On Monday morning, Mercury will slowly march across the face of the sun. The Department of Astronomy and the Astronomy Students Association will help interested viewers see this relatively rare event on the roof of RLM from 9:00 a.m. until 1:30 p.m.

A transit is an event in which a planet passes between a star and the viewer. On Monday, viewers from Earth can see Mercury transiting across the sun. The astronomy department will host a public viewing of the transit in a heliostat, a system that uses mirrors on the roof of RLM to direct a projection of the sun into a 13th floor room.  

“You’ll see, over the course of several hours, the little dot of Mercury move across the sun,” said Lara Eakins, an astronomy education and outreach coordinator.

The public will also be able to join members of the Astronomy Students Association on the roof of RLM. They will have small telescopes equipped with sunlight-blocking filters to make observing the sun safe.

“Every time we look at the sun, we usually use some sort of solar filter, so it’s really safe,” said Melissa Morris, co-president of Astronomy Students Association.

Eakins said viewers can try to see the transit with welder’s glasses or eclipse glasses that protect against UV light, although Mercury may be too small to see without a telescope. She also said viewers should use new telescopes with proper solar filters.

The only transiting planets visible from Earth are Mercury and Venus, because they are between Earth and the sun. Transits of Mercury occur about a dozen times a century, according to Eakins.

“It’s probably a factor of it being closer to the Sun, and it orbits more closely, consistently to the plane of the solar system between us,” Eakins said. “It’s not perfectly aligned, which is why you don’t see it every time it orbits the Sun.”

Although Mercury orbits the Sun every 88 days, Mercury transits are much rarer because the orbital planes of the planets don’t perfectly line up, according to astronomy research professor William Cochran. Transits are only visible when the planets are in the same orbital plane, and Mercury doesn’t line up with Earth every time.

“If they did, there would be transits all the time and it would be a dull, boring thing and nobody would care,” Cochran said. “But since they orbit in slightly different orbital planes, the transits are not all that common. They’re not tilted much, but it doesn’t take much.”

Transits outside of our solar system help astronomers find exoplanets, or planets that orbit other stars.  NASA’s Kepler mission has found thousands of exoplanets by looking at stars in the Milky Way galaxy. Astronomers can detect these exoplanets by watching for the dimming of a star caused when the planet blocks their light, according to McDonald Observatory research scientist Michael Endl. 

“To sum it up, the transit technique was incredibly useful,” Endl said. “Particularly with Kepler, that space mission revolutionized our understanding of planetary systems in the galaxy.”

The Kepler mission’s use of the transit method to detect exoplanets has given astronomers an idea of how common other planets are in our galaxy.

“It’s something that’s interesting to look at, just because it is fairly rare, but also it has some scientific use as well,” Eakins said.