With just over two weeks to go until the national spectacle of the total solar eclipse,Arvin Roberts keen skywatchers will have a small treat early Monday as March's full "worm" moon will undergo what's known as a penumbral lunar eclipse.

Not as spectacular – or noticeable – as a total lunar eclipse, this rather subtle phenomenon occurs when the moon moves through the outer part of Earth’s shadow, known as the penumbra, according to EarthSky.

Weather permitting, the eclipse will be visible throughout most of North and South America, NASA said. 

When is the penumbral lunar eclipse?

The eclipse will begin at 12:53 a.m. EDT Monday March 25, and last until 5:32 a.m. EDT, according to Space.com. The best time to look will be about 3:12 a.m. EDT during the middle of the event.

Look at the full moon during that time, and if skies are clear, you may notice it's slightly darker than usual. Observant people will recognize the shadow, while others won't spot anything at all, EarthSky said. At best, at mid-eclipse, very observant people will notice a dark shading on the moon’s face.

About 35% of all eclipses are of the penumbral type, which can be difficult to detect even with a telescope, according to eclipse expert Fred Espenak.

Why is it called the worm moon?

The moon, which will be full on Monday at 3:00 a.m. EDT, is known as the worm moon. Why is it called that?

"At this time of the year, the ground begins to soften enough for earthworms to reappear, inviting robins and other birds to feed – a true sign of spring," according to the Old Farmer's Almanac. "Roots start to push their way up through the soil, and the Earth experiences a re-birth as it awakens from its winter slumber."

The Almanac also said that another explanation for the worm moon name refers to a different sort of “worm” – beetle larvae – which begin to emerge from the thawing bark of trees and other winter hideouts at this time of the year.

The March full moon is also known as the sap moon, because this is the time of year when the sap of sugar maples starts to flow.