Astronomers around the Northern Hemisphere prepared their telescopes Monday to follow a half-millimeter-sized dot gliding across the northern edge of the sun.
At 3:19 p.m. Monday, University astronomers and students near the west side of Coffman Union’s plaza watched Mercury begin its last voyage across the sun this century.
“It’s a rare thing; we won’t be able to see it again until we’re really old,” said Chris Pulloy, an Institute of Technology freshman.
As it passes across the sun, Mercury is only visible from the Northern Hemisphere every 25 years, said John Dickey, an astronomy professor.
This time around, the silhouetted image of Mercury was only visible for 45 minutes.
Because looking directly at the sun is damaging to the naked eye, the University’s astronomy department projected a telescope image of Mercury onto a white sheet of paper, allowing many students to see it for the first time.
Fifty percent bigger than the moon and 300 times smaller than the sun, Mercury is 40 million miles from Earth, said astrophysics graduate student Ed Rhoades.
As the planet closest to the sun, Mercury completes its year — one complete orbit of the sun — faster than any other.
While Mercury, the second-smallest planet, can sometimes be seen just before sunrise or just after sunset, it crosses the sun only once every seven years. Most of its trips are not visible from the Northern Hemisphere.
But late Monday afternoon, Mercury was visible from many parts of North and South America. Earlier Monday morning, the planet was visible in parts of Japan and Australia.
“This is a way to see Mercury,” said Larry Rudnick, an astronomy professor. “It’s hard to see Mercury in the evening sky because it is always located so close to the sun.”

Amber Foley covers science and technology and welcomes comments at [email protected]. She can also be reached at (612) 627-4070 x3213.