(Reuters) – Venus and Jupiter, the two brightest planets in the sky this month, will be joined by tiny Mercury for a rare celestial show this weekend.

Typically, Venus, the second-closest planet to the sun, and Jupiter, which orbits beyond Mars, are tens of millions of miles apart. But they have been cycling together while moving ever closer to each other this month, joined by the innermost planet Mercury.

The celestial show peaks on Sunday when the trio will appear as a bright triangle of light in the western sky beginning about 30 minutes after sunset.

Triple conjunctions are relatively rare, according to NASA. The last one was in May 2011 and the next one will not occur until October 2015.

“This triple is especially good because it involves the three brightest planets in May’s night sky,” the U.S. space agency said on its website.

The formation should be visible even in places with bright city lights, though a clear view of the western horizon is a must.

Astronomers suggest sky-watchers let Venus and Jupiter be their guide. As the sky darkens the planets will be visible to the naked eye.

“They really do shine so brightly that you might mistake them for one or two approaching airplanes with their landing lights turned on,” the University of Texas’ StarDate magazine wrote on its website.

On Sunday, Mercury forms the top of the triangle. By Monday, Venus and Jupiter will be side by side, less than 1 degree apart.

“After that, Venus and Mercury will continue to climb higher into the evening sky, while Jupiter drops toward the sun,” said StarDate.