TORONTO -- When people act strangely or things go badly, you can be sure that someone might blame it on a full moon or Mercury being in retrograde.

But when it comes to the latter, what does it mean?

“If you’re looking at Mercury -- day after day after day -- you’ll see that it moves eastwards across the sky,” York University physics and astronomy professor Paul Delaney told CTV’s Your Morning.

“But then it will suddenly stop and for about three weeks it’ll go backwards -- westwards. Stop. And then resume its eastward progression,” he said, laughing that it’s “bothered astronomers for thousands of years.”

Retrograde motion is the apparent change in movement of a planet in a direction opposite to other planets within a system. Every single planet in our solar system appears to do it at some point during the year.

But the phenomenon is more often seen with Mercury because it rotates faster and closer to the sun.

Delaney explained the apparent change in motion isn’t because Mercury is “putting on the brakes and suddenly reversing its direction” but because of a simple geometric principle.

“As Mercury orbits around the sun it laps the Earth which is also orbiting the sun,” he explained.

“And there’s this three-week period where literally the geometry is such that it stops its eastward motion and moves westward. Stops again and then resumes [going east],” Delaney said.

For Mercury, retrograde motion happens three times a year because it goes around the sun every 88 days so there are more opportunities to see it from Earth’s vantage point. Venus goes into retrograde after an estimated 225 days, while outer planets such as Jupiter only do it once a year.

Delaney also stressed that -- despite Mercury’s retrograde motion being associated with astrological signs -- the phenomenon doesn’t affect people’s behaviours.

“Things are happening all the time and to pin it on poor little Mercury or any of the other planets is a bit of a copout,” he laughed. “Take ownership of the situation, people!”