North America will have a blood worm moon this week. Here's how you can see it

This week, robed in twilight, a "blood worm" will pierce the night sky. As ominous as that sounds, don't think leeches — think lunar.

Late Thursday night or Friday early morning, North Americans with clear skies can look up to see a full moon with a beautiful reddish hue. 

And while it has that eerie nickname, it's not the only one, depending on different cultures.  

This event is actually two occurrences coming together: a total eclipse of the moon by the Earth at the same time as it goes through a full moon phase for March. It's not too rare for this to happen. The last March total lunar eclipse was in 1978, according to NASA — though much of North America couldn't see it.  

The "blood" part is from the eclipse. When the Earth passes in front of the moon, it obscures the sun's light from hitting it — but some still sneaks through. To understand why the moon turns that colour, amateur astronomer Dave Chapman says you have to imagine you're an astronaut on the moon during the eclipse. 

"The sun is shining on the Earth from behind. It's being backlit," explains Chapman, a fellow at the Royal Astronomical Society of Canada. "The Earth's face will be dark, but along the edge — where the atmosphere is — there is sunlight filtering through." 

But that light comes from all the parts of the world that are experiencing sunrises or sunsets. As light during those times are more red and orange, this is the hue that hits the moon and reflects back to us. 

However, it's not always blood red. Its variations, from totally invisible to copper, are measured in the Danjon scale. We won't know where this particular eclipse rates on that scale until after it happens. 

Chapman says there's another unique quality from this "side-lighting" effect from Earth: It will also change how we perceive the shape of the moon. 

"Normally when you look at the full moon ... it doesn't look like a sphere, it just looks like a disc," Chapman told CBC News from Dartmouth, N.S. But the eclipse lighting turns it from a flat, bright, silvery disc into more of "a sphere hanging in the sky." 

The timing of this eclipse also aligns with how different cultures around the world refer to the moon during this particular month. 

Experts attribute the "worm" moniker to Indigenous tribes in the southern U.S. who would see more insects and earthworms emerge during this time of year, and it stuck through its usage in the Old Farmer's Almanac as March's full moon. 

But other cultures didn't see those worms, instead calling it by different names. For example, "more northern tribes of the northeastern United States knew this as the crow moon, with the cawing of crows signalling the end of winter," writes NASA's Gordon Johnston. 

To Mi'kmaq, whose traditional territories include Eastern and Atlantic Canada, the moon around March is known as Si'ko'ku's (pronounced see-uke-ay-we-goos), or the maple or maple sugar moon, owing to this time of year when sap starts running down trees.