Each calendar month, there’s typically just one each of the moon’s four major phases: one new moon, one first quarter moon, one full moon and one last quarter moon. But March, 2016, has two last quarter moons: March 1 and 31. Nowadays, people call a second full moon of any given calendar month a Blue Moon. Will some call Thursday’s production a “blue” last quarter moon?
Like all last quarter moons, this one rises around midnight as seen from across the Earth. Assuming you wake up early on March 31, no matter where you are, you’ll see the moon high in your predawn/dawn sky.
At last quarter, the moon appears to us as half-lit in sunshine and half-immersed in its own shadow. Of course, in the reality of outer space, it’s always half illuminated; but from Earth, we only see the moon as half-illuminated at its first and last quarter phases.
At last quarter moon, the lunar terminator – shadow line dividing the lunar night and day – shows you where it’s sunset on the waning moon. The terminator provides you with your best views of the lunar landscape through binoculars, the telescope or even the unaided eye. For optimum viewing, try looking in a twilight sky, when the moon’s glare is less obtrusive.
Now for a cool calendar trick. You know the major phases of the moon: new moon, first quarter moon, full moon, last quarter moon.
And you know we have two last quarter moons this month: March 1 and 31, 2016.
Two years from now, in 2018, the phases of the moon will fall on or near the same calendar dates that they do in 2016. However, two years from now, the same phase won’t be realigning with the same date. Instead, it’ll be the preceding lunar phase that fills the slot.
For instance, two years from now, the full moon will take the place of the last quarter moon in March, 2018, because the full moon is the major phase preceding the last quarter moon.
So, in the March of 2018, it’ll be the full moon (instead of the last quarter moon) that happens twice in one calendar month: March 2 and 31, 2018. Many people will take notice because – according to modern folklore – the second full moon of a month is the Blue Moon.
There’s also a seasonal Blue Moon, by the way – the third of four full moons in a single season. We’re currently in a season that’ll have four full moons and thus the next Blue Moon by the seasonal definition is coming up on May 21, 2016.
Later this year – in October, 2016 – we’ll have two new moons in one calendar month: October 1 and 30. Similarly, in the October of 2018, it’ll be the last quarter moon (the major phase preceding new moon) that shines on October 2 and 31, 2018.
Bottom line: The length of the months are based on the moon’s movement around Earth, and there’s typically just one of the major moon phases in each month. But March, 2016, has two last quarter moons: March 1 and 31.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.