Though the phrase “once in a blue moon” means not very often, on Wednesday, skywatchers will have their second chance this year to witness a blue moon.

The first blue moon occurred on Jan. 31.

A blue moon, explained Russell Lavery, assistant professor of astronomy, is the second full moon to occur in a calendar month.

“It’s kind of an interesting thing, but nobody gets worked up about it,” Lavery said. “It’s more of a calendar phenomenon than anything else.”

Since the average interval between full moons is 29.5 days, usually every month has exactly one full moon, Lavery said.

He said it is relatively common for a single blue moon to occur — on average, once every three years.

According to an informational Web site, www.obliquity.com /astro/bluemoon.html, two blue moons appearing in one year is a rare occurrence; it usually happens about four times per century.

The next double blue moon will occur in 2018, Lavery said.

Lee Anne Willson, professor of physics and astronomy, said people everywhere can witness the phenomenon.

“It’s one of the few astronomical phenomenon that you can see anywhere if the sky’s clear,” she said.

Despite the name, the moon doesn’t actually appear to be blue, Lavery said.

However, there have been instances when the lunar sphere appeared to be blue in hue.

Lavery said when the volcano Krakatoa erupted in 1883, it sent dust particles into the atmosphere, which made the moon appear to be blue for two years. And in 1951, forest fires in Canada sent smoke particles into the air that caused the moon to look blue to viewers in North America, he said.