Largest Moon of 2018
Astronomers at the Pisgah Astronomical Research Institute (PARI) point out that the upcoming New Years night full moon will be the largest of 2018. Didn’t the astronomers at PARI just put out an advisory that the largest full moon of the year occurred on December 4? Yes, but remember, that full moon was in calendar year 2017. A month later we have another large full moon on January 1. We start off the year with the largest full moon of 2018! Why do we have these large full moons, hyped by the media as “Super Moons,” so close together?
Let’s review the celestial mechanics of this phenomenon. The moon follows an orbital path around the earth that is an ellipse, a mathematical figure that is sort of a squashed down circle. Each time the moon orbits the earth it passes through a point called apogee where it is farthest from the earth and a point called perigee on the opposite side of its orbit where it is at its closest. The moon can pass through these points at any time of the month but, if it does so near full moon, we sometimes take notice of it. If full moon occurs near apogee, the moon will appear smaller than average. If full moon occurs close to perigee, the moon will appear a bit larger than average since it is closer to us. This is what occurred in December; we had perigee and full moon about 17 hours apart, the closest this occurred in 2017. Thus, we saw the largest full moon of the year on the night of December 3-4!
The moon takes roughly 27⅓ days to pass from one point in its orbit around and back again to the same point. This is called the sidereal month. The moon will be back at perigee at 5 p.m. EST on New Years Day. Meanwhile, the whole earth-moon system is orbiting the sun. This makes full moons about 29½ days apart; astronomers refer to this period as the synodic month. On the upcoming New Years Day, the moon will pass through perigee (356,565 km or 221,156 mi from earth) at about 5 p.m. The next full moon will be 9:24 p.m. EST that evening. Thus, the moon will be at perigee and full only 4 hours 24 minutes apart. That’s even closer than the 17 hours we had in December! This is the closest perigee and full moon will occur in 2018 and, thus, the closest a full moon will be to the earth. It will appear larger than any other full moon in 2018!