The month of March opens and closes with a full moon this year, making this the second "Blue Moon" month in 2018.
The moon first became full on Thursday, March 1, at 7:51 p.m. EST (0051 GMT) and will again on Saturday, March 31, at 8:37 a.m. EDT (1237 GMT). The first Blue Moon of 2018 was the spectacular Super Blue Blood Moon of Jan. 31.
For observers in New York City on March 31, the almost-full Blue Moon will set at 7:03 a.m. local time, or about 1.5 hours before it is full. It will rise again at 7:37 p.m., and while the moon will be past full, the difference from a full moon will not be visible to the naked eye. (On March 1 the moon rose at 5:33 p.m. local time and was well above the horizon when the satellite reaches its fullest phase. It set the following morning (March 2) at 7 a.m. local time.) [The Moon: 10 Surprising Lunar Facts]
Not every month gets two full moons. The time between full moons (known as a synodic month) averages 29.53 days, so we usually see one full moon per month. About every two to three years on average, we see a "Blue Moon" — a second full moon in one month.
Two Blue Moons in a year is relatively uncommon. According to EarthSky.org, the next year when two calendar months will each have two full moons will be 2037, when January and March will have Blue Moons. The last time it happened was in 1999.
One effect of having a full moon on Jan. 31 and March 1 is that February has no full moon at all. February is the only month in which this can happen, because the month has only 28 days (while the phenomenon can happen in a leap year, it is rare). The next time a full moon will skip February will be in 2037, according to TheSkyscrapers.org, a site run by amateur astronomers, and the phenomenon is sometimes referred to as a "Black Moon."
Moons of many names
Moons in various months have sometimes-evocative names. The March full moon, for instance, was dubbed the Full Worm Moon by some Native American tribes, because it happens when temperatures rise and the earthworms emerge, according to the Old Farmer's Almanac.
Other tribes have called the full moon of March the Sap Moon "as it marks the time when maple sap begins to flow and the annual tapping of maple trees begins," the Old Farmer's Almanac states. [Full Moon Names 2018: From Wolf Moons to Cold Moons]
What's a full moon?
A full moon is defined as the moment when the side of the moon that faces the Earth is fully illuminated. How much of the moon's face appears to be illuminated from our perspective on Earth depends on where the moon is in its orbit.
For about half of the world, the moon won't be visible at the exact moment when it is officially full. That's why the full moon is sometimes listed as happening during the day, when the moon is below the horizon, as it will be for observers in New York City on March 31. On the other hand, skywatchers in Los Angeles, where the moon reaches its fullest phase at 5:37 a.m. local time on March 31, can see it happen about an hour and a half before the moon sets at 7:04 a.m.
The moon has phases because as it revolves around the Earth, we see it from different perspectives. Like planets, the moon appears to move against the background stars. Unlike the planets, however, it does so fast enough that one can see it happen over the course of a night. The moon moves approximately one lunar diameter (about half a degree) each hour, eastward relative to the stars even as it rises in the east and sets in the west. So, the moon can move some 6 degrees to the east of its position at moonrise over the course of a 12-hour night.
This is illustrated by where the March full moons will be in the sky: On March 1, the moon will be in the constellation Leo, the lion, and about 13 degrees above the eastern horizon when it is at maximum illumination. On March 31, the moon will be in Virgo when it hits full phase (and below the horizon in the eastern U.S.). It will still be in Virgo when it rises for New York City observers that evening, but as the moon sets the next morning at 7:34 a.m. local time, it will have moved several degrees east. By the next day, it will be in Libra.
Planets on parade
The full moon tends to wash out a lot of fainter objects in the sky, but one can still see some brighter planets when the moon is full. On March 1, for example, Jupiter will rise at 11:42 p.m. local time in New York and will be about 23 degrees above the southwestern horizon at moonset on the morning of March 2. You can find the giant planet in Libra — no telescopes or binoculars necessary. From city locations, in fact, Jupiter may be the only "star" visible in that portion of the sky, as most stars in Libra are not very bright.
Saturn will rise at about 3 a.m. local time (the wee hours of March 2) and by moonset will be about 26 degrees above the horizon in Sagittarius. Mars, which rises at 2 a.m., will be in Ophiuchus and appear nearly due south. Both planets should also be easily visible without telescopes or binoculars.
Venus and Mercury are both "evening stars" — on March 1, they will both set shortly after 6:30 p.m. local time in New York City. These two planets will be no more than 5 degrees above the horizon by the time the sky gets dark enough to make them visible, so they will be very difficult to see, especially from a city location.
By March 31, the situation for observing Venus will be markedly better. Venus will be a full 12 degrees above the western horizon at the end of civil twilight (which is at 7:47 p.m. in New York) and bright enough that it should be just visible. Mercury will have set by that time, but because it is only 3 degrees away from the sun, that planet will be difficult to observe under any circumstances (and dangerous to view, without proper protective equipment to block the light of the sun).
Other planets will join the Blue Moon in the sky later that evening. Jupiter will rise at 10:40 p.m. on March 31, followed by Mars at 2:21 a.m. on April 1. Saturn will rise just 3 minutes earlier and will appear quite close to Mars in the sky. The two planets will be only about 4 degrees apart in Sagittarius.