The tiny planet Mercury is making a big splash on May 9, when it glides across the face of the sun for the first time in 10 years.
The black silhouette of the planet will take about seven and a half hours to make its trek across the solar disk, an event known as a transit. The sky show lasts from 7:12 a.m. ET until 2:42 p.m. ET.
Weather permitting, the best seats will be on the East Coast of North America as well as the bulk of South America and Western Europe, where the entire transit will happen during daylight hours. For those on the West Coast of the Americas, the transit will already be under way at sunrise, and for viewers in most of Africa, Eastern Europe, and the bulk of Asia, the transit will still be in progress at sunset.
Folks in East Asia, Australia, and New Zealand will miss out entirely, since the whole transit will happen when these regions face away from the sun.
Safe viewing of the sun’s disk is paramount at all times—never look directly at the sun without proper protection or you risk damaging your eyes. Not that you’d be able to see much with your naked eyes, anyway. From Earth, the black dot of Mercury will be just 1/158 the width of the solar disk, so you will need relatively high-powered visual aids to watch the transit.
For people using backyard telescopes, be sure to attach a solar filter that fits on the instrument’s front end. You can also use high-powered binoculars and white cardboard to create a magnified projection of the event.
And of course this transit can be enjoyed by anyone with an internet connection, thanks to live webcasts that will showcase the entire event. Both Virtual Telescope and Slooh.com promise to have coverage from beginning to end on Monday.
While Mercury does pass between Earth and the sun every four months or so, we only get to see it transit when everything lines up just right. The small planet’s orbit is tilted compared to the plane of Earth’s orbit, so most of the time the swift “messenger of the gods” passes above or below the sun's disk from our line of sight. It's the same reason we do not see solar eclipses every month.
Only the two innermost planets, Mercury and Venus, can transit the sun for Earth-based viewers. While Mercury transits happen 13 to 14 times a century, Venusian transits are even rarer, happening on average only once a century. The last Venus transit occurred in 2012, and we won’t see another one until 2117.
Don’t fret if you miss this transit of Mercury—you will get another chance in 2019. But after that, we won’t see another such event until 2032. North Americans will have an even longer dry spell to contend with after 2019, as they will have to wait until 2049 for the next Mercury transit visible from that part of the globe.
The most interesting part of a transit for many astronomers is the black drop effect, an optical illusion that happens when the planet either just enters or starts to leave the sun’s disk.
When Mercury’s leading edge first touches the sun, the planet appears to grow a narrow neck connecting it to the edge of the sun, making the silhouette look like a teardrop. This strange apparition happens again just as Mercury becomes engulfed by the sun’s disk.
Seeing a planet creep across our sun also offers a chance to see one of the ways scientists hunt down planets around distant stars. As a distant planet moves in front of its host star as viewed from Earth, our telescopes can detect the very slight dips in the brightness. From this data, astronomers are able to calculate the size, orbit, and even some physical properties of these alien worlds.
So far, NASA’s Kepler mission has been able to successfully identify and confirm 1,041 exoplanets across the galaxy using transit events like what we will be seeing up close with Mercury on May 9.
This story was updated on May 9 at 9:30 am ET.