a pretty cool day in the skies. The planet Mercury — the tiny one — is going to transit the Sun, spending about seven hours between around 7am and 3pm US Eastern (an hour earlier for US Central; two hours for US Mountain; and three for US Pacific time zones) cutting across the Sun’s face as seen from here on Earth. There are only three objects in the Solar System that can transit the Sun from our perch here (anyone know what the other two are?), and they’re very rare. The last time there was a transit of Mercury was in 2006, and the next one isn’t until November 2019.
Mercury zips around the sun in just 88 Earth days, so it passes close to and kinda sorta between the Earth and Sun fairly often. In fact, the term “Inferior Conjunction” is the point in a couple of objects’ orbits when they’re aligned on the same side of the Sun. By contrast, superior conjunction is when two objects are aligned on opposite sides of the Sun.
Transits like these happen at inferior conjunction when an object passes directly between the Sun and the Earth, and are rare because their orbits aren’t perfectly aligned. So, more often than not the planet falls above or below the Sun as we see it. In order for us to see a transit, everything needs to line up just right. If you can imagine the different view of a baseball game you get just by standing up, you might be able to see what I’m getting at. Historically, these transits have really scientifically useful because they’ve allowed astronomers to get a better idea of the size of the Solar System and objects in it.
Even though this transit will be visible to much of the world (sorry, New Zealand; maybe if your flag vote had gone differently…), the trouble is Mercury is really small, only about 3,000 miles across, and it’s over 50 million miles away. That’s so small that the Solar System has two moons (Jupiter’s Ganymede, and Saturn’s Titan) bigger than it. Also, of course, the Sun is really bright. So, in order to see it you’ll need a telescope with special filters to both magnify what’s going on and keep your eyes safe. If you can find your way to a planetarium or to someone with a good telescope, you’ll be able to see a small black dot slowly making its way across the Sun. It’ll be a pretty great sight.
I cannot emphasize this enough: DO NOT LOOK AT THE SUN WITHOUT SPECIAL SUN-SAFETY EQUIPMENT. If you do, you can permanently damage your vision.
I don’t think I’ll be able to see it, myself. I have a couple pairs of thickly filtered eclipse glasses, so I’ll have a look at the Sun anyway. It’s pretty great just knowing it’s going on.
Here’s a small video from NASA that shows what it’ll be like:
Clear skies, everyone!