Full Moon Tonight: What Is the Blue Sap Moon?

full moon blue
This photo of the full moon was taken from the International Space Station on February 24, 2005. NASA

The second full moon of the month is set to appear in the sky Saturday morning in the United States, making it a blue moon. The first full moon of March, known as the "full worm moon," happened on the first day of the month. The second will be the "full sap moon," or a blue moon.

During times when two full moons happen in the month of March, the second one is called the full sap moon. The name comes from the fact that in March, sap starts to flow from sugar maple trees, according to The Old Farmer's Almanac.

The first moon of the month gets the name "full worm moon" because it appears around the time that earthworms begin to come out of the ground during the spring season.

Whenever there's a second full moon in a single month, it's generally referred to as a blue moon. The name has nothing to do with the way the moon appears or its color at that time. The only reason the moon would appear to have a blue tint to it would be if there was some sort of particle in the atmosphere filtering the light and causing it to appear blue. This has happened following volcanic eruptions that scattered ash into the atmosphere but it's not a frequent occurrence.

Both an actual blue-appearing moon and a second full moon within one month are both uncommon. Originally, a blue moon referred to the fourth moon in a season, after the term was improperly used in a radio program in 1980 it was accepted as the second moon of the month, according to Sky & Telescope.

The full blue sap moon won't be visible in some parts of the country. It's set to become full at 8:36 a.m. EDT, by which time the moon will have set and the sun will have risen on the East Coast. On the West Coast, the moon will become full at 5:36 a.m. PDT before the moon sets, making it visible to those who live there.

The moon will appear full both Friday and Saturday nights. It may only be about 99 percent full while it's dark out, but the difference will be so small that anyone viewing with the naked eye won't be able to tell it's not completely full.