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What to know about Friday's 'black moon'

It turns out that the ominous-sounding "black moon" is just another term for a new moon, only when a new moon happens twice in the same month. You can't see a new moon, but you can see the crescent moon that appears in the days after. NASA's Marshall Space Flight Center Follow/Flickr Creative Commons

Maybe you've heard about the looming arrival of the "black moon" on Friday. What is this ominous-sounding phenomenon?

It turns out a "black moon" is simply another term for the second new moon in a month. It's similar to a "blue moon," which refers to the second full moon in a month.

In fact, the only thing mysterious about the black moon is its name.

"It is not a name which is really popular or customary in science, not even among amateur astronomers. It is chiefly a social media creation," Milan Mijic, a physics and astronomy professor at Cal State Los Angeles, tells KPCC. "In a sense, every new moon is a black moon, because a phase of the new moon is when we see the moon roughly in the direction of the sun, which means we do not see it. It's daytime. When the sun goes down, the moon goes with it."

Anthony Cook, astronomical observer at Griffith Observatory, echoes that sentiment. "It's sort of a non-event," he says. "You can't see new moons anyway unless there's an eclipse happening the same time. It's the moon you don't notice the second time in the month."

Mijic says to think of the arrival of the new moon like hands on a clock: The moon is the faster hand on the clock, the sun is the slower hand. They don't move exactly at the same time in the same direction, so the positioning of sun, Earth and moon that creates this condition generally only comes around once a month.

For it to happen twice is even rarer. Black moons occur only once about every 32 months, according to Space.com skywatching columnist Joe Rao.

The black moon is set to arrive on Friday evening at 5 p.m., but given that it's really just a moonless sky, there won't be anything to see. If you really want to see something, Mijic suggests tuning into the sky in the days after the black moon.

"On Sunday and Monday, we will see a tiny crescent moon passing by Venus. It's one of the most beautiful sights that used to hold people for millennia," Mijic said.

Venus should appear by about 7 p.m., like a white star that doesn't twinkle, and that sliver of a crescent moon will sit very low on the western horizon at sunset on Sunday, Mijic said. It will be barely visible, but by Monday evening it will be slightly easier to see.