Mercury to pass in front of sun in a rare celestial occurrence

It happens around 13 times a century: From our perspective, Mercury — the smallest planet in our solar system — will pass in front of the sun.

Most of the world, including the United States, on Monday will be able to see the planet as a tiny black dot passing slowly in front of its host star. You shouldn’t stare directly into the sun (ouch), but by watching online or using a telescope with a special filter, you can see Mercury in all its tiny glory.

If you have your own telescope, you can watch the event using a safety filter to protect your eyes from the sun. If you don’t have a filter handy, you can use a sheet of paper to rig up a safe viewing method — you can project the im- age of the sun (in the form of a white disk) onto a sheet of paper, then watch the black dot of Mercury crawl across it. You can also check out NASA’s websites and social media accounts for live image updates.

If you live near an observatory or science center, you should check out their plans — you may be able to pop in and take a peek on their telescope.

But why should you care? Because it’s a very special event for a very cool little planet.

Mercury has an orbital period of just 88 days, making it by far the fastest orbiter in the solar system. And while Mercury is orbiting, so are we — at a completely different pace. So for our planets to line up just so for us to see the other world sweep over the sun is an uncommon event. This is the first Mercurian transit since 2006, and we won’t see another until 2019. Wait, you say — that’s a lot of transits to have in just over a decade. How do we average out at just 13 for every 100 years? Mercury passes between the Earth and the sun every 116 days, but its orbital plane is skewed away from our own by a few degrees. It orbits the sun on a tilted trajectory, by our perspective. So its intersection with our orbit has to happen when it’s also intersecting with our orbital plane. Venus — the only other planet between us and the sun, so also the only other planet that transits from our perspective — orbits much more slowly and on a plane slightly less skewed than Mercury’s. Venusian transits are more rare, but also a little easier to track.