Assuming spring showers stay at bay, sky-watchers in the Northern Hemisphere will get to see a cosmic triple play on March 20, as a “super worm moon” lights up the vernal equinox. According to astronomy website EarthSky.org, a full moon has not landed this close to the first day of spring since the year 2000, and the two celestial events won’t happen less than a day apart again until 2030.
Because the moon’s orbit is egg-shaped, there are times when it is at what astronomers call perigee, or its shortest distance from Earth, and times when it is at apogee, or its farthest distance from Earth. And because the size of the moon's orbit varies slightly with each lunar cycle, perigee is not always the same distance from month to month.
At 3:45 p.m. ET (19:45 UT) on March 19, the moon will be a mere 223,309 miles away from our planet, making for an especially close perigee.
Then, at 9:43 p.m. ET on March 20 (1:43 UT on March 21), the moon will officially reach its full phase. As a result of these combined events, the full lunar disk will appear 14 percent larger and 12 percent brighter than usual—a spectacle widely known as a supermoon.... Read More: National Geographic