On Monday, May 9, 2016, the planet Mercury will pass across the face of the sun. Transits, as such events are called, occur only when a planet passes between the Earth and sun as both planets orbit the sun.

Only Venus and Mercury can transit the sun from our earthly vantage point because they are the only planets that travel inside Earth’s orbit. Transits of Mercury occur only 13 times every century. Transits of Venus are even more rare. Only eight of them have occurred since the first use of telescopes for astronomy in 1609.

The next transit of Mercury will occur on Nov. 11, 2019. After that, you’ll have to wait until 2032. The next transit of Venus won’t happen until 2117.

If we factor in the cloudiness of central Ohio, even a more-frequent Mercury transit can be one of those once-in-a-lifetime experiences that astronerds are always talking about. I’ve been observing the sky for half a century and, if the weather permits, I will be seeing only my second transit of Mercury.

The transit begins at 7:12 a.m. and ends at 2:42 p.m. Mid-transit, when Mercury is halfway across the sun’s disc, occurs at 10:57 a.m. The transit will last about 7 hours and 30 minutes.

Listen to me now. To ensure the safety of your eyes, you will need special equipment to observe the transit. If you own eclipse glasses, please note that they will not allow you to observe the transit. Mercury is far too small to be seen with eclipse glasses. You will need a specially designed solar-safe telescope.

If you do not own a solar telescope, you can attend one of two public events hosted by Perkins Observatory and the Columbus Astronomical Society on May 9.

The CAS event will occur at Homestead Park, located at 4675 Cosgray Road in Hilliard. It will begin at 11 a.m. See http://columbusastronomy.org for details.

The Perkins observing session will be from 7 a.m. to 2:42 p.m. at the Delaware State Park on the dam embankment, which is located at 3920 U.S. 23 N, north of Delaware.

You can also experience the transit on the web at the following locations:

• https://live.slooh.com/stadium/live/transit-of-mercury-2016

• http://serviastro.am.ub.edu/twiki/bin/view/ServiAstro/TransitMercuri090516

Other Internet sites may be available. They can be found by doing a search online for “Mercury Transit 2016 Live.”

As you can imagine, such relatively rare events are of some interest to astronomers, both amateur and professional.

Amateurs get a chance to use their very expensive solar equipment to see what is — for them — a spectacular astronomical event. Before you dismiss a little black dot crawling across the surface of a star as unspectacular, consider this:

Mercury is the closest planet to the sun. As such, it zips around old Sol at a stunning velocity of almost 30 miles per second. Thus, you can actually watch it move, particularly as it first enters and finally leaves the edge of the solar disc. For one of the few times in your life, you get a chance to physically observe with your own two eyes a planet orbiting the sun.

Professionals aren’t as interested in transits as they used to be but, back in the day, they were crucially important to our understanding of gravity and the exact measurement of time. Astronomer S.A. Mitchell identified those issues in a 1907 issue of Scientific American:

“Mercury has some peculiarities of motion which have cast a little doubt on the exactness of the law of gravitation, the great fundamental principle which is the foundation of exact astronomy. Transits are observed in order more correctly to determine the planet’s orbit, and so test this question, which is in reality one of the great astronomical problems. In addition these transits give a means of finding out whether the earth is a good timepiece … whether it is rotating uniformly on its axis.”

Using GPS satellites, astronomers can now measure time with exactness that does not depend on a transit Mercury.

Explaining Mercury’s “peculiarities of motion” did, in fact, change our understanding of gravity in a fundamental way that Mitchell never imagined. Gravity helps to place a planet like Mercury at a specific location on its orbit at any given time. However, precise measurements of Mercury’s position during transits revealed that Mercury was not exactly where it was supposed to be.

The solution finally came when Albert Einstein revolutionized our understanding of gravity. The pull of gravity affects not only the motions of solid bodies like Mercury but also the space around them. The sun’s substantial gravity bends the space between it and Mercury. Mercury is exactly where it is supposed to be in an Einsteinian, bent-space universe.

Old astronerds like me get the greatest thrill of all. I’ll get a chance to show a few people the strange majesty of their solar system in action. If you can sneak away from school or work for an hour or two, come and join me at the Delaware Dam.

But my heart would break if you injured your eyes trying to observe the event. Don’t look at the sun without specially designed, solar-safe equipment. Don’t look at the sun through a regular telescope. Ever. Don’t look at the sun in binoculars. Ever. Don’t look at the sun with just your eyes. Don’t look at the sun. Period.

See http://perkins.owu.edu/solar_viewing_safety.htm for more information on how to view the sun safely. If you think that you’ve come up with a safe way of doing so, please call me first at 614-886-7331. Seriously.