On Monday, May 9, you will be able to watch planet Mercury traverse the Sun. The price of admission is a telescope and special solar filter, or a friend who has said items, or a local astronomy club with festivities planned to celebrate the rare celestial event. So what's going on up there?
Conceptually, we all understand that planets rotate and orbit the Sun at varying speeds and distances. Unless you're an astronomer, however, things can get a little murky when you start trying to really visualize things. The effects of axial tilt or even the phases of the Moon can be challenging to picture without resorting to holding tennis balls next to light bulbs. The things we non-astronomers have somewhat of a grasp on (the arc of the Sun across the sky, for instance) are offset by the things that are a bit more confusing than you might think (the Moon's path across the sky over the span of a month). Even then we're sometimes locked into a very geocentric understanding of things.
In short, orbital mechanics is hard for most of us to really grasp, and so every new illustration of Newton's laws at work helps.
Enter the transit of Mercury. There, clear as day (pun intended), you will be able to watch a planet as it orbits our star, and you can finally believe Copernicus without having to learn Latin and read De Revolutionibus Orbium Coelestium.
Mercury is the closest planet to the Sun, and a Mercurial year—the time it takes to revolve around the Sun—is a scant 88 Earth days. Moreover, because of its slow rotation on its axis, one day on Mercury lasts 58.6 Earth days. This means on a Mercury colonist’s second birthday, he or she would only be three days old.
Mercury’s orbit is the most eccentric of the planets circling the Sun, and depending on the time of its year, it could be as close to the sun as 29 million miles and as far away as 43 million. This doesn’t make for consistent temperatures. At its coldest, Mercury can be -280°F. At its warmest, it can reach 800°F.
So how do you witness the transit of Mercury? Here's one way, in three easy steps.
Do not stare at the Sun. Yes, there is a giant ball of fire in the sky, and yes, it is a little weird that we evolved never to look at it, but those are the breaks. You also have an appendix whose only job is to rupture and kill you. The point is that if you stare at the Sun, you will go blind or burn a hole in your retina. Moreover, if you want to see Mercury, your sunglasses will not help you. Welder's goggles will not help you. Solar viewers will not help you. Mercury will appear to be about 150 times smaller than the Sun; it is too small for you to see unaided.
Get a telescope. Do NOT look through it—yet. First, get a solar filter designed specifically for your telescope and attach it. If the filter is damaged, do not use it. If the filter only fits "well enough," do not use it. A pristine solar-viewing filter that fits securely to your telescope is the only way to go about this task.
Now look through your telescope. Keep the lens cover on the telescope's finderscope so that you're not tempted to look through it and consequently blind yourself.
The British Astronomical Association has more information on how to safely use a telescope to view the Sun. Sky and Telescope also has a comprehensive viewing guide, offering such alternatives as the "solar projection method." If all of this is a bit much, you can watch the transit live on NASA TV or Slooh. NASA is also streaming the event live on Facebook.
Mercury will begin its transit of the Sun at 7:12 a.m. EDT and will last for 7.5 hours, completing its journey across the disc at 2:42 p.m. It will be viewable across the U.S. and indeed by most of the world. If you miss the transit, you'll have another chance on November 11, 2019. But if you miss that one, you'll have to wait until 2032.